Effects of moxifloxacin and gatifloxacin stress on growth, photosynthesis, antioxidant responses, and microcystin release in Microcystis aeruginosa

Exploring the influence of sulfadiazine-induced stress on antibiotic removal and transformation pathway using microalgae Chlorella sp

Sulfadiazine (SDZ) is a kind of anti-degradable antibiotics that is commonly found in wastewater, but its removal mechanism and transformation pathway remain unclear in microalgal systems. This study investigated the effects of initial algae concentration and SDZ-induced stress on microalgal growth metabolism, SDZ removal efficiency, and transformation pathways during Chlorella sp. cultivation. Results showed that SDZ had an inhibitory effect on the growth of microalgae, and increasing the initial algal biomass could alleviate the inhibitory effect of SDZ. When the initial algal biomass of Chlorella sp. was increased to 0.25 g L-1, the SDZ removal rate could reach 53.27%-89.07%. The higher the initial algal biomass, the higher the SOD activity of microalgae, and the better the protective effect on microalgae, which was one of the reasons for the increase in SDZ removal efficiency. Meanwhile, SDZ stress causes changes in photosynthetic pigments, lipids, total sugars and protein content of Chlorella sp. in response to environmental changes. The main degradation mechanisms of SDZ by Chlorella sp. were biodegradation (37.82%) and photodegradation (23%). Most of the degradation products of SDZ were less toxic than the parent compound, and the green algae were highly susceptible to SDZ and its degradation products. The findings from this study offered valuable insights into the tradeoffs between accumulating microalgal biomass and antibiotic toxic risks during wastewater treatment, providing essential direction for the advancement in future research and full-scale application.

The responses and tolerance of photosynthetic system in Chlorella vulgaris to the pharmaceutical pollutant carbamazepine

Screening for sensitive toxicological indicators and understanding algal tolerance to pharmaceutical contaminants (PhCs) are essential for assessing PhCs risk and their removal by microalgae. Carbamazepine (CBZ) showed adverse effects on microalgae, but the specific toxicity mechanisms on the most sensitive algal photosynthetic system (PS) remain limited. This study delved into the impact of CBZ exposure on the growth, cell viability, pigment content, and PS of Chlorella vulgaris. The findings revealed a notable inhibition of C. vulgaris growth by CBZ, with an IC50 value of 27.2 mg/L at 96 h. CBZ exposure induced algal membrane damage and cell viability. Intriguingly, CBZ drastically diminished intracellular pigment levels, notably showing "low promotion and high inhibition" of chlorophyll b (Chl b) by 72 h. Moreover, the study identified a decreased number of active reaction centers (RCs) within algal PSII alongside inhibited electron transport from QA to QB on the PSII receptor side, leading to PSII disruption. As an adaptive response to CBZ stress, C. vulgaris stimulated its Chl b synthesis, increased non-photochemical quenching (NPQ), and adapted its tolerance to bright light. Additionally, the alga attempted to compensate for the CBZ-induced reduction in electron transfer efficiency at the PSII receptor side and light energy utilization by increasing its electron transfer from downstream. Principal component analysis (PCA) further verified that the parameters on non-photochemical dissipation, electron transport, and integrative performance were the most sensitive algal toxicological indicators for CBZ exposure, and algal PS has energy protection capability through negative feedback regulation. However, prolonged exposure to high doses of CBZ will eventually result in permanent damage to the algal PS. Hence, attention should be paid to the concentration of CBZ in the effluent and the exposure time, while methods to mitigate algal photodamage should be appropriately sought for algal treatment of dense effluents.

Understanding the toxicity risk of antibiotic emissions of aquaculture from the perspective of fluctuations concentration

Organisms are generally exposed to target contaminant with stable concentrations in traditional ecotoxicological studies. However, it is difficult to truly represent the dynamics and complexity of actual aquatic pollution for risk management. Contaminants may enter nearby aquatic systems in pulsed exposure, thus resulting in that aquatic organisms will be exposed to contaminants at fluctuating concentrations. Especially during the season of summer, due to the changes in displacement or periodic emissions of veterinary antibiotics in aquaculture, algal blooms occur frequently in surrounding waters, thus leading to eutrophication of the water. Florfenicol (FFC) is currently widely used as a veterinary antibiotic, but the aquatic ecological risks of FFC under concentration fluctuations are still unknown. Therefore, the acute exposure, chronic exposure and pulsed exposure effects of FFC on Microcystis aeruginosa were investigated to comprehensively evaluate the ecological risk of FFC and raise awareness of the pulsed exposure mode. Results indicated that the toxic effects of FFC on M. aeruginosa were dominated by exposure mode, exposure duration, exposure frequency, and exposure concentration. The maximum growth inhibition rate of the 10 μg/L FFC treatment amounted to 4.07% during chronic exposure of 18 days. However, the growth inhibition rate decreased from 55.1% to 19.31% when algae was exposure to 10 μg/L FFC during the first pulsed exposure (8 h). Therefore, when the concentration of FFC was equal under chronic and pulsed exposure, FFC exhibited greater toxicity on M. aeruginosa in short pulsed exposure than in continuous exposure. In addition, repetitive pulsed exposure strengthened the resistance of M. aeruginosa on FFC. The adaptive regulation of algae was related to the duration and frequency of exposure. Above results suggested that traditional toxicity assessments lacked consideration for fluctuating concentrations during pollutant emissions, thus underestimating the environmental risk of contaminant. This investigation aims to facilitate the standardization of pulsed exposure.

A Review on Fluoroquinolones' Toxicity to Freshwater Organisms and a Risk Assessment

Fluoroquinolones (FQs) have achieved significant success in both human and veterinary medicine. However, regulatory authorities have recommended limiting their use, firstly because they can have disabling side effects; secondly, because of the need to limit the spread of antibiotic resistance. This review addresses another concerning consequence of the excessive use of FQs: the freshwater environments contamination and the impact on non-target organisms. Here, an overview of the highest concentrations found in Europe, Asia, and the USA is provided, the sensitivity of various taxa is presented through a comparison of the lowest EC50s from about a hundred acute toxicity tests, and primary mechanisms of FQ toxicity are described. A risk assessment is conducted based on the estimation of the Predicted No Effect Concentration (PNEC). This is calculated traditionally and, in a more contemporary manner, by constructing a normalized Species Sensitivity Distribution curve. The lowest individual HC5 (6.52 µg L-1) was obtained for levofloxacin, followed by ciprofloxacin (7.51 µg L-1), sarafloxacin and clinafloxacin (12.23 µg L-1), and ofloxacin (17.12 µg L-1). By comparing the calculated PNEC with detected concentrations, it is evident that the risk cannot be denied: the potential impact of FQs on freshwater ecosystems is a further reason to minimize their use.

Effects of clarithromycin exposure on the growth of Microcystis aeruginosa and the production of algal dissolved organic matter

Antibiotics are commonly found in the aquatic environment, which can affect microbial community compositions and activities, and even have potential adverse impacts on human and ecosystem health. The current understanding of the effects of antibiotics on microalgae growth and algal dissolved organic matter (DOM) remains indistinct. To understand the toxic effects of antibiotics on the microalgae, Microcystis aeruginosa was exposed to clarithromycin (CLA) in this study. Cell density determination, chlorophyll content determination, and organic spectrum analysis were conducted to show the effect of CLA exposure on the growth, photosynthetic activity, and organic metabolic processes of Microcystis aeruginosa. The findings revealed that the physiological status of algae could be significantly influenced by CLA exposure in aquatic environments. Specifically, exposure to 1 μg/L CLA stimulated the growth and photosynthetic activity of algal cells. Conversely, CLA above 10 μg/L led to the inhibition of algal cell growth and photosynthesis. Notably, the inhibitory effects intensified with the increasing concentration of CLA. The molecular weight of DOM produced by Microcystis aeruginosa increased when exposed to CLA. Under the exposure of 60 μg/L CLA, a large number of algal cells ruptured and died, and the intracellular organic matter was released into the algal liquid. This resulted in an increase in high molecular weight substances and soluble microbial-like products in the DOM. Exposure to 1 and 10 μg/L CLA stimulated Microcystis aeruginosa to produce more humic acid-like substances, which may be a defense mechanism against CLA. The results were useful for assessing the effects of antibiotic pollution on the stability of the microalgae population and endogenous DOM characteristics in aquatic ecosystems.

Effect of surfactant's charge properties on behavior, physiology, and biochemistry and the release of microcystins of Microcystis aeruginosa

Surfactant pollution is escalatitheng in eutrophic waters, but the effect of surfactant charge properties on the physiological and biochemical properties of toxin-producing microalgae remains inadequately explored. To address this gap, this study explores the effects and mechanisms of three common surfactants-cetyltrimethylammonium bromide (CTAB, cationic), sodium dodecyl sulfate (SDS, anionic), and Triton X-100 (nonionic)-found in surface waters, on the agglomeration behavior, physiological indicators, and Microcystin-LR (MC-LR) release of Microcystis aeruginosa (M. aeruginosa) by using UV-visible spectroscope, Malvern Zetasizer, fluorescence spectrometer, etc. Results suggest that charge properties significantly affect cyanobacterial aggregation and cellular metabolism. The CTAB-treated group demonstrates a ∼5.74 and ∼9.74 times higher aggregation effect compared to Triton X-100 and SDS (300 mg/L for 180 min) due to strong electrostatic attraction. Triton X-100 outperforms CTAB and SDS in polysaccharide extraction, attributed to its higher water solubility and lower critical micelle concentration. CTAB stimulates cyanobacteria to secrete proteins, xanthohumic acid, and humic acids to maintain normal physiological cells. Additionally, the results of SEM and ion content showed that CTAB damages the cell membrane, resulting in a ∼90% increase in the release of intracellular MC-LR without cell disintegration. Ionic analyses confirm that all three surfactants alter cell membrane permeability and disrupt ionic metabolic pathways in microalgae. This study highlights the relationship between the surface charge properties of typical surfactants and the dispersion/agglomeration behavior of cyanobacteria. It provides insights into the impact mechanism of exogenous surfactants on toxic algae production in eutrophic water bodies, offering theoretical references for managing surfactant pollution and treating algae blooms.

