Introduction

Mangrove forests comprise plants from diverse families of angiosperms with unique morphological, anatomical, and physiological features (Duke 1995; Friess 2016). There are nearly 73 species of “true mangroves” which harbor intertidal regions of the tropics and subtropics of around 118 countries (Spalding et al. 2010). Mangroves are one of the most productive ecosystems and protect the shorelines from cyclones, tsunamis, storms, floods, and sea waves by virtue of their extensive root system (Pillai and Harilal 2018; Lovelock et al., 2015; Garcés-Ordóñez et al. 2019). Mangrove forests are also known as blue carbon ecosystems for their ability to store huge amounts of organic carbon (Bouillon et al., 2003; FAO 2007). Despite their huge economic and ecological roles, mangrove areas are cleared on a large scale for aquaculture and other non-mangrove activities and around 50% of the mangrove area has been lost since the 1950s causing species extinction (Feller et al. 2010). Mangrove species accounting for nearly 17% have been categorized as critically endangered, endangered, or vulnerable whereas 7% are near-threatened (Spalding et al. 2010; IUCN 2020; Kumar et al. 2021). The whole mangrove biome may disappear in the next century if the rate of biomass reduction continues unabated (Duke et al. 2007). In addition to land clearance, pollution-induced climatic changes, global warming, and the resultant rise in sea level are the key stressors reducing mangrove flourishing zones (Parida and Jha, 2010). Among them, pollution caused by plastics has been recognized as a major concern to the mangroves (Owuor et al. 2019). The plastic debris gets trapped by the sweeping root system of mangroves and directly suffocates the respiring roots (van Bijsterveldt et al. 2021; Luo et al. 2021).

Mangrove areas have already become huge sinks of plastic waste entering the ecosystem either from marine or terrestrial sources (Deng et al. 2021). Every year, open oceans are dumped with an estimated 4.8 to 12.7 million tons of plastics due to improper waste management strategies (Jambeck et al. 2015). The larger plastics in turn are fragmented by chemical reactions, UV (ultra-violet) radiations, wave action, and biodegradation to form small plastic pieces, termed microplastics (MPs; Thompson et al. 2004). They are of size < 5 mm, including nanoplastics (< 0.1 μm), and also produced directly as byproducts of manufacturing industries (Neff et al. 2018). The MPs are then carried over to coastal regions by tides, water currents, and wind (Galgani et al. 2015). They have been increasingly found to be a cause of concern for the coastal vegetation as well as marine flora and fauna. MPs accumulate in plant parts and hinder nutrient uptake (Khalid et al. 2020). MPs absorb and carry organic pollutants and heavy metals which enhances its toxicity (Jeong and Choi 2020). The ingestion or absorption of the MPs enriched with organic pollutants is potentially more hazardous to animals and plants than of MPs alone (Huang et al. 2021).

The remediation of this degradation-resistant pollutant was challenging until it was found that MPs are susceptible to microbial deterioration (Shah et al. 2008; Brooks et al. 2011). Biodegradation of MPs by microorganisms is driven by the inherent capability of microbes to get adapted to newer environments. The microbial remediation of MPs is highly encouraged as their deployment causes the least harm to the environment (Shah et al. 2008; Brooks et al. 2011). From early 2020, a few authors have reviewed microbial deterioration of MPs (Yuan et al. 2020; Othman et al. 2021) and the distribution and fate of MPs in mangrove habitats (Deng et al. 2020). Recently, it has been reported that mangrove areas accumulate more MPs compared to the unvegetated areas of wetlands (Huang et al. 2021). Increased accumulation of MPs in the mangrove habitats suggests vulnerabilities of the mangroves, microbiota, and other organisms inhabiting mangrove ecosystems to MPs. However, much information on the effect of MPs on mangrove trees and the rhizosphere microbiota is completely lacking. We do not know much as to how MPs behave in the mangrove habitats and how it affects animals, other plants, and microorganisms harboring these ecosystems. Thus, the current review is an attempt to consolidate and connect the information available on the MP pollution in the mangrove wetlands with the following: (i) its effects on mangroves itself and the microbial symbionts; (ii) the possible microbial solutions including in situ biodegradation using native mangrove-associated microorganisms; and (iii) the futuristic microbial solutions to manage and remediate MPs including use of biotechnological approaches such as genetic engineering and genome editing.

