Introduction

Microalgae are photosynthetic microbial cell factories that are currently a subject of global interest as a promising and sustainable source of fuels, food, feed, bioproducts, carbon sequestration, waste mitigation, and environmental remediation (Alam and Wang 2019; Alam et al. 2020). Theoretically, microalgae are capable of achieving hyper lipid and biomass productivities which are often higher when compared to any other lipid or biomass producing crop. Besides, their biomass contains valuable metabolites including carbohydrates, lipids, proteins, and pigments and does not contain lignin, which makes them a promising feedstock for fuel and chemicals (Gonçalves, et al. 2013; Peng et al. 2020). Moreover, owing to the presence of multiple valuable metabolites, microalgae biomass can be entirely valorized in bio-refinery contexts, with biomass lipid fraction being utilized for the production of biodiesel (Hazrat et al. 2021). The remaining lipid-free residual biomass could be used for bio-methane or biogas production or can be subjected to pyrolysis to produce bio-oils (Shahid et al. 2020). In general, producing biodiesel from microalgae biomass comprises four steps including cultivation, lipid extraction, harvesting, and transesterification of lipids for fatty acid methyl esters (Siddiki et al. 2022; Peter et al. 2021; Patle et al. 2021). Hence, these four steps make the production of biofuel an energy-intensive process while the bio-refinery approach can help to achieve cost competitiveness. A reduction in the energy inputs and cost of the microalgal production process is needed to make them competitive with traditional crops (Katiyar et al. 2017). However, despite the unique characteristics of microalgae, mass cultivation for biomass productivity and biomass recovery of tiny microalgal cells presents major challenges (Ahmed et al. 2022).

Although outdoor open pond cultivation systems are low-cost yet the biomass density is often very low in open culture systems when compared to indoor cultivation (Alam et al. 2014). To overcome these challenges, various studies have already been conducted during the last few years with the majority of them focusing on the use of monoculture algal systems, in which axenic cultures of microalgae have been sustained. Maintaining a monoculture algal system necessitates a super-clean culture environment which requires intensive input in terms of skills and resources. Practically, it is difficult to maintain an axenic culture system on a mass scale especially in outdoor cultivation. In addition, the biomass production of monoculture systems is lower than that of polyculture systems because of the reduced metabolic activity that occurs throughout the night or dark period as the tricarboxylic acid cycle, glycolytic pathway and mitochondrial oxidative phosphorylation keep up high activities during illumination (Yang et al. 2000). During autotrophic cultivation, microalgae use solar energy and fix atmospheric carbon; however, it requires a cultivation system with a big surface area, shallow depth to have better light penetration, minimum contamination risks, and low biomass, all of which reduce the cost-effectiveness of the autotrophic cultivation of monocultures on large scale (Venkata Mohan et al. 2015). Alternatively, heterotrophic cultivation of microalgae is less sensitive to photoperiod allowing for higher yields as light need not permeate the algae and return high biomass yield. However, oxidative metabolism produces large amounts of carbon dioxide that are not utilized and are discharged into the atmosphere. This fragment of carbon dioxide could be further exploited by including autotrophic microalgae into the cultivation matrix (Perez-Garcia et al. 2011). Similarly, microbes including yeast, bacteria, and fungi can play a similar synergetic relationship in which carbon dioxide produced during metabolic processes is fixed by microalgae. Keeping in view these facts, binary or polyculture systems offer several benefits. Here we review the research that has been done on binary culture to date, particularly in terms of the intrinsic advantages and prospective uses of binary and polyculture systems. The mechanism of the binary and polyculture of microalgae is studied to understand the symbiosis with other microbes. The perspective of binary culture in bioflocculation is also briefly discussed with the mechanism. Microalgae binary or polyculture research gaps are also highlighted.

Binary culture for biomass and lipid production

A binary or polyculture system can be employed as a proficient and economical alternative system to reduce the cost of microalgae cultivation to produce biodiesel. The objective of the binary or poly-culture is mainly to combine two or more species in a process that a strain may hold a specific activity that compliments the strain that lacks it, which may help both strains to become mutually beneficial to achieve higher productivity of lipid and biomass. The impact of binary and/or poly cultivation systems on growth and lipid accumulation is rarely studied. However, possible relations among microalgae species and other microbes for beneficial or antagonistic, symbiotic or mutualistic have been studied. Some of the studies are discussed below to understand the challenges and opportunities in this area.

In a binary culture systems, diverse microorganisms form a synergetic interaction and coexist in harmony, benefiting from one another's contributions (Ramanan et al. 2016; Magdouli et al. 2016). Heterotrophic bacteria decompose organic carbon in a microalgae binary culture system, releasing carbon dioxide, vitamin B12, and particular hormones, which are then utilized by microalgae to stimulate their growth and development. In return, microalgae produce organic compounds and oxygen which are used by heterotrophic microbes that coexist with them (Subashchandrabose et al. 2011; Kouzuma and Watanabe 2015) (Fig. 1). It has come forward as a new technique to escalate the efficiency of lipid production and to reduce the material cost. An oxygen/carbon dioxide balance has been observed in a co-culture system. Autotrophic microalgae operate as oxygen generators, supplying oxygen to heterotrophic yeast, where yeast provides carbon dioxide for the microalgae. Moreover, yeast in the binary culture method uses dissolved organic carbon released by microalgae, which has an inhibitory effect on photosynthesis, thus stress caused by oxygen on the microalgae can be eliminated and/or alleviated (Qin et al. 2017). Binary culture of microalgae-yeast or microalgae-bacteria aid in lipid induction as nitrogen depletion begins earlier in the mixed culture compare to individual cultures, hence increasing the production of microbial (Magdouli et al. 2016). Binary cultures also induce bioflocculation process without the addition of chemicals for economical harvesting (Alam et al. 2016). According to preliminary findings from binary culture experiments, bacteria is capable to damage the microalgal cell wall, which is a promising development. If this characteristic is exploited further, it may pave the way for the production of biochemicals from microalgae biomass at a lower cost. There are a plethora of intriguing possibilities for the binary culture that should be investigated further.