Integrated physio-biochemical and transcriptomic analysis reveals the joint toxicity mechanisms of two typical antidepressants fluoxetine and sertraline on Microcystis aeruginosa

Selective serotonin reuptake inhibitor (SSRI) antidepressants are of increasing concern worldwide due to their ubiquitous occurrence and detrimental effects on aquatic organisms. However, little is known regarding their effects on the dominant bloom-forming cyanobacterium, Microcystis aeruginosa. Here, we investigated the individual and joint effects of two typical SSRIs fluoxetine (FLX) and sertraline (SER) on M. aeruginosa at physio-biochemical and molecular levels. Results showed that FLX and SER had strong growth inhibitory effects on M. aeruginosa with the 96-h median effect concentrations (EC50s) of 362 and 225 μg/L, respectively. Besides, the mixtures showed an additive effect on microalgal growth. Meanwhile, both individual SSRIs and their mixtures can inhibit photosynthetic pigment synthesis, cause oxidative damage, destroy cell membrane, and promote microcystin-leucine-arginine (MC-LR) synthesis and release. Moreover, the mixtures enhanced the damage to photosynthesis, antioxidant system, and cell membrane and facilitated MC-LR synthesis and release compared to individuals. Furthermore, transcriptomic analysis revealed that the dysregulation of the key genes related to transport, photosystem, protein synthesis, and non-ribosomal peptide structures was the fundamental molecular mechanism underlying the physio-biochemical responses of M. aeruginosa. These findings provide a better understanding of the toxicity mechanisms of SSRIs to microalgae and their risks to aquatic ecosystems.

Discovery of two novel bioactive algicidal substances from Brevibacillus sp. via metabolomics profiling and back-validation

Identifying potent bacterial algicidal agents is essential for the development of effective, safe, and economically viable algaecides. Challenges in isolating and purifying these substances from complex secretions have impeded progress in this field. Metabolomics profiling, an efficient strategy for identifying metabolites, was pioneered in identifying bacterial algicidal substances in this study. Extracellular secretions from different generations of the algicidal bacterium Brevibacillus sp. were isolated for comprehensive analysis. Specifically, a higher algicidal efficacy was observed in the secretion from Generation 3 (G3) of Brevibacillus sp. compared to Generation 1 (G1). Subsequent metabolomics profiling comparing G3 and 1 revealed 83 significantly up-regulated metabolites, of which 9 were identified as potential algicidal candidates. Back-validation highlighted the potency of 4-acetamidobutanoic acid (4-ABC) and 8-hydroxyquinoline (8-HQL), which exhibited robust algicidal activity with 3d-EC50 values of 6.40 mg/L and 92.90 µg/L, respectively. These substances disrupted photosynthetic activity in M. aeruginosa by ceasing electron transfer in PSⅡ, like the impact exerted by Brevibacillus sp. secretion. These findings confirmed that 4-ABC and 8-HQL were the main algicidal components derived from Brevibacillus sp.. Thus, this study presents a streamlined strategy for identifying bacterial algicidal substances and unveils two novel and highly active algicidal substances. ENVIRONMENTAL IMPLICATION: Harmful cyanobacterial blooms (HCBs) pose significant environmental problems and health effects to humans and other organisms. The increasing frequency of HCBs has emerged as a pressing global concern. Bacterial-derived algicidal substances are expected to serve as effective, safe, and economically viable algaecides against HCBs. This study presents a streamlined strategy for identifying bacterial algicidal substances and unveils two novel substances (4-ABC and 8-HQL). These two substances demonstrate remarkable algicidal activity and disrupt the photosynthetic system in M. aeruginosa. They hold potential as prospective algaecides for addressing HCBs.

A modified cultivation strategy to enhance biomass production and lipid accumulation of Tetradesmus obliquus FACHB-14 with copper stress and light quality induction

In this study, a two-stage culture strategy was refined to concurrently enhance the growth and lipid accumulation of Tetradesmus obliquus. The results unveiled that, during the initial stage, the optimal conditions for biomass accumulation were achieved with 0.02 mg·L-1 Cu2+ concentration and red light. Under these conditions, biomass accumulation reached 0.628 g·L-1, marking a substantial 23.62 % increase compared to the control group. In the second stage, the optimal conditions for lipid accumulation were identified as 0.5 mg·L-1 Cu2+ concentration and red light, achieving 64.25 mg·g-1·d-1 and marking a 128.38 % increase over the control. Furthermore, the fatty acid analysis results revealed an 18.85 % increase in the saturated fatty acid content, indicating enhanced combustion performance of microalgae cultivated under the dual stress of red light and 0.5 mg·L-1 Cu2+. This study offers insights into the potential application of Tetradesmus obliquus in biofuel production.

Occurrence, distribution, and risk assessment of antibiotics in a typical aquaculture area around the Dongzhai Harbor mangrove forest on Hainan Island

The trees of the Dongzhai Harbor mangrove forest suffer from antibiotic contamination from surrounding aquaculture areas. Despite this being one of the largest mangrove forests in China, few studies have focused on the antibiotic pollution status in these aquaculture areas. In the present study, the occurrence, distribution, and risk assessment of 37 antibiotics in surface water and sediment samples from aquaculture areas around Dongzhai Harbor mangrove forests were analyzed. The concentration of total antibiotics (∑antibiotics) ranged from 78.4 ng/L to 225.6 ng/L in surface water (except S14-A2) and from 19.5 ng/g dry weight (dw) to 229 ng/g dw in sediment. In the sediment, the concentrations of ∑antibiotics were relatively low (19.5-52.3 ng/g dw) at 75 % of the sampling sites, while they were high (95.7-229.0 ng/g dw) at a few sampling sites (S13-A1, S13D, S8D). The correlation analysis results showed that the Kd values of the 9 antibiotics were significantly positively correlated with molecular weight (MW), Kow, and LogKow. Risk assessment revealed that sulfamethoxazole (SMX) in surface water and SMX, enoxacin (ENX), ciprofloxacin (CFX), enrofloxacin (EFX), ofloxacin (OFX), and norfloxacin (NFX) in sediment had medium/high risk quotients (RQs) at 62.5 % and 25-100 %, respectively, of the sampling sites. The antibiotic mixture in surface water (0.06-3.36) and sediment (0.43-309) posed a high risk at 37.5 % and 66.7 %, respectively, of the sampling sites. SMX was selected as an indicator of antibiotic pollution in surface water to assist regulatory authorities in monitoring and managing antibiotic pollution in the aquaculture zone of Dongzhai Harbor. Overall, the results of the present study provide a comprehensive and detailed analysis of the characteristics of antibiotics in the aquaculture environment around the Dongzhai Harbor mangrove system and provide a theoretical basis for the source control of antibiotics in mangrove systems.

Physiological responses and removal mechanisms of ciprofloxacin in freshwater microalgae

Antibiotics, such as ciprofloxacin (CIP), are frequently detected in various environmental compartments, posing significant risks to ecosystems and human health. In this study, the physiological responses and elimination mechanisms of CIP in Chlorella sorokiniana and Scenedesmus dimorphus were determined. The exposure CIP had a minimal impact on the growth of microalgae, with maximum inhibit efficiency (IR) of 5.14% and 22.74 for C. sorokiniana and S. dimorphus, respectively. Notably, the photorespiration in S. dimorphus were enhanced. Both microalgae exhibited efficient CIP removal, predominantly through bioaccumulation and biodegradation processes. Intermediates involved in photolysis and biodegradation were analyzed through Liquid Chromatography High Resolution Mass Spectrometer (HPLC-MS/MS), providing insights into degradation pathways of CIP. Upregulation of key enzymes, such as dioxygenase, oxygenase and cytochrome P450, indicated their involvement in the biodegradation of CIP. These findings enhance our understanding of the physiological responses, removal mechanisms, and pathways of CIP in microalgae, facilitating the advancement of microalgae-based wastewater treatment approaches, particularly in antibiotic-contaminated environments.