MP pollution in the mangrove habitats

MP en route to the mangrove ecosystem

Mangrove wetlands are polluted on a large scale due to anthropogenic activities. There are multiple and complex sources of mangrove MP pollution as presented in Fig. 1. MP reaches mangrove habitats either as primary or secondary MPs based on the source point (GESAMP 2015). Primary MPs are plastic particles synthesized to have < 5 mm size and accidentally released to the environment from various manufacturing activities (GESAMP 2015; Koelmans et al. 2014). They are the constituent particles of personal care products such as bath gel, facial cleansers, toothpaste, textiles, and cosmetics (Suardy et al. 2020). Primary MPs are also formed during pre-production of resin pellets, in the air-blasting industry (Andrady 2011). On the other hand, MPs generated from chemical (freeze–thaw cycle and UV radiations), physical (water disturbance, wave strike, and abrasion), and biological degradation of larger plastics are termed secondary MPs (Dong et al. 2021; Khalid et al. 2021).

Fig. 1
figure 1

Possible sources of MP pollution in the mangrove wetlands

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Both the primary and secondary MPs are carried to the mangal (mangrove) region from the improperly managed terrestrial and marine waste streams (Deng et al. 2021). The terrestrial sources of plastic litter include agricultural runoff, sewage treatment systems, fishing activities, and industries located nearer to the streams feeding the mangrove habitats. The plastic waste dumped from coastal tourist spots can also contribute to MP pollution (Zhou et al. 2020). Other reasons for MP pollution include vast industrialization of coastal areas resulting in greater input of MP polluted water to mangrove habitats via drainage system as exemplified by the case study on Jinjiang estuary, China (Zhou et al. 2020). Greenhouse industries near coastal areas are also responsible for plastic waste accumulation in mangrove areas (de Stephanis et al. 2013). In Mediterranean basin countries, greenhouse cultivation uses plastic mulches, especially made of black plastics which subsequently accumulate as MPs and are eventually carried to nearby wetlands (de Stephanis et al. 2013). Marine-based sources of MP complement those from terrestrial sources where shipping and marine aquaculture are the major contributors of MP pollution in the mangrove ecosystem (Garcés-Ordóñez et al. 2019). Besides the external sources, in situ degradation of plastic debris retained by the mangrove root system by means of either chemical, physical, or biological forces discussed earlier can also contribute to wetland MPs (Mohamed Nor and Obbard 2014).

Evidence of MPs in mangrove wetlands around the world

As mentioned in the earlier section, plastic debris from a variety of sources are ultimately entrapped by the roots and branches of mangrove trees (Fig. 2). To assess the various polymer types and shapes of these trapped MPs within the mangrove sediments, several efforts have been made since 2014. Among them, the earliest reports on mangrove MP distribution came from Singaporean (Mohamed Nor and Obbard 2014), Indonesian (Hastuti et al. 2014), and Malaysian (Barasarathi et al. 2014) mangrove wetlands. Compared to other areas around the world, mangrove ecosystems spanning Chinese coastal regions are widely studied for the characteristics of MP pollution. Of which the very first study was conducted along seven mangrove habitats of Qinzhou Bay (Li et al. 2018). Afterwards, a total of 55 mangrove habitats of China including Futian, Maowei Sea, Pearl River Estuarine, Beibu Gulf, and Jinjiang Estuarine region were detected to be MP-contaminated (Li et al. 2019, 2020a, 2020b; Zhou et al. 2020; Zuo et al. 2020; Deng et al. 2020; Zhang et al. 2020; Duan et al. 2021). Though the dominant type of MPs varies from region to region, polypropylene (PP) contributes more towards the overall MP pollution of Chinese wetlands and fiber being the most abundant shape.

