Introduction

Nanoparticles (NPs) are widely used in commercial and industrial products such as sunscreens, batteries, and photocatalysis (Rezende et al. 2022; Dubsok et al. 2022; Simonin et al. 2016). Among the prepared nanoparticles, TiO2 NPs are undoubtedly the most widely used. The widespread use of TiO2 NPs inevitably results in their discharge into the environment. It is estimated that 2.5 million tons will be produced annually by 2025 (Robichaud et al. 2009). The content of TiO2 NPs in surface water can reach 21 ng/L; moreover, it can reach up to 4 μg/L in the wastewater after sewage treatment (Kunhikrishnan et al. 2015). In particular, the content of TiO2 NPs in the sediment is the highest, reaching mg/kg levels (Gottschalk et al. 2009). TiO2 NPs are considered a new pollutant in environmental toxicology because of their unique structural characteristics and environmental behavior (Dale et al. 2015). It can accumulate in organisms and transmitted through the food chain (Wang et al. 2016), posing a potential threat to human health and the ecological environment (Keller and Lazareva 2014).

Most organic pollutants are persistent organic pollutants that are often detected in biological samples, owing to their lipophilicity and degradation resistance (Cabrera-Rodríguez et al. 2020). They can accumulate in other animals through the transmission of food chain (Zaynab et al. 2021), posing a threat to the growth and reproduction of aquatic organisms (Du et al. 2022). Organic pollutants and NPs have ample opportunities to coexist in the surface environment (Sendra et al. 2017). When NPs coexist with other pollutants in surface water, owing to the differences in various water quality parameters (such as natural organic matter (NOM) and ions) in surface water, they affect the surface properties and environmental behaviors of NPs and coexisting pollutants (Weschenfelder et al. 2015; Domingos et al. 2009), which changes the toxic effect of NPs and coexisting pollutants. In karst areas with fragile ecological environments, the contents of Ca2+ and Mg2+ in surface water are high (Gu et al. 2017), and the hydrochemical properties are complex, making the toxic effects of NPs and coexisting pollutants unpredictable. However, few studies have explored the toxicity of NPs to aquatic organisms in surface waters (Dalai et al. 2013; Natarajan et al. 2022). Many studies on the toxicity of NP-coexisting pollutants have been carried out under specific experimental conditions, which differ significantly from those of surface water bodies with complex hydrochemical properties (Aiken et al. 2011). Therefore, exploring the toxic effects of NPs and coexisting pollutants in karst surface waters is necessary.

The purpose of this study was to explore the combined toxicities of TiO2 NPs and three different organochlorines (pentachlorobenzene (PeCB), 3,3',4,4'-tetrachlorobiphenyl (PCB-77), and atrazine) in three types of karst surface water bodies to Chlorella pyrenoidosa. In surface water with different hydrochemical properties, the toxic effects of TiO2 NPs and OCs on organisms, the effects of bioaccumulation, the hydrophobicity (CSH) of algal cells, chemical composition, and cell morphology were investigated. Our research is likely to improve our understanding of the potential risks of TiO2 NPs and OCs in the aquatic environment.

Materials and methods

Water sample collection and parameter determination

Samples were collected in June 2021. To reduce the impact of rainfall on the water samples, sampling was conducted after 7 continuously sunny days. Water samples were collected from Baihua Lake (BH, N 26°40′39″, E 106°33′6″), Hongfeng Lake (HF, N 26°26′35″, E 106°37′25″), and Huaxi Reservoir (HX, N 26°26′36″, E 106°37′42″). The temperature, pH, and conductivity of the water samples were measured on-site (Table S1). Ultrapure water (UW) was used as the control group (Zhang et al. 2023). Baihua Lake is located 22 km northwest of Guiyang City and has a well-developed karst landform. Hongfeng Lake is located in a western suburb of Guiyang, in Guizhou Province, 28 km away from Guiyang. The Huaxi Reservoir is located in Huaxi District, in Guiyang City, and is approximately 3 km downstream of Huaxi. The collected water samples were filtered with a paper filter to remove the large particles, and then filtered with a 0.45-um PTFE membrane and stored in a dark environment at 4 °C. The total organic carbon (TOC) and inorganic carbon (IC) of the different samples were determined using a TOC analyzer (TOC-L CPH, Shimadzu, Japan). The conductivity of the water samples was measured using a conductivity meter (DDS-307A, Rex Electric Chemical), and the pH of the water samples was measured using a pH meter (PHSJ-3F, Rex Electric Chemical). Cations in the samples were determined using inductively coupled plasma mass spectrometry (ICP-MS, NexlON 1000G, Japan), and total nitrogen (TN), total phosphorus (TP), and anions were determined using a multi-parameter water quality analyzer (CleverChem200, Germany). See the Supporting Information (SI) for the specific water parameters.