Assessment of sulfamethoxazole removal by three wetland plant species under hydroponic conditions: uptake, accumulation, and physiological responses

Plants play a crucial role as a removal pathway in constructed wetlands, demonstrating the ability to absorb and tolerate antibiotics from wastewater. However, the specific contribution of plants in this regard has not yet to be sufficiently established. To gain a more comprehensive insight into the associated processes, we selected three common wetland plant species, Canna indica L. (C. indica), Cyperus alternifolius L. (C. alternifolius), and Thalia dealbata Fraser (T. dealbata), to evaluate their capacity for uptake, accumulation, and physiological response in the removal of sulfamethoxazole (SMX) at varying initial concentrations (10, 30, 100, and 300 µg/L) under hydroponic conditions. The results showed that SMX removal was more efficient at lower concentrations (10 and 30 µg/L) than at higher concentrations (100 and 300 µg/L). Moreover, plant systems were found to consistently outperform unplanted systems in SMX removal. Among the assessed species, C. indica was identified as being relatively effective in the removal of SMX, whereas the performance of C. alternifolius was notably less pronounced. A positive correlation was observed between the concentration of SMX in the plant tissues and that in the external aqueous medium. However, plant tissue residues contributed only a minor fraction to the overall removal of SMX. Wetland plants absorb SMX through their roots, and we accordingly detected significantly higher concentrations in submerged plant tissues. Furthermore, we also detected reductions in net photosynthetic rates indicative of potential phytotoxicity, which is associated with the accumulation of antibiotic in the shoot tissues. Accumulation of SMX in the roots and rhizomes was also found to be associated with the development of shorter roots, with this effect becoming more pronounced with an increase in the concentration of exogenous SMX. However, despite these adverse effects, plants can detoxify antibiotics via the glutathione pathway. Of the assessed plant species, C. indica was identified as the most SMX tolerant, as indicated by Km and Vmax values, with C. alternifolius being the least tolerant. Our findings in this study reveal the potential value of wetland plants in the sequestration of antibiotics and provide evidence for the underlying mechanisms of action. These findings could make an important contribution to the implementation of phytoremediation in antibiotic-contaminated water.

Repeated Exposure Enhanced Toxicity of Clarithromycin on Microcystis aeruginosa Versus Single Exposure through Photosynthesis, Oxidative Stress, and Energy Metabolism Shift

Antibiotics are being increasingly detected in aquatic environments, and their potential ecological risk is of great concern. However, most antibiotic toxicity studies involve single-exposure experiments. Herein, we studied the effects and mechanisms of repeated versus single clarithromycin (CLA) exposure on Microcystis aeruginosa. The 96 h effective concentration of CLA was 13.37 μg/L upon single exposure but it reduced to 6.90 μg/L upon repeated exposure. Single-exposure CLA inhibited algal photosynthesis by disrupting energy absorption, dissipation and trapping, reaction center activation, and electron transport, thereby inducing oxidative stress and ultrastructural damage. In addition, CLA upregulated glycolysis, pyruvate metabolism, and the tricarboxylic acid cycle. Repeated exposure caused stronger inhibition of algal growth via altering photosynthetic pigments, reaction center subunits biosynthesis, and electron transport, thereby inducing more substantial oxidative damage. Furthermore, repeated exposure reduced carbohydrate utilization by blocking the pentose phosphate pathway, consequently altering the characteristics of extracellular polymeric substances and eventually impairing the defense mechanisms of M. aeruginosa. Risk quotients calculated from repeated exposure were higher than 1, indicating significant ecological risks. This study elucidated the strong influence of repeated antibiotic exposure on algae, providing new insight into antibiotic risk assessment.

Investigation of the antimicrobial susceptibility patterns of marine cyanobacteria in Bohai Bay: Cyanobacteria may be important hosts of antibiotic resistance genes in marine environment

Marine cyanobacteria, as widely distributed and photosynthetically autotrophic bacteria in the ocean, may contribute to the global dissemination of antibiotic resistance genes (ARGs) and develop a different antimicrobial susceptibility pattern from heterotrophic bacteria and cyanobacteria from freshwater environments. However, studies on antimicrobial susceptibility and the carriage of ARGs in marine cyanobacteria are still very limited. In this study, the antibiotic resistance characteristics of cyanobacteria in nearshore waters were examined through field monitoring and laboratory investigations, which included PCR detection and ARG transformation. The results showed a positive correlation between marine cyanobacteria and some ARGs in the nearshore waters of Bohai Bay. Moreover, most screened cyanobacteria showed high minimum inhibitory concentration (MIC) values for polymyxins, tetracyclines, kanamycin, and sulfonamides, moderate MIC values for streptomycin, chloramphenicol, rifampicin, and norfloxacin, and low MIC values for roxithromycin and cephalosporins. The blaTEM, blaKPC, sul1, sul2, strA, tetA, tetB, tetC, tetM, mdfA, and intI1 genes were detected in the screened marine cyanobacteria. The highest detection rates were observed for blaTEM (93.3 %), sul1 (56.6 %), sul2 (90 %), and strA (73.3 %). The detection rate of tetA (33.3 %) was the highest among the tetracycline resistance genes, and mdfA, a multidrug-resistant pump gene with resistance to tetracycline, also showed a high detection level (23.3 %). Overall, most of the screened marine cyanobacteria were found to tolerate multiple antibiotics in seawater, and the condition of the ARGs carriage was serious. Furthermore, the screened marine Synechocystis sp. C12-2 demonstrated the ability to accept ARGs on the RP4 plasmid through natural transformation and showed reduced sensitivity to ampicillin, suggesting the possibility that some marine cyanobacteria could acquire ARGs from the environment through horizontal gene transfer. Thus, marine cyanobacteria may play an important role in the propagation of marine ARGs.

Comprehensive study on the potential environmental risk of temporal antibiotic usage through wastewater discharges

Antibiotic residues can reach aquatic ecosystems through urban wastewater discharges, posing an ecotoxicological risk for aquatic organisms and favoring the development of bacterial resistance. To assess the emission rate and hazardousness of these compounds, it is important to carry out periodic chemical monitoring campaigns that provide information regarding the actual performance of wastewater treatment plants (WWTPs) and the potential impact of the treated wastewater in the aquatic environment. In this study, 18 of the most widely consumed antibiotics in Spain were determined by liquid chromatography-tandem mass spectrometry in both influent (IWW) and effluent wastewater (EWW) samples collected over four seasons along 2021-2022. Eleven antibiotics were detected in EWW with azithromycin, ciprofloxacin and levofloxacin showing the highest concentration levels (around 2 μg L-1 of azithromycin and 0.4 μg L-1 of quinolone compounds). Data showed that only 4 out of the 11 compounds were removed by more than 50 % in the WWTP, with sulfamethoxazole standing out with an average removal efficiency >80 %. The risk that treated water could pose to the aquatic environment was also assessed, with 6 compounds indicating a potential environmental risk by exceeding established ecotoxicological and resistance thresholds. Based on the risk assessment, the WWTP removal efficiency required to reduce such risk for antibiotics was estimated. In addition, pooled wastewater samples were screened by LC coupled to high resolution mass spectrometry with ion mobility separation, searching for metabolites and transformation products of the antibiotics investigated to widen future research. Studies like this are crucial to map the impact of antibiotic pollution and to provide the basis for designing water quality and risk prevention monitoring programs.

Biochemical responses of freshwater microalgae Chlorella sorokiniana to combined exposure of Zn(Ⅱ) and estrone with simultaneous pollutants removal

With the development of livestock industry, contaminants such as divalent zinc ions (Zn (Ⅱ)) and estrone are often simultaneously detected in livestock wastewater. Nevertheless, the combined toxicity of these two pollutants on microalgae is still unclear. Moreover, microalgae have the potential for biosorption and bioaccumulation of heavy metals and organic compounds. Thus, this study investigated the joint effects of Zn (Ⅱ) and estrone on microalgae Chlorella sorokiniana, in terms of growth, photosynthetic activity and biomolecules, as well as pollutants removal by algae. Interestingly, a low Zn (Ⅱ) concentration promoted C. sorokiniana growth and photosynthetic activity, while the high concentration experienced inhibition. As the increase of estrone concentration, chlorophyll a content increased continuously to resist the environmental stress. Concurrently, the secretion of extracellular polysaccharides and proteins by algae increased with exposure to Zn (Ⅱ) and estrone, reducing toxicity of pollutants to microalgae. Reactive oxygen species and superoxide dismutase activity increased as the increase of pollutant concentration after 96 h cultivation, but high pollutant concentrations resulted in damage of cells, as proved by increased MDA content. Additionally, C. sorokiniana displayed remarkable removal efficiency for Zn (Ⅱ) and estrone, reaching up to 86.14% and 84.96% respectively. The study provides insights into the biochemical responses of microalgae to pollutants and highlights the potential of microalgae in pollutants removal.

Unraveling the combined toxicity and removal mechanisms of fluoxetine and sertraline co-contaminants by the freshwater microalga Chlorella pyrenoidosa

Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine (FLX) and sertraline (SER), are among the most widely detected pharmaceuticals in aquatic environments, and they usually occur as mixtures. However, little is known about the combined toxicity of SSRI mixtures to microalgae and the associated removal mechanisms. This study investigated the combined toxicity of FLX and SER to the growth, photosynthetic activity, and antioxidant system of Chlorella pyrenoidosa and their removal mechanisms. The results showed that FLX and SER strongly inhibited microalgal growth with 96 h EC50 values of 493 and 61.1 μg/L, respectively. Additionally, the combined toxicity of FLX and SER towards microalgal growth exhibited an additive effect. After 4 days of short-term exposure, FLX, SER, and their mixtures caused photosynthetic damage and oxidative stress in microalgae, and the mixture's toxicity was stronger than those of individuals. However, the adverse effects on microalgal growth, photosynthetic activity, and antioxidant system were alleviated with increasing exposure time. Meanwhile, C. pyrenoidosa efficiently removed FLX (67.59%-99.08%) and SER (94.92%-99.11%) individually after 11 days of cultivation. Biodegradation (59.25%-86.21%) was the prominent removal mechanism of FLX, while both biodegradation (48.08%-88.17%) and bioaccumulation (4.74%-43.38%) contributed significantly to SER removal. The co-existence of FLX and SER lowered the removal rate and biodegradation amount of both compounds. Besides, SER inhibited C. pyrenoidosa's N-demethylation and O-dealkylation of FLX, while co-existing with FLX inhibited the excretion of the N-deamination product of SER from microalgal cells. Furthermore, the principal component analysis indicated that the removal performance of FLX, SER, and their mixtures correlated strongly to the microalgae's physiological and biochemical states. These results highlighted the significance of co-contamination during ecological risk assessments and microalgae-based bioremediation of SSRIs.