Fig. 2
figure 2

Plastics entrapped in the roots of different species of mangroves near Chithari River (12° N 75° E) (A–C) and Mogral Puthur (12° N, 74° E) (D) at Kasaragod, Kerala, India. A Aegiceras corniculatum, B Kandelia candel, C Acanthus ilicifolius, D Rhizophora mucronata

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Only a few mangrove zones of other countries have been investigated for MP pollution leaving behind most of the estuarine regions of 118 mangrove possessing countries remaining untouched. Though Singaporean coastlines are one among the earliest geographical areas of the MP pollution research, no further notable works have been reported from Singapore. Similarly one report each is available from Panama (Feldberg, 2018), Colombia (Garcés-Ordóñez et al. 2019), and South Africa (Govender et al. 2020). The second and latest MP reports from the other two earliest geographical locations of MP pollution research; Malaysia (Hamid et al. 2020) and Indonesia (Cordova et al. 2021) came very recently after a long gap. A total of 11 mangrove habitats within the Persian Gulf of Iran were explored for MP distribution in 5 different studies (Naji et al. 2017a, 2017b, 2019; Nabizadeh et al. 2019; Maghsodian et al. 2020). The detailed MP distribution of individual mangrove areas of different countries specifically with the abundant type and shape are tabulated (Table 1). Taken altogether, the dominant polymer type and shape varies from place to place depending on the respective terrestrial as well as marine sources of MPs.

Table 1 Distribution and types of MPs in global mangrove ecosystems
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Impact of MP pollution on mangroves and associated microbiota

Mangrove devastation by (micro)plastics

Even though the MP invasion of the mangrove regions is evident, the extent of deterioration caused by this pollutant is still unclear. Correlative effects of plastic pollution on respiration by aerial roots, seedling survival, and photosynthesis of mangroves have been analyzed in few studies (Williams and Simmons 1996; Smith 2012; Suyadi and Manullang 2020). But the very first and only manipulative research to date is from Java, Indonesia (van Bijsterveldt et al. 2021). The study found that constant burial of plastic debris in the topmost layer of mangrove sediment results in anoxia and suffocation. To some extent, the mangrove trees withstand this stressed condition by enhancing pneumatophore (aerial root) growth and reducing canopy (van Bijsterveldt et al. 2021). Upon higher exposure to plastics (100% plastic cover), significant leaf loss occurs and the tree survival becomes risky (van Bijsterveldt et al. 2021). More supportive scientific evidence from the mangrove zones all over the world is necessary to completely understand the mechanism of mangrove devastation by (micro)plastics. The suffocation imparted by plastics may not be the deteriorative effect of smaller-sized MPs on mangroves. So, the responses of mangroves to different polymer types of MPs should also be evaluated together with the chances of adaptive regulations the mangroves may acquire during the course of interaction with various MPs.

Mangrove microbiota are better suited to the (micro)plastic environment?

Microorganisms utilize MPs as a surface for colonization. The colonization process is influenced by physicochemical and biological factors. After colonization, a novel niche of colonizing microbes termed “plastisphere” is formed over the plastic surface (Zettler et al. 2013). Here, we suggest a related term “mangrove plastisphere” for the microbial niche on the surface of plastic debris trapped within the mangrove rhizosphere. The inhabiting microbiota within a plastisphere may either get benefit or be adversely affected by the (micro)plastic substratum. In general, a positive response showing gradual adaptation of the microorganisms to the stressed environment caused by MPs is observed (Oberbeckmann and Labrenz 2020). The overall gene expression patterns and the cell behavior alter to facilitate cell adhesion to the surface of MPs and the mobility of the microbial population over the surface (Tuson and Weibel 2013). The other adaptive responses include deviations in growth and metabolic rate and the production of protective macro-molecules (NicAogáin and O’Byrne 2016; Guan and Liu 2020). The microbial stress-responsive pathways are regulated by enzyme switches (Cooper 2000; Winkel 2017).