Fig. 1
figure 1

Interactions between microalgae and symbiotic bacteria. Heterotrophic bacteria decompose organic carbon (DOC) in a microalgae binary culture system, releasing carbon dioxide (CO2), vitamin and particular nutrients, e.g., nitrogen (N), phosphorus (P), sulphur (S), which are then utilized by microalgae to stimulate their growth and development. Consequently, microalgae produce organic compounds and oxygen which are used by heterotrophic microbes that coexist with microalgae

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In binary or polyculture systems, microbes can enter either into a competitive or co-operative interaction. Co-operative interaction appears to be advantageous for high growth rates and metabolite productivity. Aggressive activity, on the other hand, might result in reduced growth rates and metabolic output. However, it is a rarely studied perspective of microalgae research and currently, there is a scarcity of knowledge on the pathways that contribute to co-operative or competitive interactions between microalgae and other microorganisms. However, several relevant investigations, like wastewater treatment and biofilm production, have provided insight into the process of symbiosis (Ramanan et al. 2016; Higgins et al. 2016). The symbiotic relationship of microalgae with other bacteria should be investigated in greater detail to understand their interaction towards high growth and targeted metabolites production as well as bioflocculation.

Binary culture of autotrophic microalgae with heterotrophic bacteria

Microalgae-bacteria binary cultivation systems have shown to be advantageous for many purposes including lipid production, nutrients and pollutants removal from wastewater, bioremediation, bloom control, sustainable aquaculture, bioflocculation for easier harvesting, etc. (Wan et al. 2015; Ramanan et al. 2016; Magdouli et al. 2016; Alam et al. 2016). Some of the success stories of binary cultivation of bacteria and microalgae are shown in Table 1. Microalgae-bacteria binary culture has a good number of benefits including; (I) microalgae serving as photoautotrophs and producing oxygen and dissolved organic carbon in the culture system, in the meantime bacteria assimilate organic carbon heterotrophically from the culture system; (II) bacteria uptake dead microalgae cells from the culture medium and reduce unwanted contaminations; (III) bacteria produce some of the necessary vitamins and growth-promoting hormones for microalgae which would not be otherwise available in a monoculture of microalgae; (IV) In the same matrix, autotroph microalgae and heterotrophic bacteria produce aerobic and anaerobic regions, with microalgae surviving in the light-driven aerobic areas and heterotrophic bacteria surviving in the shaded (dark) areas. In a mutually beneficial relationship, this interaction helps contribute to an increase in the growth rate, lipid production, and metabolites composition; (V) during binary culture, the interaction between microalgae and bacteria occurs through the exchange of nutrients, the transmission of signals, and the transfer of genes which gives various benefits including the maintenance of algal blooms in response to some signals released by bacteria.

Table 1 Major symbiotic partners of microalgae
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The selection and characterization of microalgal development-promoting bacteria might enable a new approach of binary culture for improving large-scale microalgae cultivation for biofuel production. There are several reports on the stimulating effects of microalgae with bacteria co-culture. Forty-eight bacterial strains were isolated by Le Chevanton et al. (2013) associated with microalgae of marine organisms. The effects of the bacterial strains on Dunaliella sp. growth were also investigated. It was found that only two bacterial species, namely, Muricauda sp. and Alteromonas sp. enhanced the Dunaliella sp. growth during co-culturing. In another microalga-bacterium binary culture, the Rhizobium sp. showed positive effects on the chlorophyll, lipid and biomass content of Ankistrodesmus sp. microalgae in the binary culture where lipid productivity was increased to 112 mg/L/d (Do Nascimento et al. 2013). Two species, namely, Pseudomonas vesicularies and Psuedomonas diminuate enhanced the growth rate of Scenedesmus bicellularis and Chlorella sp. by 1.4-folds when compared to the monoculture of the same strains by lowering the photosynthetic oxygen tension in the culture (Mujtaba and Lee 2016). The microalgae–bacteria co-culture improved the carbon dioxide fixation and lipid production. The lipid productivity of 24.8 mg/L/d was significantly higher when compared to monoculture (Zhao et al. 2014). Considering all the potential advantages in the binary culture of microalgae and bacteria, more research is needed to understand the degree of interactions and mechanisms between microalgae and bacteria in large-scale cultivation, harvesting and bioprocessing point of view.

Binary culture of autotrophic microalgae and heterotrophic fungi

Symbiosis of yeast and microalgae is well studied in various combinations and modes. The association of these two microorganisms has proven to be beneficial for improving growth rate, biomass concentration, and lipid production. Some of the success stories of yeast-microalgae binary cultures are shown in Table 1. Several benefits of microalgae-yeast binary culture over individual culture have been summarized by (Qin et al. 2019); (I) in a binary culture of microalgae and yeast, both species get benefits from each other by balancing the oxygen/carbon dioxide where autotrophic microalgae act as oxygen generators while heterotrophic yeast releases carbon dioxide as a metabolic gas which is consumed by microalgae; (II) Microalgae species secret certain secondary metabolites that stimulate the growth of yeast cells; (III) Oxygen/carbon dioxide balance in binary culture systems have a synergistic effect on pH adjustment in culture for enhanced growth; (IV) Nitrogen depletion takes places early in binary culture than an individual culture which diverts the metabolic fluxes towards lipid synthesis.