Occurrence, Bioaccumulation, Metabolism and Ecotoxicity of Fluoroquinolones in the Aquatic Environment: A Review

In recent years, there has been growing concern about antibiotic contamination in water bodies, particularly the widespread presence of fluoroquinolones (FQs), which pose a serious threat to ecosystems due to their extensive use and the phenomenon of "pseudo-persistence". This article provides a comprehensive review of the literature on FQs in water bodies, summarizing and analyzing contamination levels of FQs in global surface water over the past three years, as well as the bioaccumulation and metabolism patterns of FQs in aquatic organisms, their ecological toxicity, and the influencing factors. The results show that FQs contamination is widespread in surface water across the surveyed 32 countries, with ciprofloxacin and norfloxacin being the most heavy contaminants. Furthermore, contamination levels are generally higher in developing and developed countries. It has been observed that compound types, species, and environmental factors influence the bioaccumulation, metabolism, and toxicity of FQs in aquatic organisms. FQs tend to accumulate more in organisms with higher lipid content, and toxicity experiments have shown that FQs exhibit the highest toxicity to bacteria and the weakest toxicity to mollusk. This article summarizes and analyzes the current research status and shortcomings of FQs, providing guidance and theoretical support for future research directions.

Differential toxicity of various mineral nanoparticles to Synechocystis sp.: With and without ciprofloxacin

Mineral nanoparticles (M-NPs) are ubiquitous in aquatic environments, but their potential harms to primary producers and impacts on the toxicity of coexisting pollutants are largely unknown. Herein, the toxicity mechanisms of various M-NPs (i.e., SiO2, Fe2O3, Al2O3, and TiO2 NPs) to Synechocystis sp. in absence and presence of ciprofloxacin (CIP) were comprehensively investigated. The heteroaggregation of cells and M-NPs can hinder substrate transfer or light acquisition. The attraction between Synechocystis sp. and M-NPs increased in the order of SiO2 < Fe2O3 < Al2O3 ≈ TiO2 NPs. Therefore, SiO2 and Fe2O3 NPs exerted slight effects on physiology and proteome of Synechocystis sp.. Al2O3 NPs with the rod-like shape caused physical damage to cells. Differently, TiO2 NPs with photocatalytic activities provided photogenerated electrons for Synechocystis sp., promoting photosynthesis and the Calvin cycle for CO2 fixation. SiO2, Fe2O3, and Al2O3 NPs alleviated the toxicity of CIP in an adsorption-depended manner. Conversely, the combination of CIP and TiO2 NPs exerted more pronounced toxic effects compared to their individuals, and CIP disturbed the extracellular electron transfer from TiO2 NPs to cells. The findings highlight the different effects of TiO2 NPs from other M-NPs on cyanobacteria, either alone or in combination with CIP, and improve the understanding of toxic mechanisms of M-NPs.

Occurrence, distribution, and risk assessment of quinolone antibiotics in municipal sewage sludges throughout China

A nationwide study of the occurrence, distribution, potential drivers, and ecological risks of 24 quinolone antibiotics (QNs) in 74 Chinese sludge samples from 48 wastewater treatment plants (WWTPs) was conducted. In domestic sludge, the ∑QNs concentrations were <LOD to 21,925.10 ug/kg (mean: 4808.67 ug/kg), and ofloxacin had the highest concentration (<LOD-11,138.52 ug/kg), and detection frequency (98.48%). Of four generations QNs, the levels showed the following order: 2nd-generation QNs > 3rd-generation QNs > 4th-generation QNs > 1st-generation QNs. Meanwhile, abundant veterinary and human/veterinary quinolones (<LOD-2606.96 and <LOD-12,643.47 ug/kg, respectively) were detected in municipal sludge. Interestingly, the relatively low levels of veterinary quinolones (<LOD-299.21 ug/kg) were also found in industrial sludge (the relevant WWTPs receiving ≤ 10% domestic wastewater, without other direct entry of antibiotics). The correlation analysis demonstrated QNs contamination was negatively influenced by the air temperature of sampling days. The positive correlation between moxifloxacin contents and regional economy possibly suggested local regions with relatively high economic levels face a more difficult situation of QNs antibacterial activity. Environmental risk assessment indicated ofloxacin, ciprofloxacin, and moxifloxacin posed high ecological risks to the domestic sludge. This work delineates a valuable nationwide QNs contamination profile to support their safe use and control in China.

Elucidating microalgae-mediated metabolism for sulfadiazine removal mechanism and transformation pathways

Sulfadiazine (SDZ) as a typical sulfonamide antibiotic is commonly detected in wastewater, and its removal mechanism and transformation pathways in microalgae-mediated system remain unclear. In this study, the SDZ removal through hydrolysis, photodegradation, and biodegradation by Chlorella pyrenoidosa was investigated. Higher superoxide dismutase activity and biochemical components accumulation were obtained under SDZ stress. The SDZ removal efficiencies at different initial concentrations were 65.9-67.6%, and the removal rate followed pseudo first-order kinetic model. Batch tests and HPLC-MS/MS analyses suggested that biodegradation and photodegradation through the reactions of amine group oxidation, ring opening, hydroxylation, and the cleavage of S-N, C-N, C-S bond were dominant removal mechanisms and pathways. Characteristics of transformation products were evaluated to analyze their environmental impacts. High-value products of lipid, carbohydrate, and protein in microalgae biomass presented economic potential of microalgae-mediated metabolism for SDZ removal. The findings of this study broadened the knowledge for the microalgae self-protection from SDZ stress and provided a deep insight into SDZ removal mechanism and transformation pathways.

Evaluation of Cd2+ stress on Synechocystis sp. PCC6803 based on single-cell elemental accumulation and algal toxicological response

With the development of single cell analysis techniques, the concept of precision toxicology has been proposed in recent years. Due to the heterogeneity of cells, we need to perform toxicological assessments on individual cells. Microalgae, one kind of important primary producers, play as a major pathway by which heavy metals enter the food chain and thus accumulate/transfer to higher trophic levels. Herein, the biosorption of Cd (Ex-Cd) and bioaccumulation of Cd (In-Cd) for Synechocystis sp. PCC 6803 were investigated by online 3D droplet microfluidic device combined with inductively coupled plasma mass spectrometry detection. Meanwhile, the algal toxicological responses of the algae cell to Cd²⁺ exposure under different concentration (50, 100, and 150 μg L ^- 1) and time (15 min, 24, 48 and 96 h) were studied. Combining single-cell analysis with toxicological indicators, the toxicity mechanism of Cd²⁺to algal was discussed. The single cell analysis results revealed heterogeneity in cellular uptake of Cd²⁺. The proportion of Cd-containing cells and Cd content in single algal cells all reached the maximum at 24 h. The uptake of Cd²⁺ occurred within 15 min under all tested exposure concentrations and a large part of Cd²⁺ were adsorbed on the algal cells surface. The Pearson correlation analysis showed that cell density, chlorophyll a and carotenoids were significantly negatively correlated with Cd accumulation, whereas ROS level and SOD activity were significantly positively correlated with Cd accumulation. It suggested that Cd²⁺accumulated intracellular would show toxic effects on the algal cells and oxidative stress is the main mechanism of Cd toxicity to algal cells. This work promotes our understanding of the toxicological responses of microalgae under Cd stress at single cells level.

Natural nanocolloids regulate the fate and phytotoxicity of hematite particles in water

Hematite (the most abundant iron oxide polymorph) is widely detected in the water environment and has attracted considerable attention. Natural nanocolloids (Ncs) exist ubiquitously in surface waters and play critical roles in biogeochemical processes. However, the influences of Ncs on the fate and phytotoxicity of hematite remain unknown. In this study, the infrared absorption spectra coupled with two-dimensional correlation spectroscopy analysis reveal that the specific binding interactions between Ncs and hematite primarily occur via hydrophilic effects and π-π interactions with an increase in the Ncs contact time. Moreover, binding with Ncs slightly promoted the aggregation rates of hematite particles in the BG-11 medium. Interestingly, Ncs remarkably mitigate the phytotoxicity (e.g., growth inhibition, oxidative stress, and mitochondrial toxicity) of nanosized and submicrosized hematite particles to Chlorella vulgaris after a 96 h exposure. The integrating metabolomic and transcriptomic analysis reveals that the regulated urea cycle, amino acids, and fatty acid-related metabolites (e.g., urea, serine, glutamate, and hexadecenoic acid) and genes (e.g., ACY1, CysC, and GLA) contribute to persistent phytotoxicity. This study provides new insights into the roles and mechanisms of natural Ncs in regulating the environmental risk of iron oxide minerals in aqueous media.