The adapted microbes benefit from MP attachment with a better surface area for easy access to energy sources and nutrient accumulation, facilitating cell growth (Shen et al. 2019). The metal ions adhered to the MP surface are utilized by the microbiota for cellular activities involving electron receptors (Maret 2016). Biofilm formation upon the MP surface is another favorable adaptation microbes, especially bacteria acquire. It provides mechanical strength, protection from predators, and the diffusivity is also enhanced (Tuson and Weibel 2013). The negative aspects of microbial interactions with MPs are not well understood. Surprisingly, the interplay of “mangrove plastisphere” is not even focused to date. The response of mangrove microbiota to MP is still a mystery. This gap area invites scientific initiatives addressing adaptive responses and adhesive characteristics of MP-associated mangrove microbiota.

Microbial solutions for the reduction and management of mangrove MP pollution

Though MPs are more or less resistant to microbial invasions and degradation, they are the most transformed material upon microbial attack (Rujnić-Sokele and Pilipović 2017). The different types of plastic/MP degrading microorganisms and their mechanism of action have been specifically reviewed by (Yuan et al. 2020). The following subsections discuss the MP degrading native mangrove microbes as well as microbial enzymes/biofilms from mangrove and non-mangrove sources identified so far.

Prospecting native mangrove microbes with MP degradation potential

The mangrove ecosystem harbors a wide variety of microorganisms such as algae, bacteria, cyanobacteria, and fungi. Among the mangrove bacteria, methanogenic (e.g., Methanoccoides methylutens), phosphate solubilizing (e.g., Vibrio proteolyticus), N2-fixing (e.g., Rhizobium), and sulfate-reducing groups (e.g., Desulfotomaculum) are the most dominant. The fungal groups of mangrove areas broadly include proteolytic, amylolytic, pectinolytic, cellulolytic, and ligninolytic types (Kandasamy and Bingham 2001). Cyanophyta, Rhodophyta, Phaeophyta, Chrysophyta, and Chlorophyta are the notable algal groups of the mangrove ecosystems (Sen and Naskar 2003). The microbial richness of such an extremophilic habitat like mangroves is not effectively exploited for its efficacy in biodegradation of MPs either via in vitro or in situ incubation methods. The studies can be carried out to prospect the MP degrading microbes. In vitro analysis of MP degradation is one important strategy to identify the microbes with MP degradation potential.

Evidence of MP degradation potential of native mangrove microbiota through in vitro analysis

In general, several microbes with MP degrading potential have been identified, information on microbes from mangrove habitats is limited (Yuan et al. 2020; Othman et al. 2021). Despite slow progress, recent studies have attempted in vitro investigation of plastic degrading potential of microbes isolated from the mangrove habitats. The earliest in vitro analysis identified several heterotrophic bacteria such as Vibrio, Listeria, Micrococcus, Staphylococcus, and Bacillus from the mangrove soil of Suva, Fiji Island, with degradation potential of low- and high-density polyethylene (LDPE and HDPE; Kumar et al. 2007). Mangrove-associated bacteria, viz. Streptomyces sp., Pseudomonas aeruginosa, and Staphylococcus aureus, and fungi, viz. Aspergillus niger and Rhizopus sp. of Muthupet ecosystem, were found to be capable of effectively deteriorating plastic debris especially PE (Kannahi and Sudha 2013). Indigenous bacilli namely Bacillus mycoides and Bacillus subtilis of Niger Delta mangrove ecosystem were found to biodegrade PE following an initial abiotic degradation by sunlight (Ibiene et al. 2013). Some selected isolates and consortium of fungi such as Phialophora alba, Paecilomyces variotii, Eupenicillium hirayamae, Aspergillus terreus, Aspergillus caespitosus, and Alternaria alternata of mangrove habitat (Red sea coast, Saudi Arabia) were proven to exhibit LDPE digestibility (Ameen et al. 2015). The bacterial strains namely Bacillus gottheilii, Rhodococcus sp. (36), and Bacillus sp. (27) which degrade polypropylene (PP) were also isolated from the mangrove ecosystem (Auta et al. 2017, 2018). Later, it was proved that the bacterial strains of Pichavaram, India, mangrove wetland are able to digest HDPE (Sangeetha Devi et al. 2019). Among the 109 fungal isolates collected from the Avicennia marina rhizosphere (West Coast of India), a remarkable PE degrading ability was demonstrated by Aspergillus terreus (MANGF1/WL) and Aspergillus sydowii (PNPF15/TS) (Sangale et al. 2019). These in vitro studies followed a weight loss calculation of plastic substratum and a few with morphology characterization of plastic polymers. The observations highlight the fact that plastic debris gets trapped within the mangrove wetlands and is fragmented into respective MPs. The subsequent biodegradation of MPs into smaller particles is yet to be systematically analyzed. Whether the plastic degradation by the mangrove biota enhances the MP content of the mangrove plastisphere or the subsequent and prolonged degradation of MPs eventually reduces the MP concentration is still not fully understood. To better understand the fate of mangrove MP polymers, controlled in situ experimental MP degradation and monomer tracking are needed. Further studies must focus on the mechanisms of MP degradation by the microorganisms and how microbial degradation contributes to MP elimination.