Using microalgae and yeast strains in binary culture systems with compounds of low value is a novel technique to increase lipid and biomass productivity while lowering material costs for biofuel generation. In a study by (Wang et al. (2016a) the microalga Chlorella pyrenoidosa and yeast Rhodotorula glutinis were heterotrophically co-cultured using sucrose as the source of sole carbon which resulted in a 30% increase in lipid production. However, using industrial wastewater in the photoautotrophic binary culture of Rhodotorula glutinis and Chlorella vulgaris enhanced lipid production to 117.73 mg/L from 23.1 mg/L (Zhang et al. 2017). In another study, a dual-system bubble column photobioreactor was constructed by maintaining two cultivation methods that could transport chemicals and gases while keeping microorganisms growing individually. In this system, a significant increase in biomass and lipid productivity was observed in the binary culture of C. vulgaris and R. glutinis (Zhang et al. 2014). This study indicated that the synergistic effects between yeast and microalgae are associated with the substances and gases exchange. While selecting, microalgae and yeast for binary culture, higher growth rate, high oil content, minimum competition between species, and similar nutrient requirements are the factors to be considered. According to the researchers, the inoculum ratio of autotroph microalgae to heterotroph yeast was the most critical metric, and each percentage was calculated relative to the other for each species (Papone et al. 2012).

Mutualistic interactions between fungi and algae are widespread that date back some 415 million years and this deceptive symbiosis initiates with close physical interaction and nutrient exchange (Ahmadjian and Jacobs 1981; Du et al. 2019). However, studies comprising the binaryculture of microalgae with fungal strains for improved biomass and lipid generation for biodiesel production are rare. Previously, it was known that the algal cells remain outside the hyphae of the fungus during the interactions between algae and fungi. However, recently, Du et al. (2019) reported that fungus Mortierella elongate interacts intimately with the marine microalga Nannochloropsis oceanica in co-culture, where microalgal and fungal partners establish a tight physical association and exchange of nutrients. The marine microalga P. kessleri-I and endophytic fungus Piriformospora indica were co-cultured to understand the growth, lipid production and associated metabolic changes (Bhatnagar et al. 2019). It was observed that algal and fungal cells control the lipid accumulation and growth at the latter stage of culture, where microalgal biomass was improved by 1.5-fold (to 704 mg/L from 471.6 mg/L) on the day of 12. Similarly, the co-cultivation of fungal strain Aspergillus fumigatus/GLU with Thraustochytrid sp, C. reinhardtii, C. vulgaris, D. tertriolecta, N. oculata, and D. salina showed synergistic impact on the total lipid production (Wrede et al. 2014).

The fungi-microalgae interaction has not been studied exclusively for lipid generation, but fungi-assisted microalgae flocculation has also been studied to develop easy harvesting. Several species of oleaginous fungi, namely, Trichoderma reesei, Rhodococcus opacus, Rhodococcus sp. have been co-cultured with bacteria for biological treatment of wastewaters (Huang et al. 2017) and a few species of fungi such Aerococcus viridans, Aspergillus flavus, Aspergillus oryzae, C. echinulata, etc. have been co-cultured with microalgae to induce flocculation and robust harvesting (Alam et al. 2016). However, none of the studies has been conducted so far to co-culture autotrophic microalgae with heterotrophic fungi to produce lipid for biodiesel production.

Binary culture of autotrophic microalgae and heterotrophic microalgae

Autotrophic/phototrophic microalgae grow under illuminated conditions provided either naturally or artificially where cells harvest solar energy and use carbon dioxide as the carbon source for growth and metabolites production. It is suggested that co-culturing microalgae –microalgae yield greater benefits than utilizing microalgae-bacteria for growth. The microalgae-microalgae systems have some additional advantages including; (I) offer higher natural assimilation between the species due to higher physiological closeness when compared to the microalgae-bacteria systems, (II) exhibits lesser differences in growth patterns which include specific growth rates and doubling time, (III) maintains biomass with relatively higher stable composition. As a result, the microalgae-microalgae -system-based bio-refinery is deemed a more dependable and long-term platform compared to that of monoculture and microalgae-bacteria -based bio-refinery. Different research has been carried out to demonstrate the potential of microalgae-microalgae co-cultivation, as well as to investigate the symbiotic connection and its implications on biomass composition. Ettlia sp. and Chlorella sp. HS-2 species have been extensively investigated and applied to a variety of biorefining processes. It is predicted that these species, which are found in comparable natural habitats, would create a co-operative symbiosis, resulting in high productivity of biomass and stable biomass content (Rashid et al 2019).

Microalgae-based consortia for nutrient recycling

Microalgae-bacteria consortia

Understanding the potential interactions between bacteria and microalgae has led to the usage of microalgae-bacteria consortiums for various applications, including wastewater treatment (e.g., nutrients recycling), where bacteria affect microalgae activities in various ways (Hom et al. 2015; Kouzuma and Watanabe 2015; Ramanan et al. 2016; Gonçalves et al. 2017; Quijano et al. 2017; Cheng et al. 2020). Specifically, C. vulgaris based microalgae-bacteria consortium (chlorophyll ~ 2.6 mg/L) exhibited 72–83% nitrogen removal and 100% phosphorus removal at the temperatures 15 and 25 °C, respectively, in high rate algal ponds at a large scale (Delgadillo-Mirquez et al. 2016), while NH4+–N and PO4−3–P removal rates reached 23 and 8 mg/L/d, respectively, when Chlorella sp. dominant consortium (biomass ~ 1.6 g/L) was cultivated at a lab-scale using municipal wastewater (Cho et al. 2017). On the other hand, co-immobilized of Pseudomonas putida (5 × 108 cells/mL) and C. vulgaris (5 × 107 cells/mL) showed reciprocal effects in artificial wastewater treatment, where chemical oxygen demand, NH3–N, and PO4−3–P removal efficiencies were shown to be 97%, 100%, and 100%, respectively, within 48 h (Shen et al. 2017). Such symbiotic microalgal-bacterial system also possessed comparable removal effectiveness of PO4−3–P, NH3–N, and chemical oxygen demand in the continuous culture, the leakage of cells from vectors (e.g., alginate beads), however, caused cell density decrease during wastewater treatment, hence limiting the long-term operation. It seemed to be plausible that regular replacement of vectors, a mandatory step for biomass harvesting, could solve this issue, yet the additional cost from replacement and harvesting would increase the total capital input, thus hindering the industrialization of co-immobilized systems.