Antimicrobial and the Resistances in the Environment: Ecological and Health Risks, Influencing Factors, and Mitigation Strategies

Antimicrobial contamination and antimicrobial resistance have become global environmental and health problems. A large number of antimicrobials are used in medical and animal husbandry, leading to the continuous release of residual antimicrobials into the environment. It not only causes ecological harm, but also promotes the occurrence and spread of antimicrobial resistance. The role of environmental factors in antimicrobial contamination and the spread of antimicrobial resistance is often overlooked. There are a large number of antimicrobial-resistant bacteria and antimicrobial resistance genes in human beings, which increases the likelihood that pathogenic bacteria acquire resistance, and also adds opportunities for human contact with antimicrobial-resistant pathogens. In this paper, we review the fate of antimicrobials and antimicrobial resistance in the environment, including the occurrence, spread, and impact on ecological and human health. More importantly, this review emphasizes a number of environmental factors that can exacerbate antimicrobial contamination and the spread of antimicrobial resistance. In the future, the timely removal of antimicrobials and antimicrobial resistance genes in the environment will be more effective in alleviating antimicrobial contamination and antimicrobial resistance.

Transcriptome analysis of changes in M. aeruginosa growth and microcystin production under low concentrations of ethinyl estradiol

Ethinyl estradiol (EE2) is a synthetic environmental estrogen with considerable estrogenic activity. It has been found to consequently pose a significant threat to the aquatic ecosystem. Harmful algal blooms are a major aquatic ecological issue. However, the relationship between EE2 and cyanobacterial bloom is mainly unknown. In this study, the physiological and molecular responses of Microcystis aeruginosa to EE2 exposure were investigated. A low level of EE2 (0.02 μg/L) significantly enhanced the growth of algal cells (P < 0.05), whereas higher concentrations of EE2 (0.2-200 μg/L) inhibited it. EE2 at doses ranging from 0.02 to 200 μg/L promoted the production of microcystins (MCs), with genes mcyABD playing a key role in the regulation of MC synthesis. The alterations of chlorophyll-a, carotenoid, and phycocyanin contents caused by EE2 showed the same trend as cell growth. At the molecular level, 200 μg/L EE2 significantly down-regulated genes in photosynthetic pigment synthesis, light harvesting, electron transfer, NADPH, and ATP generation. High concentrations of EE2 caused oxidative damage to algal cells on the 4th d. After 12d exposure, although there was no significant change in superoxide dismutase (SOD) content and no damage observed in membrane lipids, genes related to SOD and glutathione were changed. In addition, due to the down-regulation of pckA, PK, gltA, nrtA, pstS, etc., carbon fixation, glycolysis, TCA cycle, nitrogen and phosphorus metabolism were hindered by EE2 (200 μg/L). Gene fabG in fatty acid biosynthesis was significantly up-regulated, promoting energy storage in cells. These findings provide important clues to elucidate the effects and mechanisms of cyanobacterial blooms triggered by EE2 and help to effectively prevent and control cyanobacterial blooms.

Metabolomics and proteomics reveal the toxicological mechanisms of florfenicol stress on wheat (Triticum aestivum L.) seedlings

Although the ecological impacts of antibiotics have received attention worldwide, research on the toxicity of florfenicol is still limited. We conducted a metabolomic and proteomic study on wheat (Triticum aestivum L.) seedlings to reveal the toxicological mechanism of florfenicol. The growth of the wheat seedlings was found to be inhibited by florfenicol. Antioxidant enzyme activities (superoxide dismutase, peroxidase and catalase), malondialdehyde content and membrane permeability increased with increasing florfenicol concentration. The contents of chlorophyll and chlorophyll synthesis precursor substances (Proto IX, Mg-proto IX and Pchlide), photosynthetic and respiration rates, and chlorophyll fluorescence parameters decreased, indicating that photosynthesis was inhibited. The ultrastructure of chloroplasts was destroyed, as evidenced by the blurred membrane surface, irregular grana arrangement, irregular thylakoid lamella structure, and increased plastoglobuli number. Proteome analysis revealed that up-regulated proteins were highly involved in protein refolding, translation, oxidation-reduction, tricarboxylic acid cycle (TCA cycle), reactive oxygen species metabolic process, cellular oxidant detoxification, and response to oxidative stress. The down-regulated proteins were mainly enriched in photosynthesis-related pathways. In the metabolome analysis, the content of most of the metabolites in wheat leaves, such as carbohydrates and amino acids increased significantly (p < 0.05). Combined pathway analysis showed that florfenicol stress stimulated the TCA cycle pathway and downregulated the photosynthesis pathway.

Evaluation of growth and antioxidant responses of freshwater microalgae Chlorella sorokiniana and Scenedesmus dimorphus under exposure of moxifloxacin

As one of the fourth-generation fluoroquinolone antibiotics, moxifloxacin (MOX) has been frequently released to the aquatic environment, threatening local organisms. However, researches on its ecotoxicity to aquatic organisms are still limited. This study analyzed effects of MOX on the growth, photosynthesis and oxidative stress of two common types of freshwater microalgae, Chlorella sorokiniana and Scenedesmus dimorphus. The 96 h-EC₅₀ values of MOX for C. sorokiniana and S. dimorphus were 28.42 and 26.37 mg/L, respectively. Although variations of specific indicators for photosynthetic fluorescence intensity were different, photosystems of two types of microalgae were irreversibly damaged. The malondialdehyde content and superoxide dismutase of C. sorokiniana and S. dimorphus evidently increased, indicating that the exposure of MOX caused serious oxidative stress. Chlorophyll a, b and carotenoids contents of C. sorokiniana increased, probably resulting from the resistance to oxidative stress, whereas they were inhibited due to oxidation damage as for S. dimorphus. Risk quotients (RQs) of MOX for C. sorokiniana and S. dimorphus in wastewater were 7.882 and 8.495, respectively, which demonstrated that MOX had a considerable risk to aquatic environment, especially in the context of its increasing use in practice.

Antibiotic Pollution of Planktonic Ecosystems: A Review Focused on Community Analysis and the Causal Chain Linking Individual- and Community-Level Responses

Antibiotic pollution has become one of the most challenging environmental issues in aquatic ecosystems, with adverse effects on planktonic organisms that occupy the base of the aquatic food chain. However, research regarding this topic has not been systematically reviewed, especially in terms of community-level responses. In this review, we provide an overview of current antibiotic pollution in aquatic environments worldwide. Then, we summarize recent studies concerning the responses of planktonic communities to antibiotics, ranging from individual- to community-level responses. Studies have shown that extremely high concentrations of antibiotics can directly harm the growth and survival of plankton; however, such concentrations are rarely found in natural freshwater. It is more likely that environmentally relevant concentrations of antibiotics will affect the physiological, morphological, and behavioral characteristics of planktonic organisms; influence interspecific interactions among plankton species via asymmetrical responses in species traits; and thus alter the structure and function of the entire planktonic ecosystem. This review highlights the importance of community analysis in revealing antibiotic toxicity. We also encourage the establishment of the causal relationships between impacts at multiple scales in the future for predicting the community-level consequences of antibiotics based on the currently available individual-level evidence.

Black carbon and humic acid alleviate the toxicity of antibiotics to a cyanobacterium Synechocystis sp

Natural organic matters (NOMs), omnipresent in natural water, challenge the toxicity assessment of pollutants to aquatic organisms due to their complex interactions with chemicals and organisms. Here, we investigated the combined toxicity of one solid NOM (black carbon, BC) or one soluble NOM (humic acid, HA) with antibiotics, roxithromycin (RTM) or gatifloxacin (GAT), to the cyanobacterium Synechocystis sp.. The NOMs alleviated the toxicity of RTM and GAT to Synechocystis sp., and BC had greater alleviation effects than HA due to its stronger adsorption to antibiotics. Antibiotics disturbed the photosynthesis of Synechocystis sp. significantly, which were also mitigated by BC and HA. Proteomic analysis showed that BC up-regulated the pathway of ribosome and photosynthetic antenna protein. GAT down-regulated the pathways of ABC transporter and oxidative phosphorylation. RTM interfered the pathway of porphyrin and chlorophyll metabolism. Furthermore, the addition of BC reduced the number of differentially expressed proteins caused by antibiotics, corroborating its mitigation effects on the toxicity of antibiotics. The disturbance of HA on the pathway of ABC transporters inhibited the internalization of RTM, thus decreasing its toxicity. This study underscores the significance of NOMs in mediating the toxicity of organic pollutants to aquatic organisms in natural waters.

Toxic responses of freshwater microalgae Chlorella sorokiniana due to exposure of flame retardants

Flame retardants, such as Tetrabromobisphenol A (TBBPA), Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and tributyl phosphate (TBP), are frequently detected in surface water. However, the effects of FRs exposure on aquatic organisms especially freshwater microalgae are still unclear. In this study, the toxicities of TBBPA, TDCPP and TBP to microalgae Chlorella sorokiniana, in terms of growth inhibition, photosynthetic activity inhibition and oxidative damage, were investigated, and according ecological risks were assessed. The results showed that TBBPA, TDCPP and TBP had inhibitory effects on C. sorokiniana, with 96 h EC₅₀ (concentration for 50% of maximal effect) values of 7.606, 41.794 and 49.996 mg/L, respectively. Fv/Fm decreased as the increase of exposure time under 15 mg/L TBBPA. Under 50 mg/L TDCPP and 80 mg/L TBP exposure, Fv/Fm decreased significantly after 24 h. However, Fv/Fm rose after 96 h, indicating that the damaged photosynthetic activity was reversible. The content of chlorophyll a decreased, as the increase of TBBPA concentration from 3 to 15 mg/L. However, chlorophyll a increased first and then decreased, as the increase of TDCPP and TBP concentrations from 0 to 50 mg/L and 0-80 mg/L, respectively. Results indicated that C. sorokiniana could use the phosphorus of TDCPP and TBP to ensure the production of chlorophyll a. The risen content of reactive oxygen species, malondialdehyde as well as superoxide dismutase activity indicated that exposure to FRs induced oxidative stress. Additionally, the risk quotients showed that tested FRs had ecological risks in natural waters or wastewaters. This study provides insights into the toxicological mechanisms of different FRs toward freshwater microalgae for better understanding of according environmental risks.