In situ MP biodegradation

MP degradation by microorganisms is greatly affected by environmental factors and the interactions among them (Yuan et al 2020). Isolation of individual mangrove microorganisms and in vitro incubation with MPs may be good enough to explore the efficacy of MP degradation. But to reveal the MP degradation potential of native microorganisms within the mangrove habitats, in situ analysis is necessary where the environmental factors affecting the rate of MP degradation can also be examined. The in situ studies are even more important for practical reasons because it is easier to deploy the microbes in the MP-polluted mangrove ecosystems rather than doing in vitro degradation of the isolated MPs. The mangrove rhizosphere microorganisms are the nutrient houses of mangroves by virtue of the ability to decompose detritus material (Holguin et al. 2001). The utility of mangrove-associated microorganisms in in situ plastic degradation was recognized by Kathiresan (2003). The degradation of polymers within the mangrove rhizosphere is largely influenced by the microbe type, heat of mangrove soil, and moisture content (Kathiresan 2003). Microbes (bacteria: Pseudomonas, Moraxella, Staphylococcus, Micrococcus, and Streptococcus; fungi: Aspergillus niger and Aspergillus glaucus) of Rhizophora- and Avicennia-growing zones were found to be effectively degrading polyethylene bags and plastic cups. The bacterial and fungal candidates exhibiting the highest percentage of plastic degradation were Pseudomonas and Aspergillus glaucus respectively (Kathiresan 2003). After a long gap, a recent and unique research from Zhanjiang Mangrove National Nature Reserve, China, showed degradation of three MP substrata (LDPE, polyamide 6, and polyvinyl chloride) out of nine under in situ incubation initially for 1 month and extended up to 3 months (Xie et al. 2021). The microbial colonization in all the nine candidate MPs (others are acrylonitrile butadiene styrene (ABS), PET, PP, expanded polystyrene (EPS), PS, and polycarbonate (PC)) was specific to the polymer groups. This study also identified 1746 and 2077 unique and shared colonizing native microbes within the MP surface biofilm after one month and three months of incubation. Among them, the dominant mangrove microbes belonged to the orders Desulfobacterales, Cellvibrionales, Bacteroidales, Sphingomonadales, Steroidobacterales, Rhodobacterales, SBR1031, Campylobacterales, B2M28, Gamma proteobacteriales, Vibrionales, Clostridiales, Actinomarinales, Anaerolineales, Bacillales, and Flavobacteriales (Xie et al. 2021). These studies suggest that native microorganisms inhabiting the mangrove rhizosphere may be ideal candidates for the biodegradation of plastics. The identification of the native microbes and their deployment in the mangrove wetlands could help in the microbe-mediated remediation of the MP-polluted mangrove ecosystems. However, extensive studies are required to prospect.