Recently, microalgae-bacteria consortia were used to treat real municipal wastewater in photo-sequencing batch reactors where hydraulic retention time was reduced by 15–26 h (more than 45%), and chemical oxygen demand and Total Kjeldahl Nitrogen removal efficiencies of > 85% and 95% were achieved, respectively, along with 1.9 g/L of biomass production. When combined with real-time parameters including pH, dissolved oxygen, and oxidation–reduction potential, the peaks of dissolved oxygen, pH, and oxidation–reduction potential indicated the completion of real wastewater treatment (Foladori et al. 2018). Compared with freshwater-based wastewater, seawater toilet flushing sewage brought greater barriers on wastewater treatment plants due to its high salinity and excess eutrophic. Therefore, a marine cyanobacterium Spirulina platensis and marine bacterium Vibrio sp. Y1-5 (biomass 4.2 g/L) were tested for treating synthetic seawater toilet flushing, where 98% of the chemical oxygen demand (from 1600 mg/L), 91% total phosphorus (from 40 mg/L), and 86% of total nitrogen (from 200 mg/L) were removed (Li et al. 2018a). Except for nitrogen and phosphorus recycles, the hormone 17β-estradiol, commonly found in urban wastewater, was almost completely removed (> 94%) by microalgae-bacteria consortia (total suspended solids 248–544 mg/L) under favorable seasonal conditions, while harsh conditions (e.g., low temperature at 2 °C) significantly reduced the removal efficiency (50%) and greatly affected microbial diversity (Parlade et al. 2018). It should be noted that the microalgae-bacteria symbiosis system (mixed liquor suspended solids 3 g/L) with the aeration rate of 0.08 m3 air/h showed the total phosphorus and total nitrogen removal efficiencies in a photobioreactor increased to 69% and 95%, respectively, even though the TN removal largely occurred at non-aeration phase (Tang et al. 2018).

Microalgae-fungi consortia

Co-cultivation of C. vulgaris and fungi Pleurotus geesteranus, Ganoderma lucidum, and Pleurotus ostreatus, respectively, were evaluated in nutrients recycling from biogas slurry, and it was found that G. lucidum/C. vulgaris (biomass ~ 1.8 g/L) showed the best removal efficiencies on the chemical oxygen demand (70%), total nitrogen (76%), total phosphorus (78%), and carbon dioxide (60%) when such co-pellets were incubated under the red and blue illumination with a ratio at 5:5, accompanied with the highest economic efficiency of the energy consumption (Cho et al. 2017). On the other hand, pellets composed of C. vulgaris UMN235 and Aspergillus sp. UMNF01 (initial inoculation at 100 pellets per 100 mL) exhibited nutrient removal efficiencies at 23, 45, 85, and 70% for ammonium, total nitrogen, total phosphorus, and chemical oxygen demand when cultured in diluted swine manure wastewater (Zhou et al. 2012). Similarly, pellets composed of C. vulgaris 211/11B (inoculation at 137 mg/L) and Aspergillus niger ATCC16888 (inoculation at 127 mg/L) was applied in cadmium recycling, and an efficiency of ~ 60% of cadmium-removal was achieved with the initial concentration at 1 µg/L (Bodin et al. 2017).

Microalgae-microalgae consortia

Cultivation of microalgae in open reactor systems (i.e., raceway ponds) is estimated to yield a higher rate of return on energy investment than closed photobioreactor systems. Unfortunately, single algal cultures are frequently vulnerable to environmental change in irradiance and temperature growth circumstances, as well as biotic invasion, causing instability of the algal biomass yield. Several studies have been attempted to manipulate poly-algae culture to overcome drawbacks that are associated with pure algal cultures. It has been reported that polyculture including Ankistrodesmus falcatus, Chorella sorokiniana, Scenedesmus acuminatus, Pediastrum duplex, Selenastrum capricornutum, and Scenedesmus ecornis exhibit stable capability in biocrude production overtime under the environment with temperature 17–27 °C. Particularly, this effect was obvious when the polycultures contained four to six species (Narwani et al. 2016).