Experimental materials and organism

Anatase TiO2 nanoparticles (TiO2 NPs) with a particle size of 5–10 nm and purity of 98% were purchased from Zhejiang Hongsheng Material Technology Co., Ltd. The basic properties of TiO2 NPs have been characterized in other studies (Ji et al. 2011). Chlorella pyrenoidosa was purchased from the Wuhan Institute of Aquatic Biology, Chinese Academy of Sciences, cultured using BG11 (See the SI for the specific formula). Atrazine (97% purity) was purchased from Beijing Bailingwei Technology Co., Ltd. The purities of 3,3′,4,4′-tetrachlorobiphenyl (3,3′,4,4′-tetrachlorobiphenyl, PCB-77) and pentachlorobenzene (PeCB) were 99% and 98%, respectively. Both were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Acetone was used as a co-solvent for the three OCs, and the acetone concentration was less than 100 uL/100 mL (It was confirmed in the preliminary experiment that acetone was not toxic to algal cells). Due to the volatility of OCs, they were supplemented every 24 h to maintain the OC concentration consistent with the initial concentration (Zhang et al. 2017).

Individual and combined toxicity of TiO2 NPs and OCs to algae

The algal species were washed with pure water thrice and centrifuged (4000 rpm, 25 °C, 10 min). The culture medium used in the experiment was 100 mL of surface water, which was placed in a 250-mL conical flask for culture. UW was used as the control group, and the initial concentration of algae was 2.5 × 106 cells/mL. The culture conditions on the illumination shaking table were a temperature of 25 ℃, light intensity of 100 μE/m2/s, light–dark ratio of 14:10 h, shaking speed of 110 rpm, and a culture time of 96 h. The number of algae was counted using a blood cell–counting plate under an optical microscope (BM1000, China), and the 96 h-IC50 of the pollutants was calculated by linear interpolation.

Individual toxicity

The concentration range of TiO2 NPs or OCs in the individual toxicity test was TiO2 NPs (0–130 mg/L), atrazine (0–0.72 mg/L), PeCB (0–0.66 mg/L), and PCB-77 (0–0.01 mg/L). OCs were replenished every 24 h. The 96 h-IC50 of TiO2 NPs in UW was small; therefore, the concentration gradients of TiO2 NPs were increased within 0–10 mg/L.

Combined toxicity

In the combined toxicity experiment, the initial concentration ratios of TiO2 NPs and OCs (atrazine, PeCB, and PCB-77) were (0a, 0b), (0.2a, 0.2b), (0.4a, 0.4b), (0.6a, 0.6b), (0.8a, 0.8b), and (a, b), where a and b represent the reference concentrations of OCs (PeCB, atrazine, and PCB-77) and TiO2 NPs, respectively, obtained from individual toxicity experiments. The reference concentrations of TiO2 NPs and atrazine were the 96 h-IC50, and the 96 h-IC50 of TiO2 NPs and atrazine in UW, BH, HF, and HX were TiO2 NPs (0.900, 65, 61, 67) mg/L, atrazine (0.300, 0.552, 0.522, 0.580) mg/L, respectively. As the inhibition rates of PeCB and PCB-77 were lower than 50% in their respective maximum solubility range in the individual toxicity experiment, the maximum solubilities of PeCB and PCB-77 were selected as reference concentrations, which were 0.66 and 0.01 mg/L, respectively. See Table S4 for specific reference concentrations. Concentration addition (CA) was used to evaluate the type of combined toxic effects (Belden et al. 2007).

$${EC}_{\mathrm{x},\mathrm{min}}={\left(\sum_{i=1}^{n}\frac{{P}_{\mathrm{i}}}{{EC}_{x,i}}\right)}^{-1}$$

where ECx,min is the corresponding concentration (mol/L) when the toxic effect of mixed contaminants reaches x %; ECx,i represents the corresponding concentration (mol/L) of component i in mixed pollutants when its individual toxicity reaches x %; Pi represents the proportion of component i in the mixed contaminants.

