Cyanobacterium Microcystis aeruginosa response to pentachlorophenol and comparison with that of the microalga Chlorella vulgaris

Effects of PAHs on nitrogen uptake by phytoplankton species: Implications for environmental risk assessment of micropollutants and algal bloom formation

Polycyclic aromatic hydrocarbons (PAHs) are of great concern because they threaten primary productivity, but their specific effects on ecosystem functioning are scarce, hindering a comprehensive understanding of their ecological risks, especially in eutrophic waters. The present study was conducted by adding PAHs to four marine phytoplankton species and showed that naphthalene (Nap) and phenanthrene (Phe) induced both stimulatory and inhibitory effects (>50 %) on urea and NO3- uptake by phytoplankton species. In addition, the apparent stimulative effects (>50 %) for NH4+ were also observed. Overall, 38.9 % of the samples exhibited stimulation effects after 24 h exposure, which increased to 61.1 % after 96 h exposure. This suggested the existence of a lag period, during which a tolerant cell population could adapt to PAHs. Significant positive correlations (P < 0.01) between low and high concentrations of PAH individuals demonstrated that the mode of action for both pollutants on nitrogen uptake by phytoplankton was the same. Species-specific responses were also observed, with 19.0 % of Thalassiosira sp. and 24.0 % of Tetraselmis sp. exhibited inhibition effects greater than 50 %, while 40.9 % of Karlodinium veneficum and 27.3 % of Rhodomonas salina demonstrated stimulation effects exceeding 50 %, providing a unique perspective for exploring the harmful algal bloom of the mixotrophic K. veneficum, in addition to the original consideration of nutrients. The internal mechanisms may lie in differences in energy consumption between N-forms, exposure time and chemical concentrations, as well as morphological characteristics and biochemical structures of the species, which require further investigation.

Physiological and molecular mechanisms of the inhibitory effects of artemisinin on Microcystis aeruginosa and Chlorella pyrenoidosa

Artemisinin, a novel plant allelochemical, has attracted attention for its potential selective inhibitory effects on algae, yet to be fully explored. This study compares the sensitivity and action targets of Microcystis aeruginosa (M. aeruginosa) and Chlorella pyrenoidosa (C. pyrenoidosa) to artemisinin algaecide (AMA), highlighting their differences. Results indicate that at high concentrations, AMA displaces the natural PQ at the QB binding site within M. aeruginosa photosynthetic system, impairing the D1 protein repair function. Furthermore, AMA disrupts electron transfer from reduced ferredoxin (Fd) to NADP+ by interfering with the iron-sulfur clusters in the ferredoxin-NADP+ reductases (FNR) domain of Fd. Moreover, significant reactive oxygen species (ROS) accumulation triggers oxidative stress and interrupts the tricarboxylic acid cycle, hindering energy acquisition. Notably, AMA suppresses arginine synthesis in M. aeruginosa, leading to reduced microcystins (MCs) release. Conversely, C. pyrenoidosa counters ROS accumulation via photosynthesis protection, antioxidant defenses, and by regulating intracellular osmotic pressure, accelerating damaged protein degradation, and effectively repairing DNA for cellular detoxification. Additionally, AMA stimulates the expression of DNA replication-related genes, facilitating cell proliferation. Our finding offer a unique approach for selectively eradicating cyanobacteria while preserving beneficial algae, and shed new light on employing eco-friendly algicides with high specificity.

Responses of cyanobacterium Microcystis aeruginosa under single and repeated ofloxacin exposure

Antibiotics are omnipresent and pseudo-persistent in the environment. Yet, their potential ecological risks under repeated exposure, which is more environmentally relevant, are understudied. Therefore, this study used ofloxacin (OFL) as the probe chemical to investigate the toxic effects of different exposure scenarios-single dose of high concentration (4.0 µg/L) and multiple additions of low concentrations-towards the cyanobacterium Microcystis aeruginosa. Flow cytometry was employed to measure a collection of biomarkers, including endpoints related with biomass, single cell properties and physiological status. Results showed that the single dose of the highest OFL level inhibited cellular growth, chl-a content and cell size of M. aeruginosa. In contrast, OFL induced stronger chl-a autofluorescence and higher doses tended to have more remarkable effects. Repeated low OFL doses can more significantly increase the metabolic activity of M. aeruginosa than a single high dose. Viability and cytoplasmic membrane were not affected by OFL exposure. Oxidative stress was observed for the different exposure scenarios, with fluctuating responses. This study demonstrated the different physiological responses of M. aeruginosa under different OFL exposure scenarios, providing novel insights into the toxicity of antibiotics under repeated exposure.