Microalgae consortia with different microalgae species have also been used for nutrient recycling coupled with wastewater treatment, mostly at a lab-scale (Gonçalves et al. 2017). For example, Chlorella sp., C. zofingiensis, and Scenedesmus spp. were also used to construct microalgae consortia and further applied into wastewater treatment, the chemical oxygen demand and total phosphorus were removed by 57–63% and 91–96%, respectively, while the biomass concentration reached 5.1–5.4 g/L (Qin et al. 2016). Nevertheless, the microalgae consortium mainly consisted of Pediastrum boryanum and Desmodesmus opoliensis at an initial concentration of 260 mg/L in 15-L plastic buckets, reached 91% and 33% removal of total nitrogen (from 35 g/m3) and total phosphorus (from 4 g/m3), respectively, in a high rate algal pond without carbon dioxide control, even though microalgal productivity remarkably increased with highly frequent augmentation of carbon dioxide (Sutherland et al. 2014). Further, synthetic consortia with microalgae Ulothrix zonata, Rhizoclonium hieroglyphicum, U. aequalis, and Oedogonium sp. were able to remove 60% total nitrogen (from 64 mg/L) and 93% total phosphorus (from 14 mg/L) in 220 L biofilm reactor, while the biomass yielded to 5.5 g/m2/d and the total nitrogen annual uptake rate was estimated to be 1430 kg/ha/year (Wilkie and Mulbry 2002), enlightening the feasibility of integrated biomass production and simultaneous nutrients recycling in wastewater treatment via microalgae consortia at large scale. It is advisable that highly settable microalgae consortia (Total suspended solids 6 ~ 12 g/L with settle efficiency > 97%), dominated by green algal species (Scenedesmus sp. and Chlorella sp.), followed by cyanobacteria Chroococcus sp., Oscillatoria sp., and diatom Melosira sp., were shown the removal efficiencies of PO43− (from 1.6 mg/L) and NO3− (from ~ 11 mg/L) of > 99% and 61–79%, respectively (Hu et al. 2017).

In another study, it was demonstrated that poly-cultures containing algae species, namely, Chlorella vulgaris UTEX 2714, Chlamydomonas reinhardtii UTEX 90, Scenedesmus obliquus UTEX 393, and Chlorococcum sphacosum UTEX 1787, and two cyanobacteria species such as Synechococcus leopoliensis UTEX B 625 and Microcystis aeruginosa UTEX LB 2386 exhibited higher grazing resistance when compared to that of monocultures. Importantly, data indicated that greater species diversity noticeably improved yield by 41% when compared to the average among monocultures; nonetheless, no polyculture outperformed the most productive monoculture in terms of biomass (Thomas et al. 2019a). In another extending study, the artificial polyculture (Scenedesmus obliquus UTEX 393, Microcystis aeruginosa UTEX LB 2386, and Synechococcus leopoliensis UTEX B 625) and natural polyculture (Chlorella, Scenedesmus, Selenastrum, Synechococcus, Monoraphidium, and naviculoid diatom) were grown in an anaerobic digester effluent in a raceway system. Under a semi-continuous culture, the natural polyculture achieved 25% and 19% higher average biomass productivity and nitrogen removal efficiency, respectively than the artificially constructed polyculture. Remarkably, the natural polyculture exhibited higher grazing residence, higher mean biomass productivity of 60% as well as higher algal community diversity when compared to the artificial polyculture, under stress conditions (Thomas et al. 2019b).

Binary culture to improve bioflocculation for robust harvesting

Bioflocculation has been recommended as a viable technique for harvesting microalgae due to the significant reduction of energy consumption when compared to centrifugation, and binary cultures often induce bioflocculation and it has been comprehensively reviewed in previous studies (Wan et al. 2015; Alam et al. 2016; Wang et al. 2016a; Rashid et al. 2018). The binary cultures have been applied in enhancing bioflocculation for efficient microalgae biomass harvest. Specifically, an actinomycete Streptomyces sp. hsn06 exhibited high-efficiency harvesting of C. vulgaris via forming mycelial pellets with the help of calcium (Li et al. 2017a). Besides, bacterial cells of Micrococcus sp. hsn80 flocculated more than 60% of C. vulgaris (4.9 × 107 cells/mL) with the addition of 9% concentration, and N-acetylmuramic acid with chains of amino acids were mainly responsible for the flocculation via ions bridging with the help of calcium (Li et al. 2018b).

Another enhanced bioflocculation was reported in a binary culture including C. pyrenoidosa (~ 0.98 g/L) and A. fumigatus (7 g/L), where the flocculation reached 99% within 3 h, while such consortium also exhibited 95% flocculation within 3.5 h when treated with wastewater (Bhattacharya et al. 2017), which indicated the feasibility of integrating the flocculation-based biomass harvesting and nutrients recycling coupled with wastewater treatment. Similarly, a filamentous fungus Isaria fumosorosea and microalga C. sorokiniana CCAP 211/8 k were co-cultured with the inoculum at 105 blastospores/mL and ~ 0.14 g/L, respectively, the final biomass of I. fumosorosea and C. sorokiniana reached 265.3 and 776.0 mg/L, with rapidly formed and stable pellets (1–2 mm), and the pellets could be easily harvested with large inexpensive filters for hydrothermal gasification (Mackay et al. 2015). Interestingly, the aqueous phase of hydrothermal gasification supported the fungi growth after appropriate dilution, suggesting another possible way to save energy input in consortia cultivation. In addition, microalgae-fungi consortia were successfully applied in harvesting a marine microalga Nannochloropsis sp., where 94–97% cells of Nannochloropsis sp. (0.4 g/L) were immobilized onto the mycelium of A. nomius CCK-PDA 7#6 within 3 h when the biomass ratio was 4:1 between mycelium and microalgae (Talukder et al. 2014).

Further investigation revealed that surface proteins with low molecular in mycelial pellets of A. niger played an essential role in capturing microalgae, while calcium addition simultaneously bound the surfaces of mycelium and algal cells, indicating that calcium bridging was mainly accounted for mycelial pellets flocculation (Li et al. 2017b). Similar observations were also reported in Chlorella sp. and Penicillium sp. (Chen et al. 2018), N. oceanica CCMP1779 and Mortierella elongate AG77 (Du et al. 2018). Nevertheless, separate cultures of bacteria, fungi, and microalgae were used to generate bacteria-microalgae or fungi-microalgae consortia, and the organic carbon source was critical for maintaining viable consortia during the flocculation process, demonstrating the potential for microalgae-based mixotrophic consortia in wastewater treatment.