When the sum of the toxic effects produced by individual pollutants in mixed contaminants is less than the toxic effects of mixed contaminants, it is synergistic; if the toxicity is equal to the toxic effects of mixed contaminants, it is additive; and when the toxicity is higher than the toxic effect of mixed pollutants, it is antagonistic.

Determination of accumulation of TiO2 NPs and OCs by algae

This study measured the bioconcentration factors of TiO2 NPs and OCs to explore the accumulation of OCs and TiO2 NPs by algal cells in surface waters. The individual and combined exposure concentrations of OCs and TiO2 NPs were 0.8 times of the reference concentration (the concentration was conducive to observe the effect of pollutants on the algal cells), and the algal concentration was 2.5 × 107 cells/mL. The consumed OCs were replenished every 24 h, and 20 mL algal liquid was collected at 6, 12, 24, 48, 72, and 96 h, respectively. The number of algae was counted using a blood cell–counting plate under an optical microscope, then centrifuged (4000 rpm, 25 °C, 15 min). See SI for the specific experimental steps. The biologically enriched amounts (BEA, μg/mg) of OCs by algae were calculated based on the dry weight of the algae, and the concentration (M, μg/mL) of OCs in the supernatant was measured so as to obtain the bioconcentration factor (BCF) of OCs by algae. The calculation formula is as follows.

$$\mathrm{BCF}=\frac{BEA}{M}$$

Determination of algal cell properties

Sedimentation of algae

To explore the influence of OCs on the homogeneous aggregation of TiO2 NPs and algal cells, and the heterogeneous aggregation of TiO2 NPs-algal cells in different water bodies, the individual and combined exposure concentrations of OCs to TiO2 NPs were as described in Sect. 2.3. The initial concentration of algae was expressed as the concentration of algae solution with an absorbance of 0.2 at 660 nm, and it was measured hourly for 13 h. The absorbance A of each suspension was measured using an ultraviolet spectrophotometer (UV-2450, Japan) at a wavelength of 660 nm. The self-settling and co-settling curves of algae and pollutants were expressed using the absorbance A at each time point compared with the previous initial absorbance A0. The settling curves represent the absorbance of the mixture and the sum of the absorbance of the individual suspensions over time. When the co-deposition curve was lower than the additive curve, TiO2 NPs-algal cells were heteroaggregated, and this difference reflected the degree of heteroaggregation.

Hydrophobicity measurement

In the algal hydrophobicity experiments, the individual and combined exposures of TiO2 NPs and OCs were 96 h-NOEC each (“no observed effect concentration” (NOEC) calculated from individual and combined toxicities). The method of hydrophobicity measurement has been reported in the relative literature (Zhang et al. 2016a, b). In an individual toxicity experiment, the 96 h-NOECs for TiO2 NPs, atrazine, PeCB, and PCB-77 in UW, BH, HF, and HX were TiO2 NPs (0.3, 4, 4, 6 mg/L), atrazine (0.009, 0.014, 0.016, 0.019 mg/L), PeCB (0.076, 0.112, 0.090, 0.106 mg/L), and PCB-77 (0.002, 0.003, 0.004, 0.003 mg/L), respectively. To analyze the hydrophobicity of algal cells in different water bodies, when TiO2 NPs and OCs were exposed together, the minimum 96 h-NOECs of TiO2 NPs, atrazine, PeCB, and PCB-77 in different water bodies were 0.168, 0.005, 0.058, and 0.001 mg/L, respectively. The algal concentration was 2.5 × 106 cells/mL. Hydrophobic experiments were conducted at this concentration in different water bodies. See the SI for the specific experimental steps.

Determination of chemical composition

The changes in the chemical composition of algal cells were determined using a Fourier transform infrared spectrometer (Thermoscientific Nicolettis 5, USA) after the individual and combined exposure to TiO2 NPs and OCs. The culture conditions, the concentration of algae, and the concentration of pollutants were the same in the section “Individual and combined toxicity of TiO2 NPs and OCs to algae”. After 96 h of culture, the TiO2 NPs and algal cells were separated using the density gradient method. After washing the algal cells with PBS (pH = 7.0) three times, the algal cells were ground after freeze drying. After mixing 1-mg algae powder with 100 mg of KBr, grinding, and tabletting, the resolution was 4 cm−1, and the wave range was 4000–400 cm−1. Finally, the infrared spectrum data were normalized, and then the different spectra were analyzed.