Temporal dynamics of microcystins in two reservoirs with different trophic status during the early growth stage of cyanobacteria

Harmful cyanobacterial blooms are increasing in frequency and severity, which makes their toxic secondary metabolites of microcystins (MCs) have been widely studied, especially in their distribution and influence factors in different habitats. However, the distribution of MCs on the early growth stage of harmful cyanobacteria and its influence factors and risks are still largely unknown. Thus, in the present study, two reservoirs (Lutian Reservoir and Lake Haitang) with different trophic status in China have been studied weekly from March to May in 2018, when the cyanobacteria communities were just in the early growth stage, to investigate the variation of MCs concentration and the relationships between MCs and environmental parameters. During the investigation, Lutian Reservoir and Lake Haitang were found to be mesotrophic and light eutrophic, respectively. In Lutian Reservoir, the concentration of EMCs (extracellular MCs) was obviously higher than that of IMCs (intracellular MCs) with a mean value of 0.323 and 0.264 μg/L, respectively. Meanwhile, the concentration of EMCs also fluctuated more sharply than that of IMCs. Congeners of IMC-YR and EMC-LR were respectively dominant in total concentrations of IMCs and EMCs. Unsurprisingly, in Lake Haitang, the concentrations of IMC and EMC were both significantly higher than that in Lutian Reservoir with a mean concentration of 0.482 and 0.472 μg/L, respectively. Differently, the concentration of MC-YR was dominant in both IMCs and EMCs, followed by MC-LR. In correlation analysis, the IMCs were significantly and positively correlated with the density and biomass of phytoplankton phyla and potential MCs-producing cyanobacteria and the parameters of water temperature (WT), nutrients, and organic matters. Similar results were also observed for EMCs. While the different variations of MCs in the two reservoirs might be primarily caused by the differences in WT, nutrients (especially phosphorus), organic matters, and the composition of MCs-producing cyanobacteria. In addition, the coexistence of the dominant species of Pseudoanabaena sp., which can produce a taste-and-odor compound of 2-methylisoborneol (2-MIB), might have a significant impact on the concentration and toxicity of MCs. Our results suggested that the risks posed by MCs at the early growth stage of cyanobacteria should also deserve our attention, especially in mesotrophic water bodies.

Fabrication of g-C3N4@Bi2MoO6@AgI floating sponge for photocatalytic inactivation of Microcystis aeruginosa under visible light

In this work, a floating photocatalyst was constructed by loading g-C₃N₄@Bi₂MoO₆@AgI (GBA) nanocomposite on a modified polyurethane sponge via a simple dip-coating method and applied for the inactivation of Microcystis aeruginosa under visible light. GBA ternary photocatalyst was fabricated successfully and the morphology, structure, chemical state, and optical properties were characterized systematically. The floating catalyst achieved near 100% removal efficiency of algae cells under 6 h visible light irradiation and also could be retrieved and used at least three times repeatedly. The influences of various conditions on photocatalytic performance such as loading content of nanoparticles, algae density, and concentration of natural organic matters were also studied, which revealed that the GBA floating catalyst exhibited excellent photocatalytic performance of algae removal under different conditions. Furthermore, the physiological characteristics of algae cells during the photocatalytic process, including cell morphology, membrane permeability, Zeta potential, photosynthetic system, antioxidant system, and the metabolic activity were investigated. Results confirmed that the algae cells were severely damaged during the photocatalytic inactivation and the normal physiological functions were significantly affected, which resulted in the death of algae cells at last. Finally, a possible photocatalytic inactivation mechanism of algae cells was proposed. In summary, GBA floating catalyst can effectively inactivate Microcystis aeruginosa under visible light, which confirmed the high efficiency of the novel photocatalytic algae removal technology. Meanwhile, the recyclable floating material also makes the practical application in eutrophic waters of the algae removal technology possible.

Mixotrophic cultivation of Chlorella pyrenoidosa under sulfadiazine stress: High-value product recovery and toxicity tolerance evaluation

Sulfadiazine (SDZ) as a common sulfonamide antibiotic is frequently detected in wastewater, but there is little information on the high-value product recovery and toxicity tolerance evaluation of mixotrophic microalgae under SDZ stress. In this study, effects of SDZ on growth, photosynthesis, cellular damage, antioxidant capacity and intracellular biochemical components of Chlorella pyrenoidosa were investigated. Results showed that the growth of C. pyrenoidosa was inhibited by about 20% under high SDZ stress, but there was little impact on photosynthesis. Cellular damage and antioxidant capacity were evaluated using malondialdehyde (MDA) content and superoxide dismutase (SOD) activity to further explain the toxicity tolerance of mixotrophic microalgae. The SDZ stress not only increased lipid and carbohydrate content, respectively attaining to the maximum of 390.0 and 65.4 mg/L, but also improved the biodiesel quality of C. pyrenoidosa. The findings show the potential of mixotrophic microalgae for biodiesel production and wastewater treatment.

Ecotoxicological effects of the antidepressant fluoxetine and its removal by the typical freshwater microalgae Chlorella pyrenoidosa

The antidepressant fluoxetine (FLX) has gained increasing attention due to its frequent detection in aquatic environments and negative effects on non-target organisms. However, knowledge on the ecotoxicological effects of FLX and its removal by microalgae is still limited. In this study, the ecotoxicological effects of FLX (10 -1000 μg/L) were assessed using batch cultures of the freshwater microalgae Chlorella pyrenoidosa for 10 days based on changes in growth, antioxidant response, and photosynthetic process. The removal efficiency, removal mechanism, and degradation pathway of FLX by C. pyrenoidosa were also investigated. The results showed that the growth of C. pyrenoidosa was inhibited by FLX with a 4 d EC₅₀ of 0.464 mg/L. Additionally, FLX significantly inhibited photosynthesis and caused oxidative stress on day 4. However, C. pyrenoidosa can produce resistance and acclimatize to FLX, as reflected by the declining growth inhibition rate, recovered photosynthetic efficiency, and disappearance of oxidative stress on day 10. Despite the toxicity of FLX, C. pyrenoidosa showed 41.2%- 100% removal of FLX after 10 days of exposure. Biodegradation was the primary removal mechanism, accounting for 88.2%- 92.8% of the total removal of FLX. A total of five metabolites were found in the degradation processes of FLX, which showed less toxicity than FLX. The main degradation pathways were proposed as demethylation, O-dealkylation, hydroxylation, and N-acylation. Our results not only highlight the potential application of microalgae in FLX purification, but also provide insight into the fate and ecological risk of FLX in aquatic environments.

Graphene-Mediated removal of Microcystin-LR in chitosan/graphene composites for treatment of harmful algal blooms

Water quality can be severely impacted by algal blooms alone, yet cyanotoxins, such as microcystin (MC), are potent underlying hazards produced by various species of cyanobacteria. Currently there is a need for environmentally compatible and economically viable media to address large scale application for HAB impacted waters. This study evaluated the interactions of chitosan/graphene (CSG) composites with three different species of cyanobacteria: Anabaena sp, Synechocystis sp, and Microcystis aeruginosa for both removal of algal optical density and toxins. Although results suggest that CSG has an algae dependent removal of density with a range of 40-90% removal, graphene/CSG is highly effective at MC toxin removal, removing >94% of MC-LR produced by Microcystis aeruginosa. Characterization by SEM and XRD revealed that 750 m²/g surface area graphene, imparts graphene morphology and functionality into the chitosan matrix surface, potentially enabling π-π interactions between graphene and the aromatic ring of microcystin. This proposed π-π removal mechanism of microcystin via the CSG chitosan biopolymer substrate offers a promising sustainable and selective media suitable for deployable treatment of HAB impacted waters.

Efficient inhibition of cyanobacteria M. aeruginosa growth using commercial food-grade fumaric acid

The control of cyanobacteria blooms is a global challenge. Here, we reported the efficient inhibition of M. aeruginosa by fumaric acid (FA), an intermediate metabolite of the tricarboxylic acid cycle. FA showed strong algicidal activity with an inhibition rate of 90.5% on the 8th day at a dose of 40 mg/L. The presence of FA caused severe membrane damage, as suggested by the fluorescence flow cytometry and morphology analysis. FA inhibited the formation of chlorophyll a, interrupting the photosynthesis system. It also induced oxidative stress in cells. Principal component analysis of the indicators suggested that the FA-treated sample had a significantly different inhibitory pattern than the acid-treated sample. Thus, the inhibitory effect was not solely caused by the pH effect. Untargeted metabolomic analysis revealed that 31 metabolites were differentially expressed in response to FA stress, which were mainly involved in the metabolite processes and the membranes. A commercial food-grade FA was able to inhibit the growth of M. aeruginosa similar to the analytical-grade FA. Our results suggest that FA can be potentially an efficient and low-risk chemical for inhibiting M. aeruginosa growth, which may find future applications in cyanobacteria bloom control.