Self-flocculating microalgae such as C. vulgaris, Ankistrodesmus falcatus, Scenedesmus obliquus, and Ettlia texensis were also used as the flocculants to harvest non-flocculating microalgae (Salim et al. 2012; Guo et al. 2013; Alam et al. 2014). Even though such flocculation was not constructed as consortia, it had, however, provided the insight that co-culture of self-flocculating and non-flocculating microalgae at the beginning could also be used for efficient nutrient recycling and biomass harvest. No additional organic carbon was needed during the cultivation since microalgae were autotrophic, which was superior to fungi- and bacteria-microalgae consortia where the source of organic carbon was essential for maintaining the stable consortia. However, the slower growth of microalgae-microalgae consortia could be an issue for its rapidly economical application when compared with bacteria-microalgae consortia.

Mechanisms of mutualist interaction in binary cultures

Microalgae grow with other types of microorganisms in binary cultures and trigger each other’s growth, modify the biomass composition, improve metabolic products and bio-refinery applications. The symbiotic relationship with other microalgae, bacteria, fungi, or yeast in binary cultures is species-dependent, and therefore, cannot be generalized across the microalgae species. Even though the performance of microalgae-based consortia is species-dependent, the mechanism of maintaining stable consortia is still recapitulative, where carbon, macro-and micro-nutrients (e.g., vitamins, nitrogen, carbon, and phytohormones) seem to play a central role during the interaction among the species in the consortia (Ramanan et al. 2016; Wang et al. 2016a, b; Rashid et al. 2018; Yao et al. 2019). For instance, a synthetic mutualism between the green microalga Chlamydomonas reinhardtii and the yeast Saccharomyces cerevisiae, two model eukaryotes, arises spontaneously in a microcosm that necessitates reciprocal exchange of nitrogen and carbon. In this metabolic circuit, S. cerevisiae metabolizes glucose and release carbon dioxide, which is photosynthetically digested by C. reinhardtii and released as oxygen, while C. reinhardtii consumes nitrite as well as releases ammonia as a source of nitrogen for S. cerevisiae (Hom and Murray 2014).

On the other hand, indole-3-acetic acid and tryptophan serve as signaling molecules in a bacterium-microalga consortium consisting of Sulfitobacter sp. SA11 and Pseudo-nitzshia multiseris PC9, where the bacterium Sulfitobacter sp. SA11 promoted cell division of the diatom P. multiseris PC9 via secreting indole-3-acetic acid, synthesized by the bacterium via using both endogenous tryptophan and diatom-secreted (Amin et al. 2015). It was reported that inhibition of N-acyl-homoserine lactones led to reduction of the protein content in algal-bacteria granular sludge and further structural stability of such granules (Zhang et al. 2017), indicating that N-acyl-homoserine lactones -based quorum sensing might also be applicable in explaining the mechanism of maintaining the stable consortia. Moreover, advanced techniques inherited from extensively researched gut microbiota are expected to expand our understanding of functionalities, active drivers, implications, and the potential applications from microalgae-bacteria, microalgae-fungi, and microalgae-microalgae association. Co-culture systems allow cells to communicate with one another via quorum sensing as well as release regulating metabolites that regulate internal and exterior interactions. Light pattern, aeration, light intensity, nutrients, carbon supply, temperature, and pH are all parameters that influence the co-culture system’s performance in the same way that they affect the performance of a monoculture system. The co-cultivation system is heavily influenced by the co-habitation qualities of the cells.

Poly-microalgae cultures for carbon dioxide sequestration and production of biomass and biodiesel

Natural polycultures have demonstrated to own a capability of stabilizing biomass productivity and algal diversity, several research groups have utilized natural algal polycultures for carbon dioxide sequestration and biomass production. Mixed algae cultures originated from freshwater, brackish water, and stormwater were cultivated photographically in Basal Medium and enriched seawater F/2 medium under 110 µmol/m2·s with 16:8 light/dark cycles at 25–28 °C and continuous aeration of air at 11 L/min to enrich the algae population. Six microalgal polycultures that exhibited good growth under the laboratory conditions were selected to domesticate under different carbon dioxide concentrations including 1% and 5%. The mixed cultures of microalgae were subjected to a series of experiments to better domesticate the polyculture and enable it to endure 100% flue gas from an unfiltered coal-fired power plant with 11% carbon dioxide content. The tolerance of growth of the polyculture with SOx and NOx was attained by a slower adaption rate over several months, by gradually increasing flue gas supplementation from 1 to 10% and phosphate buffering at higher concentrations. The decline of algae biodiversity could be observed in the first few months until reached stable dominant species of Desmodesmus spp. This study demonstrated that mixed microalgal communities can be slowly adapted to grow in 100% unfiltered coal-fired power station flue gas. Furthermore, upscaling as well as serial flue gas passages in several closed or open photobioreactors are required to achieve remarkable carbon dioxide uptake from flue gas (Aslam et al. 2017).