Microbial enzymes for MP remediation

The polymeric MPs are specifically digested by microbial enzymes into component monomeric units which serve as a carbon source for the degrading microorganisms (Ganesh et al. 2020; Bollinger et al. 2020; Liu et al. 2021). The enzymatic mechanisms of microbial degradation of MP were reviewed in detail by Othman et al. (2021). Briefly, the microbial enzymes act via surface modification (Vertommen et al. 2005) or polymer degradation (Gómez-Méndez et al. 2018). Although several plastic degrading microbial enzymes have been identified from other environments, the microbial enzymes from mangrove ecosystems are not yet identified. Auta et al. (2018) have found enzymatic degradation of MPs; still, the identity of the plastic degrading enzymes is not yet established. The bacterial strains examined for polypropylene degradation were efficiently reducing the polymer mass (Auta et al. 2018). Table 2 summarizes the microbial enzymes, their sources, and the respective MP substrates. Similar enzymes might be existing in the microbial communities of the mangrove ecosystems. Further in-depth studies must be carried out to harvest the potential native microbial enzymes from the mangrove habitats.

Table 2 Microbial enzymes and their respective MP substrates
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Microbial biofilms and their role in MP degradation

Microbial consortia assemble and unite under varying settings to form wide varieties of biofilms which respond quickly to environmental cues (Hall-Stoodley et al. 2004). Microorganisms ranging from smaller viruses to larger fungi colonize and form biofilms over the MP surface (Oberbeckmann et al. 2015). The whole process of MP degradation by biofilms may occur in four stages. During the first stage, the microbes attach to the MP surface and mask surface properties like adhesive characteristics and hydrophilicity. In the second and third stages, MP degrading microbial enzymes are secreted out causing leaching of monomers and additives out of MP. In the fourth stage, deep penetration of MP by microbial filaments and further degradation takes place (Flemming 1998; Miao et al 2019). MP degradation mediated by bacterial biofilms and the polymer specificity of the degrading microorganisms is better studied in recent years as compared to fungal biofilms. For example, bacterial biofilms of Escherichia coli, Burkholderia cepacia, and Acinetobacter calcoaceticus specifically deteriorate PP (Hossain et al. 2019). On the other hand, MPs are supposed to selectively enrich the type of microorganism growing on its surface (Miao et al. 2019). The most favored biofilm-forming microbial families on the MP surfaces are Cyclobacteriaceae, Phycisphaerales, Pirellulaceae, and Roseococcus than those established on natural substrates (Miao et al. 2019). However, the complexity of mechanisms that lead to biodegradation specificity of MPs by biofilms is yet to be studied. There is no information available to date regarding the MP attached biofilms of mangrove microbiota and the subsequent MP degradation.

Futuristic microbe-mediated strategies for remediation of MPs from mangrove wetlands

Microbial biofilm traps: clear out already polluted mangrove zones

As evident from the earlier discussion, several studies have been done to find out suitable microbial enzymes and biofilms for MP degradation. As these microbial mechanisms specifically or preferably target individual MP types, removal of MP mixture containing different types and sizes of polymers is not possible (Liu et al. 2021). Moreover, the process of biodegradation is comparatively slow which leaves behind the MP traces for longer durations (Miao et al. 2019). Bioaggregation, which has been proven in the remediation process of pesticides and metalloids, is a potent and emerging strategy for the efficient removal of MP mixtures (Chua et al. 2015; Wang 2016). The major advantage of this biological trap is that it is possible to recycle the MP aggregates rather than opting for landfill disposition and incineration (Deshpande et al. 2020). Liu et al. (2021) engineered P. aeruginosa biofilm to trap and aggregate (“trap-and-release” bioaggregation mechanism) MPs within its sticky matrix which could be released and recycled later. The trap-and-release mechanism is independent of material type and size and easily removes the resultant MP clusters due to its aggregation at high concentrations (Liu et al. 2021). The removal of MP aggregates is also supported by lowering of cyclic diguanylate (c-di-GMP) levels (Chua et al. 2015). The native microflora constantly being exposed to the mangrove MPs might have adapted better potential to form biofilms on their surface and to serve as efficient MP traps. Future applied studies aimed at remediation of wetland MPs should also incorporate in situ biofilm traps made of mangrove microbiota to eliminate MP pollutants.