Determination of oxidative damage and observation by transmission electron microscope

In the experiment of oxidative damage and transmission electron microscope, the different exposure concentrations of TiO2 NPs and OCs and the algal concentration were the same in the section “Individual and combined toxicity of TiO2 NPs and OCs to algae”, and the culture time was 24 h (The preliminary experiment showed that 24 h was conducive to observing the damage of algal cells). In this study, the superoxide dismutase activity (SOD), catalase activity (CAT), malondialdehyde content (MDA), and reactive oxygen species (ROS) in the algal cells were determined, and the changes of algal cell structure were observed. See the SI for the specific experimental steps.

Data processing and correlation analysis

Data were processed in Excel, and all experiments were performed in triplicate. The results were expressed as the mean ± standard deviation. The drawing software used in this study was Origin 2018. The differences in water quality parameters were analyzed in advance to evaluate the correlation between water parameters and the toxicities of pollutants better. Because the difference in pH and Mg2+ among the water bodies was small, and, compared with the ionic strength, conductivity was redundant in reflecting the general situation of ions in water bodies. Finally, the correlation analysis did not include the conductivity, pH, and Mg2+. In addition, the ionic strength was calculated from the cations and anions. While TOC, TN, and TP could affect the algal growth, Ca2+ is a typical ion in karst surface water. Finally, five water parameters, i.e., ionic strength, Ca2+, TOC, TN, and TP, were selected for correlation analysis.

Results and analysis

Toxicities of TiO2 NPs and OCs to algae

As shown in Fig. 1A–D, the growth inhibition of algae by TiO2 NPs and OCs in the three surface waters was lower than that of UW. Within the experimental concentration range, TiO2 NPs and atrazine significantly inhibited algal growth, whereas PeCB and PCB-77 showed little inhibition (less than 50%). The growth inhibitions of PeCB and PCB-77 on algae in different water bodies all were UW > HF > BH > HX (Fig. 1B and D). Due to the lack of regularity in the physical and chemical properties of surface waters in this study, there was no obvious difference in the algal growth by the individual exposure of TiO2 NPs and atrazine in different surface waters.

Fig. 1
figure 1

Combined toxicities of TiO2 NPs and OCs to algae

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As shown in Fig. 1E–P, the effect of the combined exposure to TiO2 NPs and OCs on algal growth was evaluated by comparing the measured combined toxicities with those predicted by the CA model. The combined exposure of TiO2 NPs and OCs in UW had the highest inhibitory effect on algal growth, indicating that the algal cells were greatly damaged. When co-exposed to TiO2 NPs and PeCB, the measured combined toxicities in BH, HF, and UW were higher than those calculated by the CA model, which was synergistic. While the measured combined toxicity in HX was equal to that calculated by the CA model, the effect was additive. The growth inhibition of algal cells by TiO2 NPs and PeCB in different water bodies was in the order of UW > HF > BH > HX from large to small. When co-exposed to TiO2 NPs and atrazine, the measured combined toxicity of pollutants to algae in different water bodies was higher than that calculated by the CA model, displaying a synergistic effect. TiO2 NPs and atrazine had the smallest inhibition of algal growth in HX, and their inhibition of algal growth was similar in BH and HF. When co-exposed to TiO2 NPs and PCB-77, the measured combined toxicities in different water bodies were less than the calculated value of the CA model, and the effect was antagonistic. The inhibition of algal growth by TiO2 NPs and PCB-77 in HF was greater than that in HX or BH.

Bioaccumulation of OCs and TiO2 NPs by algal cells

As shown in Fig. 2, in the presence of OCs, the BCFs of algal cells to TiO2 NPs were larger in different water bodies, indicating that the bioaccumulation trend of TiO2 NPs by algal cells was stronger, which has enhanced the toxicity of TiO2 NPs to algae. Over time, the BCF of algal cells to TiO2 NPs decreased overall, with the greatest decrease observed after 12 h. The BCFs of algal cells to TiO2 NPs in surface waters were significantly lower than those in UW. In addition, the presence of atrazine and PeCB (except for PeCB in HX) significantly increased the BCFs of the algal cells to TiO2 NPs in different waters. However, in the presence of PeCB, the BCF of TiO2 NPs by algal cells in HX increased slightly, resulting in no significant increase in the combined toxicity. In addition, the presence of PCB-77 reduced the BCF of the algal cells to TiO2 NPs. The above results indicated that the degree of bioaccumulation of pollutants by algal cells was due to the differences in the hydrochemical and pollutant properties.