A study on fatty acid methyl esters quantification revealed that the esters and total lipid contents of the polyculture reached maximum levels of 280.3 µg/L and 14.03 µg/mL/d, respectively, under the cultivation with 1% carbon dioxide (flue gas) (Aslam et al. 2018). In an extending study, natural polycultures of microalgae obtained from freshwater algal and stormwater samples were mixed and utilized to remove heavy metals from coal-fired flue gas under various CO2 concentrations (Aslam et al. 2019). Desmodesmos sp., filamentous cyanobacteria, Scenedesmus sp., and Chlorella sp. dominated the culture composition. The mixed culture was examined to grow in the nutrient-rich medium and carbon dioxide-rich flue gas (5.5%, 3% and 1% carbon dioxide) containing detected heavy metals of copper, aluminum, manganese, iron, and zinc. It was reported that the maximum fraction of zinc, boron and manganese was captured by algal cells at 355.5 g/L, 46.8 g/L, and 253.66 g/L, respectively, under 30% flue gas containing 3% carbon dioxide during the entire course of the cultivation (16 days). Moreover, freshwater algal culture and stormwater algal cultures obtained high biomass concentrations of 0.8 g/L and 0.87 g/L, respectively, under 1% carbon dioxide when compared to that under 3% carbon dioxide. Iron and aluminum were the two heavy metal pollutants found in the greatest concentrations in harvested biomass under a 5.5% carbon dioxide dose. The study demonstrated that bioremediation of carbon dioxide and heavy metals can be implemented practically in conjunction with algal biomass production via natural microalgal polycultures.

Recently, it has been reported that the injection of pure carbon dioxide caused a significant change in the pH of the algae culture, leading to the variation of the composition of the algae community. The growth of larger microalgae (Chlamydomonas sp. > 5 μm) was favoured at pH 6, whereas smaller microalgae (Chlorella stigmatophora and Nannochloris sp. < 2 μm) achieved maximum growth rates at pH 7 and 8, respectively. At pH 7, carbon dioxide was converted into carbon biomass at a maximum rate of 40% (Galès et al. 2020).

Mixed algal culture originated from the secondary basin of the local municipal wastewater treatment plant was utilized for simultaneously capturing carbon dioxide and wastewater treatment. It was reported that the mixed culture grew well in wastewater under a carbon dioxide concentration of 2.5–15%, reaching a growth rate of 0.48–0.86/day, which is comparable to 0.45–0.79/day obtained for a single strain of Spirulina platensis. The biomass productivity and carbon dioxide fixation rates reported for the mixed algal culture and Spirulina platensis were 0.384 g/L/d and 0.460 gC/L/d and 0.246 g/L/d and 0.360 gC/L/d, respectively. In terms of pollutant removal capability, the mixed algal culture exhibited a better removal efficiency of organic matter, total inorganic nitrogen, and total phosphorous than that of the single strain with respective results determined of 97.2%, 99.6% and 99.41%. The microscopic data revealed that that the algal community and biodiversity in the mixed culture was heavily dependent on wastewater sources used as growth media and carbon dioxide concentration supplement. For example, Colonial Green Algae and Blue-Green Algae were found to be more prevalent in wastewater cultured at a 15% carbon dioxide dose, indicating that these species prefer high inorganic content. For mixed cultures grown in primary effluent, the Blue-Green Algae dominate, with Microspora, Oscillatoria, Aphanocapsa, and Aphanizomenon accounting for 10%, 11–12%, 9%, 5–6%, respectively. The next dominant genera in primary effluent were Vaicheria 7–9% and Yellow–Green Algae with Tribonema 7–8%. Other genera such as Diatoms 10–12% and Colonial Green Algae 5–17% existed at a low percentage. Yellow–Green Algae, Green Algae, and Diatoms were the most common genera found in secondary and primary effluents, accounting for 41–51%, Green Algae 14–25%, and Diatoms 10–19%, respectively (Almomani et al. 2019).

Monitoring growth, interactions, populations, and construction of binary cultures

Monitoring the growth and population of polyculture of microalgae and bacteria is a very important aspect of microalgae cultivation research. Visually, microscopic measurements can be taken regularly to identify the dominant species in the cultures over the cultivation time (Almomani et al. 2019). Traditionally, total biomass growth can be determined by the gravimetric method, which has been widely reported in the literature. For natural mixed culture, microalgal community structure and the population dynamic can be also determined by molecular biology tool using 18S rRNA gene analysis (Galès et al. 2020). Interestingly, mathematical modeling is an effective tool to model and predict over-yielding of microalgal polyculture as a response that is dependent on a variety of variables including species, light, culture medium composition, pH, and temperature (Di and Yang 2018; Supriyanto et al. 2019).

The interaction of algal species in a community of mixed culture is an extremely critical factor that is needed to be considered. It is because of positive interactions that increase biomass and negative interactions that decrease biomass when algal species are co-cultured. Fundamentally, a natural mixed culture is ecologically established, thus most of the species in the natural mixed culture exhibits positive interaction (Shurin et al. 2014). However, artificially mixed cultures are designed by researchers do not always achieve positive interaction (Bohutskyi et al. 2016). In this regard, it needs to create mixed cultures by combining two to more different single algal species, followed by a screening procedure to obtain desirable combinations which exhibit over-yielding biomass productivity (Narwani et al. 2016; Thomas et al. 2019a, 2019b). However, over a million species of microalgae are estimated to exist, making clarifying the relationships that might be helpful for biofuel production exceedingly resource and time-intensive (Guiry 2012). Thus, developing a high-throughput screening of algal mixed cultures for over-yielding biomass production is critically important.

Group research led by Guiry (Guiry 2012) developed a microfluidic technology to manufacture millions of parallel, nanoliter-scale algal mixed cultures for evaluating biomass output and devised a strategy for quick, high-throughput screening of algal community combinations. Principally, interaction and biomass production of mixed algal cultures were firstly experimented in flask scales, following a translation and examination of their growth in microfluidic droplets with daily fluorescence observation using a Spectramax M5 spectrophotometer (Molecular Devices). The study has validated that positive interactions between C. sorokiniana and A. falcatus as well as S. minutum and C. sorokiniana (increased biomass when compared to the monoculture) and negative interaction between S. capricornutum and S. ecornis (reduced biomass when compared to the monoculture) were observed in microfluidic droplets as the same of flask-size over-yielding experiments. This lays the groundwork for using microdroplet co-cultivation for high-throughput screening of a large number of microalgal combinations for the complex algal library (Pan et al. 2011; Carruthers et al. 2017).