Bioplastics from mangrove microbes

Ordinary plastics and their MP counterparts are petroleum byproducts which are non-biodegradable. To save our environment from being a plastic sink, it is necessary to replace petroleum based plastics with biodegradable plastics (bioplastics). Bioplastics are synthesized from microbial degradation of renewable biomasses such as corn and sugarcane, or from microbes such as yeast (Alzubaidy et al. 2016; Hong et al. 2019). The use of mangrove microorganisms as reservoirs of a wide range of resources like antimicrobial substances, enzymes, medicines, and food is well discussed (Lin et al. 2001; Maria et al. 2005). But their contributions to bioplastic production are least documented. The biopolymer poly-beta-hydroxybutyrate (PHB) was isolated from the estuarine microorganisms long ago (Herron et al. 1978). Later, Alarfaj et al. (2015) extracted and characterized PHB from a mangrove bacterium (Bacillus thuringiensis KSADL127) of Saudi Arabia. A hydroxyvalerate additive of PHB was also characterized from a halotolerant mangrove Bacillus (Moorkoth and Nampoothiri 2016). Among the four mangrove rhizospheric bacteria isolated from Al-Marabi and Al-Madhaya, Erythrobacter aquimar accumulates greater concentrations of PHB (Mostafa et al. 2020). Taken together, the research exploring native mangrove microbes in bioplastic production is comparatively less and it requires screening more candidates for their capability to produce bioplastics in near future.

Biotechnological tools for microbe-mediated MP remediation

Microbial degradation is an eco-friendly strategy to overcome the after-effects of deadly MP pollution in the global mangrove wetlands. The characterization of the mangrove plastisphere microbiome is important to increase our understanding of the plastic-microbe interactions. Its understanding is important because MPs offer unique surfaces as substratum for the microbes to colonize leading to alteration of microbial composition/functioning within the microbial population (Bhagwat et al. 2021). The interactions between MPs and microorganisms are established mainly through enzymatic activities but most of them remain uncharacterized. The recent developments in omics technologies have revolutionized genomic analysis of environmental samples which facilitates identification of plastic degrading microbes from various ecosystems (Danso et al. 2019; Jaiswal et al. 2020). It is easier and cheaper to sequence the genomes of organisms including microorganisms to decipher the structure and functions of the genes (Loman et al. 2012). The combined application of genomic, proteomic, metabolomic, and ionomics approaches could help identify the gene-protein-metabolite and ion networks that are involved in the biodegradation of MPs (Wei et al. 2021). Genetic dissection of the enzymes and identification of other regulators including miRNAs, lncRNAs that mediate the functioning of the enzymes would be helpful in devising and deploying highly efficient microbes for bioremediation of the MPs (Nazarov et al. 2013; Deng et al. 2018). The multi-omics tools can also help identify and pinpoint the exact enzymes and the genes that encode them for easier manipulation using gene modification/editing tools (Jaiswal et al. 2020; Wei et al. 2021). The candidate genes encoding the most important enzymes can be easily identified through the multi-omics approaches (Blimkie et al. 2020). Moreover, the microbial interactions within a biofilm which might be influencing the rate of MP degradation process can also be elucidated (Yu et al. 2019). Meyer-Cifuentes et al. (2020) studied the biodegradation of copolyester blend (PBAT-based blend film, PF) by the microbial consortium from the marine samples of three different countries, viz. Germany, Greece, and Italy. Using multi-omics approaches such as meta-transcriptomics, metaproteomics, and metagenomics, this study further identified important genes encoding PF degrading enzymes such as PETase-like enzymes (Ples) and MHETase-like enzymes (Mles) which are crucial for the degradation of PF (Meyer-Cifuentes et al. 2020). One important candidate genes with MP degradation potential are characterized, biotechnological tools such as genetic engineering and gene editing can be applied to create over-expressing, genetically modified, or genetically edited microbes with enhanced biodegradability potential. The availability of limited information on the mangrove microbes and the MP degrading enzymes shows that mangrove microorganisms are the least exploited microbiota for their potential in bioremediation of MPs. The current review proposes several possible futuristic strategies utilizing mangrove microbiota to tackle mangrove MP pollution as depicted in Fig. 3.