Fig. 2
figure 2

The BCF of algae to TiO2 NPs in the presence of OCs; lowercase letters indicate significant differences between the different water bodies at the same time (p < 0.05)

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As shown in Fig. 3, the presence of TiO2 NPs in different water bodies increased the BCFs of OCs by algal cells. However, after 12 h, the BCFs of algal cells to OCs decreased rapidly, but after 24 h, the change was not significant, and the number of algal cells increased rapidly. Regardless of the presence of TiO2 NPs, algae in different waters had the smallest BCF to atrazine, which might be related to its low hydrophobicity. In addition, the BCF difference of algae to atrazine was small in the three surface waters, which might be the reason for the similar toxicity results in different surface water bodies. Although the BCFs of algal cells to PeCB and PCB-77 were higher than that of atrazine (Fig. 3A and E), as the toxicities of PeCB and PCB-77 were relatively low in the experimental concentration range, no significant growth inhibition was observed in the toxicity experiment (Fig. 1B and D). When TiO2 NPs were co-exposed with PCB-77 and PeCB, respectively, the BCFs of algal cells to PCB-77 and PeCB were generally larger in different water bodies. Because the BCF of algae to PeCB was the smallest in HX, it showed lower toxicity in the combined toxicity experiment (Fig. 1G).

Fig. 3
figure 3

The BCF of algae to OCs in the presence of TiO2 NPs; lowercase letters indicate the significant differences between the different water bodies at the same time (p < 0.05)

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TiO2 NPs can destroy the structure of the algal cell membrane (Loosli et al. 2013), allowing the OCs to enter algal cells more easily, thus increasing the bioaccumulation of OCs by algal cells (Figs. 3 and S2). The presence of atrazine and PeCB in different water bodies slowed down the co-precipitation rate of TiO2 NPs and algal cells to varying degrees, thus increasing the contact probability between TiO2 NPs and algal cells, which correspondingly increases the BCF and BEA of algae to TiO2 NPs (Figs. 2 and S1). However, the increase of BCF and BEA of algal cells to TiO2 NPs and PeCB in HX was small, which led to little change in the toxic effects of both in the combined exposure. The difference was that PCB-77 accelerated the deposition rate of TiO2 NPs and, thus, reduced the contact between TiO2 NPs and algal cells. This reduced the BCF and BEA of algal cells to TiO2 NPs, thus reducing the damage caused by TiO2 NPs to algal cells. In addition, the ionic strength and NOM in different water bodies affect the stability of NPs (Keller et al. 2010), thus affecting the aggregation of NPs with organisms. However, the ions and NOM in UW were almost completely removed, which made it easier for TiO2 NPs and OCs to adsorb on algal cells and showed greater toxicity. The BCF and BEA of algal cells to TiO2 NPs and OCs were basically consistent with the results of the combined toxic effect.

Changes in algal cell properties

Effects of TiO2 NPs and OCs on algal settlement

As shown in Fig. S3, atrazine and PeCB in different water bodies slowed the sedimentation rate of TiO2 NPs to different degrees but did not significantly affect the sedimentation rate of algal cells. PCB-77 slightly increased the sedimentation rate of TiO2 NPs but slowed the sedimentation rate of algal cells. Without the addition of pollutants, the sedimentation rates of algae in different water bodies were almost the same. As shown in Fig. 4, the additive curve (sum) of TiO2 NPs and algal cells in surface water was lower than the co-deposition curve (mix1); the results showed that the aggregation formation of TiO2 NPs-algal cells in three types of surface water bodies was inhibited. However, the additive curve and co-deposition curves in UW almost coincided in the later stage, indicating that the formation of TiO2 NPs–algal cell aggregates was not obviously inhibited. It has been reported that DOM in surface water helps inhibit the heterogeneous aggregation of TiO2 NPs–algal cells (Zhang et al. 2016a, b). OCs had little effect on the heterogeneous agglomeration of TiO2 NPs–algal cells but affected the homogeneous agglomeration of TiO2 NPs and algal cells, respectively. In general, the risk of contact between TiO2 NPs and algal cells was higher in UW.

