As the first step of methylmercury (MeHg) entry into the aquatic food webs, MeHg uptake by phytoplankton is crucial in determining the final human MeHg exposure risks. MeHg availability to plankton is regulated by dissolved organic matter (DOM) in the water, while the extent of the impacts can vary largely based on the sources of DOM. Here, we investigated impacts of DOM sources on MeHg bioconcentration by three freshwater phytoplankton species (i.e. S. quadricauda, Chlorella sp., Microcystis elabens) in the laboratory system. We found that algae-derived DOM would prohibited the cellular MeHg bioconcentration by a percent up to 77–93%, while the soil-derived DOM didn't show similar inhibition effects. DOM characterization by the excitation‒emission matrices, Fourier transform infrared spectrum, ultra‒high performance liquid chromatography‒tandem quadrupole time of flight mass spectrometry shown that the molecular size of S-containing compound, rather than thiol concentration, has played a crucial role in regulating the MeHg uptake by phytoplankton. Climate change and increasing nutrient loadings from human activities may affect plankton growth in the freshwater, ultimately changing the DOM compositions. Impacts of these changes on cellular MeHg uptakes by phytoplankton should be emphasized when exploring the aquatic Hg cycling and evaluating their risks to human beings and wild life.

The 6:2 chlorinated polyfluoroalkyl ether sulfonic acids (6:2 Cl-PFAES), 2,3,3,3-tetrafluoro-2-(1,1,2,2,3,3,3-heptafluoropropoxy)-propanoic acid (HFPO-DA) and perfluoroethylcyclohexane sulfonate (PFECHS) are emerging per- and polyfluoroalkyl substances (PFASs) that are being applied to replace phased-out PFASs, which have high persistency, high bioaccumulation potential and high toxicity. Recently, these emerging PFASs were observed in estuary and marine areas with a pollution level of ng/L. In this study, three levels (10 ng L⁻¹, 100 ng L⁻¹ and 1000 ng L⁻¹) for these PFASs were selected to investigate the response of marine Chlorella sp. to 14 days of exposure. The growth of Chlorella sp. was significantly inhibited by each PFAS over time. Treatments with 1000 ng/L exposure caused the most severe reduction in growth for each PFAS treatment. For the first half of the experimental period (from Day 0 to Day 6), the influence of each PFAS was not significant (p > 0.05). However, treatments with all concentrations of 6:2 Cl-PFAES, HFPO-DA and 1000 ng L⁻¹ PFECHS significantly reduced the growth of Chlorella sp. from Day 8. The superoxide dismutase (SOD) activities in Chlorella sp. were significantly increased (p < 0.05) when exposed to 6:2 Cl-PFAES, HFPO-DA and PFECHS. The catalase (CAT) and peroxidase (POD) activities in Chlorella sp. were significantly inhibited (p < 0.05) by each PFAS. The glutathione (GSH) contents in Chlorella sp. were significantly increased by each PFAS. However, the increases in GSH concentration in Chlorella sp. were low. The inhibition of algal growth was primarily due to the reduction of the activities of CAT and POD. PFECHS had the lowest toxicity among the three PFASs, and it induced less oxidized damage to Chlorella sp. In conclusion, as alternatives to phased-out PFASs, the emerging PFASs are not safe in aquatic environment, and attention should be paid to the management and restriction of these emerging PFASs.

Antibiotics have been widely detected in the ocean and have various impacts on the environment, while knowledge of their chronic influence on phytoplankton, especially red tide algae, is still limited. Dinoflagellates and green algae are common phytoplankton in marine ecosystems. The former is the main red tide algae, and the latter is an important primary producer. We investigated the long-term responses of two representative algae, Prorocentrum lima and Chlorella sp., to two common antibiotics (sulfamethoxazole (SMX) and norfloxacin (NFX)) at environmentally relevant levels (10 and 100 ng/L) during simulated natural conditions. The cell density and activities of three antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD)) were analyzed. The results showed that the influence of each antibiotic on Chlorella sp. was not significant (p > 0.05) during the first 10 days, but the influence of the antibiotics later began to show significant inhibition (p < 0.05) compared with the control group, especially during mixed exposure. P. lima was not inhibited, but its cell density increased. SMX had a superior stimulation effect on P. lima. The three enzymes activities of P. lima increased, and the antioxidant mechanism was not seriously impacted. However, for Chlorella sp., the activity of SOD increased while the activities of CAT and POD decreased, suggesting that this algae’s antioxidant system was unbalanced due to oxidative stress. Based on our results, the growth of P. lima was different from green algae Chlorella sp. as well as other inhibited marine algae (such as diatom, golden algae) studied in previous studies. Therefore, as a typical pollutant in the ocean, antibiotics may play a positive role in the bloom of dinoflagellate red tides.