Low-frequency ultrasound and nitrogen limitation induced enhancement in biomass production and lipid accumulation of Tetradesmus obliquus FACHB-12

The two-stage cultivation strategy was optimized in this study to simultaneously promote the growth and lipid accumulation of Tetradesmus obliquus. Results showed that the optimal dual-stress conditions were nitrogen concentration at 25 mg N·L^-1 and low-frequency ultrasound at 200 Watt, 1 min, and 8 h interval. The biomass and lipid content of Tetradesmus obliquus were increased by 32.1% and 44.5%, respectively, comparing to the control, and the lipid productivity reached 86.97 mg^-1·L^-1·d^-1 at the end of the cultivation period. The protein and photosynthetic pigment contents of microalgae decreased by 22.4% and 14.0% under dual stress comparing to the control environment. In addition, dual stress cultivation of microalgae presented higher level of antioxidant capacity to balance to oxidation level in microalgal cells. This study provides a new insight for microalgae growth and lipid accumulation with dual stress stimulation.

Adsorption of Pesticides, Antibiotics and Microcystin-LR by Graphene and Hexagonal Boron Nitride Nano-Systems: A Semiempirical PM7 and Theoretical HSAB Study

In recent years, graphene (CC) and hexagonal boron nitride (h-BN) have been widely used in water purification and environmental remediation because of their unique physical and chemical properties. Therefore, based on the reaction enthalpy, equilibrium structure, atomic charge, molecular, orbital and electronic spectrum provided by a semiempirical PM7 method, the adsorption of pesticides, antibiotics and microcystin-LR on graphene and hexagonal boron nitride (h-BN) nano-systems was examined. For the adsorption of diazinon, parathion, oxacillin and ciprofloxacin, the results show that as the bond length decreases and the atomic partial charge increases, the adsorption energy increases. The removal efficiency for antibiotics is higher than that for pesticides. Regarding the co-adsorption of pesticides/antibiotics and microcystin-LR on nano-systems, hydrogen bonds play a crucial role in stabilizing the whole structure. In addition, the non-covalent interaction (NCI) diagrams show the adsorption strength of the nano-systems to the pesticides/antibiotics. The energy gap and HSAB global descriptors are calculated based on the energy values of HOMO and LUMO. It is proved that the graphene nano-system has excellent electron-accepting ability, and suitable sensor materials can be designed.

Toxicity, biodegradation of moxifloxacin and gatifloxacin on Chlamydomonas reinhardtii and their metabolic fate

The novel fourth-generation fluoroquinolones (FQs) were developed to improve the antimicrobial activity and their utilization has rapidly increased in recent years. However, knowledge of the ecotoxicity and microalgae-mediated biodegradation of these novel FQs is limited. In this research, the toxic effects of moxifloxacin (MOX) and gatifloxacin (GAT) on Chlamydomonas reinhardtii as well as their biodegradation and metabolic fate were investigated. The results showed that the toxicity of MOX to C. reinhardtii was higher than that of GAT, and increased with culture time. Chlorophyll fluorescence and pigment content analyses suggested that the decrease in photosynthetic efficiency was primarily caused by the inhibition of electron transport after Q_A in PSII complex. These FQs induced oxidative damage in cells, and the antioxidation mechanisms of C. reinhardtii were analyzed. The maximum MOX removal of 77.67% by C. reinhardtii was achieved at 1 mg/L MOX, whereas the maximum GAT removal of 34.04% was attained at 20 mg/L GAT. The different hydrophilicity and lipophilicity of these FQs resulted in distinct findings in biodegradation experiments. Identification of the transformation products suggested that the likely biodegradation pathways of FQs by C. reinhardtii were hydroxylation, demethylation, and ring cleavage.

Transcriptomics unveiled metabolic perturbations in Desmodesmus quadricauda by sulfacetamide: Key functional genes involved in the tolerance and biodegradation process

Antibiotic contamination in the environment has significant adverse effects on benthic microorganisms, which causes dysfunction of normal ecological processes. However, in-depth molecular mechanisms underlying the potential ecological impacts of these emerging pollutants are poorly understood. In this study, metabolic perturbations in a freshwater microalga, Desmodesmus quadricauda by sulfacetamide (SFM) were investigated using transcriptomics. The results found 28 genes in the tricarboxylic acid cycle and oxidative phosphorolysis pathways were significantly downregulated by 3.97 to 6.07, and 2.47 to 5.99 folds by 0.1 and 1 mg L^-1 SFM, respectively. These results indicated that SFM disrupted the microalgal cellular activities through inhibition of energy metabolism. Whilst, the upregulated genes have been most enriched in porphyrin and chlorophyll metabolism (hemE, hemL, hemY, chlD, chlP, PAO, and CAO), and arachidonic acid metabolism (GGT1_5 and gpx). Expression of these genes was significantly upregulated by up to 3.36 times for tolerance against SFM. Moreover, the genes encoding decarboxylase, oxidoreductases, α-amylase, hydrolases, O-acetyltransferase, and lyase were upregulated by >2 folds, which can induce di/hydroxylation, decarboxylation, bond cleavage and deamination. These findings provide insights into the molecular mechanisms of the ecotoxicological effects of antibiotics on microalgae, and supply useful information for their environmental risk assessment and management.

Joint effects and mechanisms of luteolin and kaempferol on toxigenic Microcystis growth-Comprehensive analysis on two isomers interaction in binary mixture

Allelochemicals are widely accepted as promising algaecide to mitigate Microcystis-dominated cyanobacterial blooms (MCBs). Allelopathic algicidal effect of single luteolin or kaempferol against Microcystis had been confirmed, but their joint effect against Microcystis was unclear. This study comprehensively explored time-dependent joint effect and mechanisms of luteolin and kaempferol on Microcystis growth during 14 day-test. The 50%-inhibitory threshold of their mixture (IC_{50 mix}) was verified as 4.872 and 5.211 mg/L at equitoxic ratio, and 5.167 and 4.487 mg/L at equivalent ratio, respectively, on day 8 and 14. Using toxicity unit, isobologram and predictive models, results revealed that luteolin and kaempferol at equivalent ratio interacted additively at lower, median and higher dosages, while at equitoxic ratio interacted additively at lower dosage but synergistically at median and higher dosages in Microcystis on day 8 and 14, implying that their equitoxic mixture posed better algicidal effect against Microcystis. Various dosages of equitoxic mixture concurrently decreased aqueous and total microcystins (MCs) contents along test. Thus, luteolin and kaempferol could be jointly applied as high-efficacy and eco-safe algaecide with declined MCs pollution risks. As mixture dosage elevated, more strongly weakened cellular MCs retention and inhibited cellular photosynthetic pigments content during late stage, as well as decreased aqueous MCs content long test, jointly explained increasing growth inhibition ratio with rising mixture dosage. Yet, cell damage was gradually repaired due to early stimulated antioxidant defense at each mixture dosage, thus cell damage might not be a major reason for inhibited growth under mixture stress. This study provided novel insights and guidance to coupled application of luteolin and kamepferol for mitigating MCBs and decreasing MCs pollution risks.

Instantaneous synthesis and full characterization of organic-inorganic laccase-cobalt phosphate hybrid nanoflowers

A novel approach termed the "concentrated method" was developed for the instant fabrication of laccase@Co₃(PO₄)₂•hybrid nanoflowers (HNFs). The constructed HNFs were obtained by optimizing the concentration of cobalt chloride and phosphate buffer to reach the highest activity recovery. The incorporation of 30 mM CoCl₂ and 160 mM phosphate buffer (pH 7.4) resulted in a fast anisotropic growth of the nanomaterials. The purposed method did not involve harsh conditions and prolonged incubation of precursors, as the most reported approaches for the synthesis of HNFs. The catalytic efficiency of the immobilized and free laccase was 460 and 400 M⁻¹S⁻¹, respectively. Also, the enzymatic activity of the prepared biocatalyst was 113% of the free enzyme (0.5 U mL⁻¹). The stability of the synthesized HNFs was enhanced by 400% at pH 6.5–9.5 and the elevated temperatures. The activity of laccase@Co₃(PO₄)₂•HNFs declined to 50% of the initial value after 10 reusability cycles, indicating successful immobilization of the enzyme. Structural studies revealed a 32% increase in the α-helix content after hybridization with cobalt phosphate, which improved the activity and stability of the immobilized laccase. Furthermore, the fabricated HNFs exhibited a considerable ability to remove moxifloxacin as an emerging pollutant. The antibiotic (10 mg L⁻¹) was removed by 24% and 75% after 24 h through adsorption and biodegradation, respectively. This study introduces a new method for synthesizing HNFs, which could be used for the fabrication of efficient biocatalysts, biosensors, and adsorbents for industrial, biomedical, and environmental applications.