Fig. 4
figure 4

Algal co-precipitation and addition curves; sum is addition curves of TiO2 NPs and algae, mix1 is TiO2 NPs + algae, mix2 is TiO2 NPs + algae + PeCB, mix3 is TiO2 NPs + algae + atrazine, mix4 is TiO2 NPs + algae + PCB-77

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Hydrophobicity changes of algae

The hydrophobicity (CSH) of algae is closely related to the surface properties of the algal cells. The adsorption capacity of hydrophobic organic contaminants (HOCs) by algal cells increases with an increase in CSH (Garg et al. 2014). The CSH of algal cells could affect their interaction of them with organic substances, TiO2 NPs, and NOM. Organic pollutants can affect the CSH of algal cells (Zhang et al. 2015). As shown in Fig. 5, the CSH of algal cells in surface waters was lower than that in UW. The presence of OCs in different water bodies increased the CSH of algal cells, and the variation tendency over time was similar to that in the blank control group. The CSH of algal cells in the presence of PeCB and PCB-77 was generally larger than that of atrazine, which might be related to the higher hydrophobicity of PeCB and PCB-77. Owing to the low hydrophobicity of atrazine, the adsorption of atrazine by algal cells was low, and the CSH of algal cells was also low. In addition, during the algal growth cycle, CSH showed a downward trend as a whole. It was speculated that the decrease in the CSH of algal cells in the late stage might be related to the reproduction strategy of algal cells, which was more helpful for algae to absorb nutrients in the culture medium.

Fig. 5
figure 5

Effects of TiO2 NPs and OCs on CSH of algae; lowercase letters indicate significant differences of different pollutants at the same time (p < 0.05)

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Effects of TiO2 NPs and OCs on the chemical composition and oxidative damage of algal cells

As shown in Fig. 6, the peaks at 1740 cm−1 and 878 cm−1 were caused by C \(\equiv\) O and C \(=\) O stretching vibrations in DNA/RNA (Lu et al. 2016), and C \(=\) C, C \(=\) N, and C-H bonds of the nucleotide ring structure (Yu and Irudayaraj 2005). The absorption peak at 650 cm−1 is the characteristic peak of protein lactam band I (Gelfand et al. 2015). The absorption peak at 2920 cm−1 is attributed to the –CH3/–CH2 group in the lipid. When TiO2 NPs were exposed alone or in combination with OCs, the algal cells in different waters decreased to different extents at the above peaks, especially in the UW. This indicates that the synthesis of DNA/RNA, polysaccharides, and proteins in algal cells was affected. As shown in Fig. S4, the cell structure and starch granules were intact in the control group. However, in the presence of TiO2 NPs, the structure of algal cells was damaged, plasmolysis occurred, and the volume of organelles decreased. It has been reported that, after absorption and internalization of TiO2 NPs by cells, it was easy to combine with the cell membrane to form aggregates in vesicles and vacuoles (Singh et al. 2007). In this study, UW3, UW4, BH3, BH4, BH5, and HX5 exhibited electron-dense or vacuolar structures. This indicated that the algal cells brought TiO2 NPs into the interior of algal cells through endocytosis; this effect has also been observed in previous research (Lin et al. 2012).

Fig. 6
figure 6

Infrared spectra of algae in the presence of TiO2 NPs and OCs

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As shown in Fig. S5, the presence of PeCB and PCB-77 in OCs did not significantly change the oxidative stress in algal cells, except that atrazine caused a slight increase in oxidative damage in algal cells. However, in the presence of TiO2 NPs, the ROS in algal cells was significantly increased. In contrast, the MDA was basically in line with the changes of ROS, and SOD and CAT were also increased under higher oxidative pressure. It has been reported that, when mussels are exposed to TiO2 NPs, the levels of SOD, CAT, and GSH increased (Huang et al. 2018). When TiO2 NPs and OCs co-existed, the ROS in the atrazine group increased most significantly, whereas the changes of ROS in the PeCB group and PCB-77 groups were small. Overall, TiO2 NPs exerted the greatest oxidative pressure on algal cells, suggesting that TiO2 NPs were the main cause of the greater damage to algal cells. The above results indicated that, in the presence of TiO2 NPs and OCs, the chemical composition of algal cells was changed; the synthesis of DNA/RNA, polysaccharides, and proteins all were affected; the oxidative stress on algal cells increased to different degrees; and the structure of algal cells was damaged, with the change in UW being the most significant. The above results indicated that in the presence of TiO2 NPs and OCs, the chemical composition of algal cells was changed; the synthesis of DNA/RNA, polysaccharides, and proteins all were affected; and the structure of algal cells was damaged.