Low Levels of Contaminants Stimulate Harmful Algal Organisms and Enrich Their Toxins

A widespread increase in intense phytoplankton blooms has been noted in lakes worldwide since the 1980s, with the summertime peak intensity amplifying in most lakes. Such blooms cause annual economic losses of multibillion USD and present a major challenge, affecting 11 out of the 17 United Nations Sustainable Development Goals. Here, we evaluate recent scientific evidence for hormetic effects of emerging contaminants and regulated pollutants on Microcystis sp., the most notorious cyanobacteria forming harmful algal blooms and releasing phycotoxins in eutrophic freshwater systems. This new evidence leads to the conclusion that pollution is linked to algal bloom intensification. Concentrations of contaminants that are considerably smaller than the threshold for toxicity enhance the formation of harmful colonies, increase the production of phycotoxins and their release into the environment, and lower the efficacy of algaecides to control algal blooms. The low-dose enhancement of microcystins is attributed to the up-regulation of a protein controlling microcystin release (McyH) and various microcystin synthetases in tandem with the global nitrogen regulator Ycf28, nonribosomal peptide synthetases, and several ATP-binding cassette transport proteins. Given that colony formation and phycotoxin production and release are enhanced by contaminant concentrations smaller than the toxicological threshold and are widely occurring in the environment, the effect of contaminants on harmful algal blooms is more prevalent than previously thought. Climate change and nutrient enrichment, known mechanisms underpinning algal blooms, are thus joined by low-level pollutants as another causal mechanism.

Stimulatory and inhibitory effects of phenanthrene on physiological performance of Chlorella vulgaris and Skeletonema costatum

The effects of polycyclic aromatic hydrocarbons on phytoplankton have been extensively documented, but there is limited knowledge about the physiological responses of marine primary producers to phenanthrene at environmentally relevant levels. Here, we investigated the toxicity of phenanthrene (0, 1, and 5 or 10 μg L⁻¹) to the physiological performance of two cosmopolitan phytoplankton species: the green alga Chlorella vulgaris and bloom-forming diatom Skeletonema costatum. The specific growth rates of both species were remarkably inhibited at both low (1 μg L⁻¹) and high phenanthrene concentrations (5 or 10 μg L⁻¹), while their tolerance to phenanthrene differed. At the highest phenanthrene concentration (10 μg L⁻¹), the growth of C. vulgaris was inhibited by 69%, and no growth was observed for S. costatum cells. The superoxide dismutase activity of both species was enhanced at high phenanthrene concentration, and increased activity of catalase was only observed at high phenanthrene concentration in C. vulgaris. Interestingly, the low phenanthrene concentration stimulated the photosynthetic and relative electron transport rates of S. costatum, whereas hormetic effects were not found for growth. Based on our results, phenanthrene could be detrimental to these two species at a environmentally relevant level, while different tolerance levels were detected.

Time- and dose-dependent allelopathic effects and mechanisms of kaempferol on toxigenic Microcystis growth

This study determined time-dependent IC₅₀ and confirmed 3.5 mg/L as IC₅₀ value for kaempferol inhibiting toxigenic Microcystis growth, based on which algicidal effects and mechanisms against toxigenic Microcystis exposed to various kaempferol doses (0.5-2 × IC₅₀) were explored along 14 day-test. Results showed that growth inhibition ratio (GIR) almost elevated with increasing kaempferol dose, and at each dose GIR elevated firstly and fluctuated around 17.8%- > 40%, 53.6%-65.6% and 84.8%-89.3% at 1.75, 3.5 and 7 mg/L kaempferol during mid-late stage, respectively. With rising kaempferol dose, photosynthetic pigments contents (chlorophyll-a, phycobiliproteins), antioxidant response (superoxide dismutase and catalase (CAT) activities, glutathione (GSH) contents) and microcystins (MCs) production were almost increasingly stimulated as cellular protective responses during early-mid stage. However, these parameters (excluding CAT and GSH) were almost increasingly inhibited at late stage by prolonged stress and Microcystis cell was still more severely damaged as dose elevated along test, which could be reasons for increasing GIR with rising kamepferol dose. Persistent stimulation of CAT and GSH at each dose could alleviate cell damage until late stage, thus GIR no longer increased at late stage at each kaempferol dose. Moreover, fewer MCs release under kaempferol stress than control suggested kaempferol as eco-safe algaecide for migrating toxigenic Microcystis-dominated blooms (MCBs) and decreasing MCs risks. Compared with our previous data for luteolin inhibiting toxigenic Microcystis, this study supported formerly-proposed 'flavonoids structure - algicidal activity' relationship that the only OH-location difference between kaempferol and luteolin could affect algicidal activity and mechanisms against toxigenic Microcystis. Also, kaempferol and luteolin was revealed to exert additive effect on toxigenic Microcystis growth at equitoxic ratio. Our findings gave novel algicidal scenario of flavonoids and were greatly implicated in eco-friendly migrating toxigenic MCBs.