Perspective

Despite limited understandings of ecological and evolutionary approaches in microalgae-based consortia, feasible and economical applications of microalgae-based binary culture systems in nutrient recycling from wastewater are calling for comprehensive studies (Ramanan et al. 2016; Rashid et al. 2018; Yao et al. 2019). Specifically, more microalgae species are needed to be tested in binary culture systems for wastewater treatment rather than model microalgae, such as C. vulgaris. Meanwhile, the energy input, such as organic carbon, of bacteria or fungi cultivation in consortia generation should be also considered in the life cycle assessment of microalgae-based consortia application, while microalgae-microalgae consortia could grow photo-tropically with a relatively lower growth rate when compared with microalgae-bacteria and microalgae-fungi system, which further address the importance of microalgae species screening.

Wastewater could serve as an inexpensive source for nutrients, and it has been reported that significantly increased biomass, lipid, and bioproduct yields are obtained in the microalgae-fungi system (Xie et al. 2013), suggesting that it is possible to integrate wastewater treatment with valuable chemicals production from microalgae-based consortia. The chemicals could also expand to food and feed supplements, nutraceuticals, cosmetics, and polyhydroxy butyrate. Secondly, the development of easy biomass harvest after wastewater treatment is explored to a limited extent. Fortunately, bacteria-associated microalgae harvest has been documented, such as Bacillus sp. RP1137 and N. oceanica IMET1 (Powell and Hill 2014), Streptomyces sp. hsn06 and C. vulgaris (Li et al. 2017a, b), as well as fungi-associated microalgal harvest, such as Penicillium sp. and Chlorella sp. (Chen et al. 2018), Mortierella elongate AG77 and N. oceanica CCMP1779 (Du et al. 2018).

Together with self-flocculating microalgae-associated microalgae, bioflocculation could be a promising strategy for cost-effective biomass harvest (Wan et al. 2015; Alam et al. 2016), and more efforts should be made in the application of these consortia in simultaneous wastewater treatment and biochemical production. The composed biomass can be either used for aquaculture and animal feed (Ramanan et al. 2016) or subjected to hydrothermal liquefaction for bio-oil production, where the post hydrothermal liquefaction aqueous could be reused as nutrient sources for consortia cultivation (i.e., nitrogen, carbon, and phosphorus) (Patel et al. 2016). Thirdly, the scarcity of omics approaches has constrained our comprehensive understanding of microbial interactions in the binary culture system. Recently, transcriptomics and metabolic analysis have contributed to unraveling the indole-3-acetic acid as an important element in interaction and signaling between a diatom P. multiseries PC9 and associated bacteria Sulfitobacter sp. SA11 (Amin et al. 2015), while metabolic fingerprinting has revealed the IAA is also involved in the interaction between bacteria and a marine diatom Haslea ostrearia (Lépinay et al. 2018).

Advanced tools also inherited from an extensively studied area (e.g., gut microbiota) are expected to inspire more solutions to the following issues (Hom et al. 2015; Ramanan et al. 2016; Rashid et al. 2018): (1) How microalgae and other microorganisms recognize and communicate (2) how specific and dynamic are microalgae-microbes interactions, (3) how to predict the interaction or directly evolve to generate tunable microalgae-microbes consortia, (4) how to expand the successful examples to other microalgal species. Finally, it should be noticed that genetic modification may occur in the microalgae-based consortia, and unexpected gene escape or leakage could cause biosafety concerns (Hom et al. 2015). Thus, screening native microalgae-microbe consortia for wastewater treatment and biochemicals production arises to be crucial.

The scheme of microalgae-based consortia development and application is proposed in Fig. 2. More precisely, after screening robust candidates for consortia development, especially native consortia, robust microalgae species and microbial partners will be used for tunable and robust microalgae-based consortia development. Model microalgae such as Chlamydomonas and Nannochloropsis could serve as candidates for various biochemicals production, while specific microalgae such as Haematococcus could be used for astaxanthin production. In terms of microbial partners, despite promoting microalgal cells growth, they could also facilitate biomass harvest, and the biomass is ready to generate bio-crude via hydrothermal liquefaction. The developed consortia with microalgae and microbial partners should exhibit efficient nutrients recycles and simultaneous biochemical production during wastewater treatment. Eventually, overcoming the aforementioned issues will significantly drive the industrialization and commercialization of tunable and robust microalgae-based consortia on large scale.

Fig. 2
figure 2

Microalgae-based consortia development and application. Chlamydomonas and Nannochloropsis could serve as candidates for various biochemicals production, while specific microalgae such as Haematococcus could be used for astaxanthin production

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Conclusion

Microalgae-based binary cultures have exhibited great potential in wastewater treatment and biochemical production, yet limited understanding of the interaction between microalgae and microbes have constrained directly engineering the binary system for the desired application. In this review, increased biomass and lipid production, efficient nutrient recycling, enhanced bioflocculation for easy biomass harvest in microalgal-based consortia are summarized with the focus on microalgae species screening. The identification of the perfect symbiotic partner organism for binary culture with microalgae requires additional research and will have important ramifications for the future use and manipulation of microalgae in bioenergy and other biotechnological applications. Another technique that might be pursued is the generation of genetically manipulated strains by classifying the various stress-reactive traits that could be integrated into the strains to ensure higher production of value-added products for ensuring an efficient algal bio-refinery. More large-scale tests of demonstrated microalgal-microbial binary systems on a lab-scale are necessary for accelerating the commercialization, while omics approaches are expected to expand our understanding of the interaction between microalgae and microbes in consortia.