Fig. 3
figure 3

Futuristic strategies involving incorporation of mangrove plastisphere microbes for microbe-mediated remediation of MPs from mangrove wetlands. The microbial inhabitants of the mangrove plastisphere may be either used directly or with genetic modifications for onsite biodegradation of MPs. Bioengineered biofilms from the plastisphere may serve as efficient traps for bio aggregation of wetland MPs. Finally, the efficiency of native or genome-edited plastisphere microorganisms may be analyzed for bioplastic production. The multi-omics approaches may contribute more towards the understanding of MP-biofilm interactions and prospection of new MP degrading enzymes

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Conclusions

Considering the ecological and economic importance of the mangroves and the vulnerabilities due to multiple threats, it is essential to take concerted efforts to tackle various types of pollution affecting mangroves. MP pollution being one of the serious issues has recently received little attention from environmental scholars. Although not much literature is available from nearly 118 countries where mangroves are found, the limited studies from some of the mangrove-dominated areas point towards the seriousness of the MP pollution not just for the mangroves per se but for a number of other organisms that shelter in the mangrove habitats. It points towards how vulnerable mangrove ecosystems are. A future without mangroves is not imaginable especially for the coastal communities who are protected by the mangroves especially during cyclones, sea waves, floods, and tsunamis. The current review concludes that the mangrove habitats are contaminated with (micro)plastics dumped from marine as well as coastal sources. It is also evident that MP pollution results from the lack of basic sanitation and anthropogenic activities. The previous studies reporting global distribution and composition of MPs in the mangrove ecosystem are not balanced. Most of the data comes from the Chinese coastal regions with only a handful of reports from the other parts of the world. Of the various solutions to MP pollution, sooner or later microbial mitigation forms the key strategy for MP-free mangrove wetlands. Genomic investigation of multiple mechanisms regulating microbial degradation of MPs using multi-omics tools will increase our understanding of genetic regulation of microbe-mediated MP degradation. The genetic underpinnings of microbe-MP interactions will help in devising futuristic strategies aimed at the removal of MPs from the vulnerable mangrove wetlands.

Future perspectives

The current article emphasizes deployment of microbes, especially the native microbiota of “mangrove plastisphere” for bioremediation of increasing MPs in the mangrove ecosystem. The future applied research must take advantage of advancements in multi-omics, genetic engineering, and gene editing tools to bridge the key gaps in addressing the issue of wetland MP pollution. Other areas of research in the future surrounding biodegradation of MP pollution in the mangrove wetlands are presented in the following points:

  • Characterization of MP-polluted mangrove wetlands: It is high time to evaluate the abundance, sources, and types of MPs in the global mangrove inhabiting countries to get the actual picture of MP distribution which will guide us to design accurate prevention/management strategies.

  • Studies focusing on microbial degradation must consider aspects such as (i) identification of the potential microbes capable of MP degradation, (ii) identification of the genes, enzymes that aid MP degradation, and (iii) application of gene editing and engineering tools to increase MP degrading efficiency of the microbes.

  • As the mangrove ecosystems have already been reported as MP sinks, the mangrove microbiota might have acquired adaptive characteristics upon constant exposure which is yet to be studied and documented.

  • Though the microbial aspects of MP degradation have been discussed by many authors, the possibility of using mangrove microbiota for in situ MP biodegradation strategy is not widely adopted. The review invites wider attention of the scientific community towards this end.

  • Devising integrative strategies to mitigate MP pollution: Multi-omics studies should be encouraged to unravel genes, enzymes, and novel and efficient MP-biofilm adhesions. Multi-omics studies must be deployed to investigate the genes, enzymes important for MP biodegradation including deciphering the efficacy of biofilms in MP biodegradation.

  • The remarkable future prospect from the current review is the use of biofilm traps for addressing MP pollution. Using biofilm traps for bioremediation of MPs from the mangrove habitats is perceived as one of the promising strategies in the future.

  • Mangrove-associated microbes are expected to possess peculiar characteristic features due to the extremophilic habitat they share. This peculiarity might be an added advantage while utilizing mangrove microbiota for bioplastic production. Based on the studies reported so far, more scientific research is required to validate the efficiency of mangrove microorganisms in mass production of bioplastics and also tune their genome to enhance MP degradation efficacy and efficiency.