Correlation analysis between different water parameters and algal cytotoxicity

The surface water contains different material compositions and contents, which leads to the differences in chemical properties of the water body, and increases the difficulty of toxicity analysis of pollutants in the surface water. Correlation analysis was used to analyze the correlation between five indicators (ionic strength, Ca2+, TOC, TN, and TP), and the toxicity of different pollutants in this study. As shown in Table S5, TiO2 NPs had a significant negative correlation with both ionic strength and conductivity (p < 0.05), indicating that, among the three water bodies, the toxicity of TiO2 NPs was reduced in surface water with higher ionic strength, However, the effects of TOC, Ca2+, TP, and TN on TiO2 NPs were not significant. PeCB, atrazine, and PCB-77, either alone or in combination with TiO2 NPs, were significantly positively correlated with TOC (p < 0.05), indicating that higher TOC content in the three surface water bodies increased their inhibition of algal growth, which was consistent with the variation in algal cell concentration in the three surface water bodies. The BEA of algae to OCs or TiO2 NPs in HF with higher TOC content was generally larger (Figs. S1 and S2), but the change rule was the opposite in HX with lower TOC content, resulting in a positive correlation between TOC and its toxicity.

Discussion

Differences in surface water bodies’ physical and chemical properties would affect the bioaccumulation and toxicity of NPs and coexisting pollutants. For example, the ionic strength and pH would affect the surface properties of the NPs (Gunawan et al. 2011), thus affecting the release of reactive oxygen species (Wu et al. 2014). This would eventually lead to changes in the toxicity to organisms. It has been shown that pH, ionic strength, and the structure and concentration of NOM can all affect the behavior of TiO2 NPs in the aquatic environment and, thus, affect the stability of the NPs (Freixa et al. 2018). In addition, after absorbing on TiO2 NPs, NOM can increase its suspension time by electrostatic effect, which also increases the exposure of TiO2 NPs to algae, thereby increasing the algal toxicity of the NPs (Li et al. 2016). The surface waters in the karst area have a higher ionic strength than UW (Fig. 1), which compresses the electric bilayer around TiO2 NPs and increases the chance of collision of TiO2 NPs, thus promoting the homogeneous agglomeration of TiO2 NPs (Lin et al. 2012). The karst surface water is rich in Ca2+ and Mg2+. It was reported that Ca2+ could interact with NPs and humic acid in the solution, thus significantly promoting the agglomeration and settlement of NPs; however, Mg2+ did not have such interactions (Chen and Elimelech 2007). In addition, the presence of Ca2+ accelerates the agglomeration of TiO2 NPs and forms mm-sized aggregates, and the valence state of the ions affectes the agglomeration of TiO2 NPs (Ji et al. 2010). This reduced the damage caused by TiO2 NPs to algal cells, leading to a negative correlation between the ionic strength and the toxicity of TiO2 NPs in this study (Table S5). Because of the complex physical and chemical properties of surface water, the toxicity of pollutants in different water bodies to algal cells was not clear, making it challenging to explore the toxic effect of NPs-coexisting pollutants in surface water bodies. In the future, more surface water bodies with different hydrochemical properties should be selected to explore the combined toxicity of different pollutants.

The effective cumulative concentration of pollutants by organisms determines their toxicity to organisms (Long et al. 2012). Algae initially rapidly adsorbed OCs and then slowly desorbed it (Koelmans 2014). This resulted in higher bioaccumulation to OCs by algal cells in the early stage, but the bioaccumulation to OCs by algal cells was decreased at a later stage. In this study, the BCF and BEA of algal cells to TiO2 NPs and OCs were mainly concentrated before 12 h (Figs. 2 and 3 and Figs. S1 and S2), and the number of algae increased slowly because the algal cells were greatly inhibited. However, owing to changes in algal cell reproduction strategies in the later, such as the secretion of extracellular polymeric substances (EPS) by algae under the action of TiO2 NPs to resist the attack of pollutants (Gao et al. 2020), and the gradual decrease of the hydrophobicity of algal cells, the chance of contact with pollutants was reduced. Moreover, this was conducive for algal cells to obtain nutrients from the surrounding environment, thus enabling the rapid proliferation of algal cells. Hence, the BCF and BEA of the corresponding algal cells for TiO2 NPs and OCs were also reduced. It has been reported that the direct interaction of NPs-OC hardly affected the bioaccumulation and toxicity of TiO2 NPs (Zhang et al. 2017). However, complex material components (such as Ca2+ and NOM) in surface water bodies affected the bioaccumulation of TiO2 NPs in algal cells by affecting the suspension stability of TiO2 NPs (Romanello and Cortalezzi 2013), thus affecting the bioaccumulation of TiO2 NPs and OCs by algal cells.