Biodegradation of hydrophobic pesticides by microalgae: Transformation products and impact on algae biochemical methane potential

Intensive and extensive use of pesticides has contributed to their wide distribution in soil, air, and water. Due to their detrimental effects on non-target organisms, different technologies have been considered for their removal. In this work, three hydrophobic pesticide active compounds, namely, chlorpyrifos, cypermethrin, and oxadiazon, were selected to study the potential for their removal from aqueous media by a microalgae consortium. An abiotic and a killed control (thermally inactivated dead microalgae biomass) were employed to clarify their removal pathways, and pesticide content was quantified in liquid and biomass phases for 7 days. At the final time, total degradation (biodegradation plus photodegradation) contributed to the removal of 55% of oxadiazon, 35% of chlorpyrifos, and 14% of cypermethrin. Furthermore, more than 60% of chlorpyrifos and cypermethrin were removed by sorption onto microalgae biomass. Overall, the three pesticides showed high removal from the liquid phase. O,O-diethyl thiophosphate was identified in the liquid phase as a transformation product of chlorpyrifos formed by microalgae degradation. Phycoremediation was coupled with anaerobic degradation of the microalgae biomass containing the retained pesticides by sorption through biochemical methane potential tests. Anaerobic digestion was not inhibited by the pesticides as verified by methane production yields. The removal efficiency of the pesticides in the digestate was as follows: chlorpyrifos > cypermethrin > oxadiazon. These results highlight the potential of low-cost algal-based systems for the treatment of wastewater or effluents from agrochemical industries. The integration of wastewater treatment with biogas production through anaerobic digestion is a biorefinery approach that facilitates the economic feasibility of the process.

Phenanthrene and pyrene disturbed the growth of Microcystis aeruginosa as co-cultured with Chlorella pyrenoidosa

Significant levels of polycyclic aromatic hydrocarbons (PAHs) were detected in lakes. The competition between algae would be disturbed by PAHs resulted in variations of algal growth. For controlling the cyanobacterial blooms, it is important to understand this disturbed competition between Microcystis aeruginosa and other algae. A 6-day cultivation experiment was designed to investigate the responses of M. aeruginosa to PAHs in presence of green algae. A popular green alga Chlorella pyrenoidosa was used as a representative of green algae, and phenanthrene and pyrene were selected as representatives of PAHs. The results showed that M. aeruginosa outcompeted C. pyrenoidosa under PAH contamination, and PAHs and M. aeruginosa significantly inhibited the survival of C. pyrenoidosa. PAHs disturbed the growth of algae by influencing photosynthetic pigments and phycobiliproteins, and the different alteration of F_v/F_m ratios implied that shifted algal community composition would be induced by PAHs. The F_v/F_m of the two algal mixture and individual C. pyrenoidosa was significantly negatively correlated with phenanthrene levels. However, there were no significant correlations between the F_v/F_m of M. aeruginosa and the exposure levels of phenanthrene or pyrene. Remarkably, the F_v/F_m significantly increased in M. aeruginosa at 0.15 mg L^-1 pyrene, suggesting that PSII resistance to pyrene was enhanced in M. aeruginosa. Our results pointed out an increasing frequency and intensity of cyanobacterial blooms could be induced by PAHs in contaminated waters.

Antioxidant responses in cyanobacterium Microcystis aeruginosa caused by two commonly used UV filters, benzophenone-1 and benzophenone-3, at environmentally relevant concentrations

Benzophenone-type ultraviolet filters (BPs) have recently been recognized as emerging organic contaminants. In the present study, the cyanobacterium Microcystis aeruginosa was exposed to environmentally relevant levels (0.01-1000 μg L^-1) of benzophenone-1 (BP-1) and benzophenone-3 (BP-3) for seven days. A battery of tested endpoints associated with photosynthetic pigments and oxidative stress was employed for a better understanding of the mode of action. The tested cyanobacterium could uptake the two BPs (27.4-54.9%) from culture media. The two BPs were able to inhibit the production of chlorophyll a (chl-a) and promote the accumulation of carotenoids, leading to unaffected chl-a autofluorescence. Slightly increased malondialdehyde (MDA) contents suggested that BP-1 and BP-3 caused moderate oxidative stress. BP-1 stimulated the activities of superoxide dismutase (SOD), glutathione reductase (GR) and glutathione S-transferase (GST) in M. aeruginosa while BP-3 increased the activities of SOD, GST, and glutathione (GSH), showing a concentration- and time-dependent relationship. The activities of other biomarkers, such as catalase (CAT) and glutathione peroxidase (GPx) fluctuated depending on exposure time and concentration. The overall results suggested that the two BPs can trigger moderate oxidative stress in M. aeruginosa and the tested cyanobacterium was capable of alleviating stress by different mechanisms.