China has been faced with severe haze pollution, which is hazardous to human health. Among the air pollutants, PM2.5 (particles with an aerodynamic diameter ≤ 2.5 μm) is the most dangerous because of its toxicity and impact on human health and ecosystems. However, there has been limited research on PM2.5 particle toxicity. In the present study, we collected daily PM2.5 samples from January 1 to March 31, 2018 and selected samples to extract water-soluble species, including SO42−, NO3−, WSOC, and NH4+. These samples represented clean, good, slight, moderate, and heavy pollution days. After extraction using an ultrasonic method, PM2.5 solutions were obtained. We used Chlorella as the test algae and studied the content of chlorophyll a, as well as the variation in fluorescence when they were placed into the PM2.5 extraction solution, and their submicroscopic structure was analyzed using transmission electron microscopy (TEM). The results showed that when the air quality was relatively clean and good (PM2.5 concentration ≤ 75 μg m−3), the PM2.5 extraction solutions had no inhibiting effects on Chlorella, whereas when the air quality was polluted (PM2.5 concentration > 75 μg m−3) and heavily polluted (PM2.5 concentration > 150 μg m−3), with increasing PM2.5 concentrations and exposure time, the chlorophyll a content in Chlorella decreased. Moreover, the maximum photochemical quantum yield (Fv/Fm) of Chlorella obviously decreased, indicating chlorophyll inhibition during polluted days with increasing PM2.5 concentrations. The effects on the chlorophyll fluorescence parameters were also obvious, leading to an increase of energy dissipated per unit reaction center (DIo/RC), suggesting that Chlorella could survive when exposed to PM2.5 solutions, whereas the physiological activities were significantly inhibited. The TEM analysis showed that there were few effects on Chlorella cell microstructure during clean days, whereas plasmolysis occurred during light- and medium-polluted days. With increasing pollution levels, plasmolysis became more and more apparent, until the organelles inside the cells were thoroughly destroyed and most of the parts could not be recognized.

The present study examines the effect of carboxyl-CdSe/ZnS quantum dots (QDs) on Cu and Pb availability to microalgae with different cell wall characteristics: Chlorella kesslerii possessing a cellulosic cell wall and two strains of Chlamydomonas reinhardtii, a wall-less and a walled strain containing glycoproteins as the main cell wall component. Results demonstrated that QDs decreased Pb and Cu intracellular contents ({Cu}ᵢₙₜ and {Pb}ᵢₙₜ) in walled strains by a factor of 2.5 and 2, respectively, as expected by the decrease of about 70% and 40% in the dissolved Cu and Pb concentrations. QDs increased {Cu}ᵢₙₜ and {Pb}ᵢₙₜ in wall-less strain by a factor of 4 and 3.5. These observations were consistent with the observed association of QDs to the wall-less C. reinhardtii, and lack of association to walled algal strains. Suwannee River humic acid did not influence metal association to QDs, but decreased {Cu}ᵢₙₜ and {Pb}ᵢₙₜ in all microalgae.

Transformation products usually differ in environmental behaviors and toxicological properties from the parent contaminants, and probably cause potential risks to the environment. Toxicity evolution of a labile preservative, bronopol, upon primary aquatic degradation processes was investigated. Bronopol rapidly hydrolyzed in natural waters, and primarily produced more stable 2-bromo-2-nitroethanol (BNE) and bromonitromethane (BNM). Light enhanced degradation of the targeted compounds with water site specific photoactivity. The bond order analysis theoretically revealed that the reversible retroaldol reactions were primary degradation routes for bronopol and BNE. Judging from toxicity assays and the relative pesticide toxicity index, these degradation products (i.e., BNE and BNM), more persistent and higher toxic than the parent, probably accumulated in natural waters and resulted in higher or prolonging adverse impacts. Therefore, these transformation products should be included into the assessment of ecological risks of non-persistent and low toxic chemicals such as the preservative bronopol.

The present study addresses the key issue of linking the chemical speciation to the uptake of priority pollutants Cd(II) and Pb(II) in the wastewater treatment plant effluents, with emphasis on the role of the colloidal organic matter (EfOM). Binding of Cd(II) and Pb(II) by EfOM was examined by an ion exchange technique and flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry in parallel to bioassays with green microalga Chlorella kesslerii in ultrafiltrate (<1 kDa) and colloidal isolates (1 kDa to 0.45 μm). The uptake of Cd by C. kesslerii was consistent with the speciation analysis and measured free metal ion concentrations, while Pb uptake was much greater than that expected from the speciation measurement. Better understanding of the differences in the effects of the EfOM on Cd(II) and Pb(II) uptake required to take into account the size dependence of metal binding by EfOM. Colloids isolated from WWTP effluents decrease Cd uptake, but increase Pb uptake by microalga Chlorella kesslerii.