NPs attach to the surface of algal cells, resulting in the formation and settlement of NPs-algal aggregates, which is an important process affecting the toxic effect of NPs. Simultaneously, NPs would generate a series of toxic mechanisms, such as physical damage, shading effects, oxidative stress, and absorption internalization (Ma and Lin 2013). It was reported that TiO2 NPs had high activity and easily produced a series of ROS (Clément et al. 2013). In addition, NPs could cause oxidative stress in algal cells, resulting in lipid, protein, and nucleic acid damage (Huang et al. 2018). In this study, the presence of TiO2 NPs and atrazine led to an increase in ROS in algal cells, which resisted higher oxidative stress by increasing the activities of CAT and SOD (Fig. S5). In addition, the structural integrity of algal cells in different water bodies reduced the volume of organelles and damaged the proteins, DNA/RNA, and lipids in algal cells (Fig. 6). TEM showed that the plasmolysis of algal cells occurred, especially in UW (Fig. S4). Studies have shown that the fulvic acid in NOM can not only remove the oxidative free radicals in organisms, detoxify pollutants, and resist oxidative damage. This can help organisms to eliminate harmful substances in the body, promote metabolism, maintain electrochemical balance, and prevent cell imbalance (Li et al. 2016). This results in that the damage of algal cells caused by TiO2 NPs and OCs in surface water was generally smaller than that caused by UW.

In summary, the toxicities of TiO2 NPs and OCs in surface waters were different. Because of the high ion and NOM concentrations in surface water, the stress of TiO2 NPs and OCs on algal cells was alleviated. The existence of TiO2 NPs and atrazine both increased the bioaccumulation of the other in algal cells and increased the oxidation pressure of algal cells; thus, they showed greater toxicity, and were synergistic effects. Similarly, TiO2 NPs and PeCB both promoted the bioaccumulation of algal cells to different degrees. Especially in BH, HA, and UW, the bioaccumulations of pollutants by algal cells were significant, indicating a synergistic effect. However, the bioaccumulation of TiO2 NPs in algal cells was not significantly increased in HX. Additionally, the toxicity in the co-exposure of TiO2 NPs and PeCB showed little change compared to the sum of their individual toxicities in HX, which was an additive effect. Although TiO2 NPs increased the bioaccumulation of algae to PCB-77, within the experimental concentration range, PCBs -77 showed little inhibition on the growth of algal cells, and PCB-77 reduced the bioaccumulation of algal cells to TiO2 NPs. The increased toxicity of PCB-77 to algae cells was less than the reduced toxicity of TiO2 NPs to algae, which was an antagonistic effect. Owing to the complex water quality parameters and lack of regularity in the hydrochemical properties of surface waters, the oxidative stress and bioaccumulation of algae induced by TiO2 NPs and OCs in BH, HF, and HX did not change significantly. However, compared with surface water, algal cells were more damaged in UW.

Conclusion

Compared with UW, all three types of surface water bodies reduced the algal growth inhibition by TiO2 NPs and OCs. Correlation analysis showed that the toxicities of TiO2 NPs and OCs to algal cells in surface waters were mainly related to ionic strength and TOC. The presence of OCs increased the hydrophobicity of algal cells, which could increase the risk of OCs bioaccumulation by algal cells. In addition, the bioaccumulation of TiO2 NPs and OCs by algal cells could affect their combined toxic effects. Co-exposure to TiO2 NPs and atrazine increased the bioaccumulation of TiO2 NPs and atrazine by algae, and their combined toxicity was synergistic. The co-exposure of TiO2 NPs and PCB-77 decreased the bioaccumulation of TiO2 NPs by algae. Although it increased the bioaccumulation of PCB-77 by algae, the decreased toxicity of TiO2 NPs could exceed the increased toxicity of PCB-77, thus, their combined toxicity of them was antagonistic. When algal cells were co-exposed to TiO2 NPs and PeCB, the bioaccumulation of TiO2 NPs and PeCB by algae increased in BH, HF, and UW, and their combined toxicity was synergistic. However, the bioaccumulation of TiO2 NPs did not significantly increase in HX, and the toxicity of PeCB within the experimental concentration range was not high. Therefore, their combined toxicity was additive in HX. Owing to the lack of regularity in the hydrochemical properties of different surface waters, the effects of TiO2 NPs and OCs on organisms in surface waters were irregular. We hope that our research can provide a reference for the toxic effects of TiO2 NPs and OCs on organisms in real water environments.