Salt-alkalization may potentially promote Microcystis aeruginosa blooms and the production of microcystin-LR

The development of saline-alkali lands has contributed to the increasing discharge of alkaline salt-laden wastewater, which poses a threat to aquatic organisms. However, the comprehensive effect of alkaline salt on Microcystis aeruginosa, a harmful cyanobacterium, remains unclear. In this study, the growth, physiology, cell ultrastructure and production of microcystin-LR (MC-LR) in Microcystis aeruginosa exposed to four levels of alkaline salt stress were evaluated. The growth of Microcystis aeruginosa was stimulated at an electrical conductivity (EC) of 2.5 mS/cm compared to the control, as supported by the increased cell density, photosynthetic pigment and protein contents. Microcystis aeruginosa could tolerate a certain level of alkaline salt (i.e., EC of 5 mS/cm) via increasing photosynthetic pigment contents to protect cells from alkaline salt stress, but the antioxidant defence system and cell ultrastructure were not affected. When EC increased to 7.5 mS/cm, alkaline salt caused oxidative stress and toxicity in Microcystis aeruginosa, as evidenced by analysis of the integrated biomarker response (IBR). Furthermore, the photosynthetic pigment and protein contents decreased, and cell apoptosis associated with ultrastructural changes was observed. Therefore, we propose that EC of 7.5 mS/cm is a threshold for growth of Microcystis aeruginosa. Additionally, the intracellular MC-LR content was stimulated by alkaline salt, and the highest value was observed at EC of 2.5 mS/cm. The extracellular MC-LR content increased with the increasing alkaline salt concentration. When EC was 7.5 mS/cm, the extracellular MC-LR content was significantly higher than in the control and was associated with the upregulated mcyH gene. This study recommends that more attention should be paid to the risk of Microcystis aeruginosa bloom and microcystin-LR pollution in lakes located in salinization regions.

Photosynthetic Toxicity of Enrofloxacin on Scenedesmus obliquus in an Aquatic Environment

Aquaculture facilities are a potential source of antibiotics in aquatic environments, having adverse effects on the algae species. In this study, the toxicity induced by enrofloxacin (ENR) on the algae Scenedesmus obliquus was evaluated. The uptake of ENR and the change in the growth and photosynthesis of algae were analyzed. At the exposure doses of 10-300 μg/L, the accumulated levels of ENR in algae were 10.61-18.22 μg/g and 12.09-18.34 μg/g after 48 h and 96 h of treatment, respectively. ENR inhibited the growth of algae, with a concentration for 50% effect of 119.74 μg/L, 53.09 μg/L, 64.37 μg/L, and 52.64 μg/L after 24 h, 48 h, 72 h and 96 h of treatment, respectively, indicating the self-protection and repair ability of algae in a short period of time. Furthermore, the chlorophyll contents decreased in all treatment groups, and the photosynthetic system Ⅱ parameters decreased in a dose-dependent manner under ENR stress, suggesting that ENR caused a disorder in the electron transport of the photosynthesis of algae, and the carbon fixation and assimilation processes were thus damaged. These results indicate that ENR poses a considerable risk to aquatic environments, affects the carbon sinks, and even has an adverse effect on human health.

Toxicity mechanism of cerium oxide nanoparticles on cyanobacteria Microcystis aeruginosa and their ecological risks

The extensive application of cerium oxide nanoparticles (CeO₂ NPs), a type of rare earth nanomaterial, led to pollution into aquatic environments. Cyanobacteria, a significant component of freshwater ecosystems, can interact with CeO₂ NPs. However, little attention has been paid as to whether CeO₂ NPs will have adverse effects on cyanobacteria. In the present study, Microcystis aeruginosa (FACHB-942) was exposed to different concentrations (0, 1, 10, and 50 mg/L) of CeO₂ NPs. Results showed 50 mg/L CeO₂ NPs inhibited algal growth (11.48% ± 5.76%), suppressed photosynthesis and induced the generation of reactive oxygen species (ROS) after 72 h exposure. The toxicity mechanism is the adsorption of CeO₂ NPs on cell surface, the ROS formation and the intracellular Ce. Additionally, the intracellular microcystins (MCs) content was significantly induced (11.84% ± 1.47%) by 50 mg/L CeO₂ NPs, while no significance was found in 1 and 10 mg/L CeO₂ NP treatments. Results indicated high concentrations of CeO₂ NPs could be toxic to algae through the adverse effects on algal growth and photosynthesis. Moreover, the promoted MCs production could also pose a threat to freshwater ecosystems due to the possible release into the environment.

Microplastics impacts in seven flagellate microalgae: Role of size and cell wall

The toxicity of microplastic particles (MPs) on aquatic environments has been widely reported; however, their effects on protists are still contradictory. For example, it is unclear if cell size and cell wall have a role in shaping the response of flagellates to MPs. In this study, seven marine flagellated microalgae (six Dinoflagellates and one Raphidophyceae) were incubated with 10 mg L^-1 MPs (polystyrene plastic micro-spheres, 1 μm diameter) to address the above question by measuring different response variables, i.e., growth, optimal photochemical efficiency (F_v/F_m), chlorophyll-a (Chl-a) content, superoxide dismutase (SOD) activity, and cell morphology. The effect of MPs on growth and F_v/F_m showed species-specificity effects. Maximum and minimum MPs-induced inhibitions were detected in Karenia mikimotoi (76.43%) and Akashiwo sanguinea (10.16%), respectively, while the rest of the species showed intermediate responses. The presence of MPs was associated with an average reduction of Chl-a content in most cases and with a higher superoxide dismutase activity in all cases. Seven species were classified into two groups by the variation of Chl-a under MPs treatment. One group (Prorocentrum minimum and Karenia mikimotoi) showed increased Chl-a, while the other (P. donghaiense, P. micans, Alexandrium tamarense, Akashiwo sanguinea, Heterosigma akashiwo) showed decreased Chl-a content. The MPs-induced growth inhibition was negatively correlated with cell size in the latter group. SEM images further indicated that MPs-induced malformation in the smaller cells (e.g., P. donghaiense and K. mikimotoi) was more severe than the bigger cells (e.g., A. sanguinea and P. micans), probably due to a relatively higher ratio of the cell surface to cell volume in the former. These results implicate that the effect of MPs on marine flagellated microalgae was related to the cell size among most species but not cell wall. Thus plastic pollution may have size-dependent effects on phytoplankton in future scenarios.

Interactions between antibiotics and heavy metals determine their combined toxicity to Synechocystis sp

Co-pollution of antibiotics and metals is prevailing in aquatic environments. However, risks of coexisted antibiotics and metals on aquatic organisms is unclear. This study investigated the combined toxicity of antibiotics and metals towards Synechocystis sp. PCC 6803, a cyanobacterium. We found that the joint toxicity of antibiotics and metals is dependent on their interplays. The complexation between chlortetracycline (CTC) and copper/cadmium (Cu(II)/Cd(II)) resulted in their antagonistic toxicity. Contrarily, an additive toxicity was found between florfenicol (FLO) and Cu(II)/Cd(II) due to lack of interactions between them. CTC facilitated the intracellular uptake of Cu(II) and Cd(II) by increasing the membrane permeability. However, FLO had no obvious effects on the internalization of metals in Synechocystis sp. Proteomic analysis revealed that the photosynthetic proteins was down-regulated by CTC and FLO, and ribosome was the primary target of FLO. These results were verified by parallel reaction monitoring (PRM). Cu(II) induced the up-regulation of iron-sulfur assembly, while Cd(II) disturbed the cyclic electron transport in Synechocystis sp. The co-exposure of CTC and metals markedly alleviated the dysregulation of proteins, while the co-exposure of FLO and metals down-regulated biological functions such as ATP synthesis, photosynthesis, and carbon fixation of Synechocystis sp., compared with their individuals. This supports their joint toxicity effects. Our findings provide better understanding of combined toxicity between multiple pollutants in aquatic environments.

Proteomic mechanisms for the regulation of growth, photosynthetic activity and nitrogen fixation in Nostoc sp. PCC 7120 exposed to three antibiotic contaminants

This study investigated the influences of three frequently detected antibiotics in surface waters, ciprofloxacin, tetracycline and sulfamethoxazole, on the growth, photosynthetic activity, nitrogen-fixing capacity and proteomic expression profiles of Nostoc sp. PCC 7120, through a 15-day exposure test at environmentally relevant exposure doses of 50-200 ng/L. Cyanobacterial growth was stimulated by 100 ng/L and 200 ng/L of ciprofloxacin and sulfamethoxazole as well as 50-200 ng/L of tetracycline. The nitrogenase synthesis ability in each cyanobacterial cell was stimulated by 50-200 ng/L of ciprofloxacin while inhibited by 100 ng/L and 200 ng/L of tetracycline and sulfamethoxazole. At the exposure dose of 100 ng/L for each antibiotic, the variation of total nitrogen in the culture medium indicated that the nitrogen-fixing capacity of Nostoc sp. was determined by total nitrogenase concentration calculated by cell density × nitrogenase synthesis ability. Therefore, ciprofloxacin enhanced nitrogen fixation through the stimulation of both cyanobacterial growth and nitrogenase synthesis, while tetracycline and sulfamethoxazole enhanced nitrogen fixation merely through growth stimulation. At the exposure dose of 100 ng/L, only two downregulated proteins, a phosphonate ABC transporter and a methionine aminopeptidase, as well as one upregulated protein, the phenylalanine-tRNA ligase alpha subunit, were commonly shared by three antibiotic-treated groups. Ciprofloxacin upregulated proteins related to nitrogen fixation, carbon catabolism and biosynthesis, but downregulated photosynthesis-related proteins. In contrast, tetracycline and sulfamethoxazole increased the photosynthetic activity of Nostoc sp. through upregulating photosynthesis-related proteins, but downregulated proteins related to nitrogen fixation, carbon catabolism and biosynthesis. The resistance of Nostoc sp. PCC 7120 to three target antibiotics were related with the responses of RNA synthesis regulatory proteins. Stimulation of cyanobacterial nitrogen fixation by antibiotic contaminants could aggravate eutrophication in aquatic environments.