The influence of pH, ionic strength, presence of humic or alginic acids, extracellular polymeric substances (EPS), or freshwater microalga Chlorella kesslerii on the stability and transformation of carboxyl-PEG-CdSe/ZnS core/shell quantum dots (QDs) in terms of number, hydrodynamic size and fluorescence of individual particles, was studied by fluorescence correlation spectroscopy. Obtained results demonstrated that QDs form stable dispersions at nanomolar concentrations under conditions typical for freshwaters. The presence of 5 or 15 mg C L−1 of humic acid or 50 mg C L−1 EPS did not significantly affect these parameters. In contrast, 5 or 50 mg C L−1 alginate at ionic strength of 10 mM shifted the hydrodynamic radius toward larger values, suggesting a possible capture of QDs by the linear alginate chains. The addition of microalga to the QD dispersions resulted in a slight reduction of the number of QDs and a significant decline in the fluorescence of individual QDs. Carboxyl-PEG-CdSe/ZnS core/shell quantum dots form stable dispersions under conditions representative of freshwaters.

Nowadays, the ubiquitous distribution and increasing abundance of P⁺ᴵᴵᴵ in waterbodies have caused serious concerns regarding its bioavailability and potential toxicity. However, our knowledge on these issues is relatively limited. We addressed previously unknown effects of P⁺ᴵᴵᴵ on three dominate algae species i.e. Microcystic aeruginosa (M. aeruginosa), Chlorella pyrenoidesa (C. pyrenoidesa) and Cyclotella. sp in eutrophic waterbodies in China. Remarkable declines in biomass, specific growth rate and Chl-a of algae cells treated with 0.01–0.7 mg/L P⁺ᴵᴵᴵ as sole or an alternative P source were observed, indicating P⁺ᴵᴵᴵ had an inhibitory effect on the algal growth. Besides, the intracellular enzyme activities e.g superoxide dismutase (SOD) and malondialdehyde (MDA) were significantly increased with P⁺ᴵᴵᴵ stress. M. aeruginosa and Cyclotella. sp cells seemed to be more sensitive to P⁺ᴵᴵᴵ toxicity than C. pyrenoidesa since cell membrane suffered more serious stress and destruction. These findings combined, it confirmed P⁺ᴵᴵᴵ could not be utilized as bioavailable P, but had certain toxicity to the tested algae. It indicated that the increased P⁺ᴵᴵᴵ abundance in eutrophic waterbodies would accelerate the algal cell death, which could have a positive effect against algal blooms. Our results provide new insights into assessing the ecological risks of P⁺ᴵᴵᴵ in aquatic environments.

Australian tropical freshwaters can experience extreme seasonal variability in rainfall and run off, particularly due to pulse events such as storms and cyclones. This study investigated how seasonal variability in dissolved organic matter (DOM) quality impacted the chronic toxicity of copper to a tropical green alga (Chlorella sp.) in the presence of two concentrations of DOM (low: ∼2 mg C/L; high: ∼10 mg C/L) collected from three tropical waters. Copper speciation and lability were explored using diffusive gradients in thin-films (DGT) and modelled maximum dynamic concentrations (cᵈʸⁿₘₐₓ) using data derived from the Windermere Humic Aqueous Model (WHAM VII). Relationships between copper lability and copper toxicity were assessed as potential tools for predicting toxicity. Copper toxicity varied significantly with DOM concentration, source and season. Copper toxicity decreased with increasing concentrations of DOM, with 50% growth inhibition effect concentrations (EC₅₀) increasing from 1.9 μg Cu/L in synthetic test waters with no added DOM (0.34 mg C/L) up to 63 μg Cu/L at DOM concentrations of 9.9 mg C/L. Copper toxicity varied by up to 2-fold between the three DOM sources and EC₅₀ values were generally lower in the presence of wet season DOM compared to dry season DOM. Linear relationships between DGT-labile copper and dissolved copper were significantly different between DOM source, but not concentration or season. Modelled cᵈʸⁿₘₐₓ consistently under-predicted labile copper in high DOM treatments compared to DGT measurements but performed better in low DOM treatments, indicating that this method is DOM-concentration dependent. Neither speciation method was a good surrogate for copper toxicity in the presence of different sources of natural DOM. Our findings show that DOM source and season, not just DOM concentration, affect copper toxicity to freshwater biota. Therefore, DOM quality should be considered as a toxicity-modifying factor for future derivation of bioavailability-based site-specific water quality guideline values.