Growth, physiological responses and microcystin-production/-release dynamics of Microcystis aeruginosa exposed to various luteolin doses

By testing time-dependent IC₅₀ of luteolin against Microcystis growth, this study revealed 6.5 mg/L as nearly IC₅₀ value during prolonged stress until day 14, and explored chlorophyll-a (CLA) and phycobiliproteins (PBPs) contents, antioxidant responses and microcystin (MC)-production/-release dynamics at rising luteolin doses (0.5~2-fold IC₅₀). Growth inhibition ratio (GIR) generally rose at rising luteolin dose, while at each dose GIR firstly increased and then leveled off or dropped. In early stage, CLA, allophycocyanin (APC), phycoerythrin (PE) and glutathione (GSH) contents, and superoxide dismutase (SOD) and catalase (CAT) activities, were increasingly stimulated at rising luteolin dose to enhance energy yield and antioxidant defense, but Microcystis was damaged more severely at rising dose, due to stress-repair imbalance. Such more severe damage in early stage, coupled with stronger PBPs-inhibition in mid-late stage, at rising dose could jointly account for rising GIR at rising dose. The CAT/GSH-stimulation persisting until late stage could alleviate cell damage in late stage, which explained for why GIR no longer increased in late stage at each luteolin dose. Besides, more MCs were produced and retained in cell to exert protective roles against luteolin-stress in early stage, but intracellular MCs decreased following inhibited MC-production by prolonged stress to decrease cell protectant. Extracellular MCs detection showed that less MCs amount existed in water phase than control along luteolin-stress, implying luteolin as eco-friendly algaecide with promising potential to remove MPM blooms and MC-risks. This is the first study to reveal the effect of various luteolin doses on MC-production/release and PBP-synthesis dynamics of Microcystis during prolonged stress. The findings shed novel views in anti-algal mechanisms of luteolin, and provided direct evidence for luteolin applied as safe agent to remediate Microcystis-dominant blooms.

A trigger mechanism of herbicides to phytoplankton blooms: From the standpoint of hormesis involving cytochrome b559, reactive oxygen species and nitric oxide

The cause of phytoplankton blooms has been extensively discussed and largely attributed to favorable external conditions such as nitrogen/phosphorus resources, pH and temperature. Here from the standpoint of hormesis response, we propose that phytoplankton blooms are initiated by stimulatory effects of low concentrations of herbicides as environmental contaminants spread over estuaries and lakes. The experimental results revealed general stimulations by herbicides on Microcystis aeruginosa and Selenastrum capricornutum, with the maximum stimulation in the 30-60% range, depending on the agent and experiment. In parallel with enhancing stimulation, the ratio of HP (high-potential) form to LP (low-potential) form of cytochrome b₅₅₉ (RHL) was observed decreasing, while intracellular reactive oxygen species (ROS) were observed increasing. We propose that the ROS originated from the thermodynamic transformation of cytochrome b₅₅₉, enhancing the stimulatory response. Furthermore, the results also proved that thermodynamic states of cytochrome b₅₅₉ could be modulated by nitric oxide, thus affecting cellular equilibrium of oxidative stress (OS) and correspondingly causing the inhibitory effect of higher concentrations of herbicides on phytoplankton. This suggests that hormesis substantially derives from equilibrium shifting of OS. Moreover, it is reasonable to infer that phytoplankton blooms would be motivated by herbicides or other environmental pollutants. This study provides a new thought into global phytoplankton blooms from a contaminant perspective.

Effects of Phenolic Pollution on Interspecific Competition between Microcystis aeruginosa and Chlorella pyrenoidosa and their Photosynthetic Responses

The demand for phenolic compounds has been increasing rapidly, which has intensified the production and usage of phenol at a commercial scale. In some polluted water bodies, phenol has become one of the typical aromatic contaminants. Such water bodies are inescapably influenced by nutrients from human activities, and also suffer from nuisance cyanobacterial blooms. While phenolic pollution threatens water safety and ecological balance, algal cells are ubiquitous and sensitive to pollutants. Therefore, effects of phenolic pollution on interspecific competition between a bloom-forming cyanobacterium and other common alga merit quantitative investigation. In this study, the effects of phenol on Microcystis aeruginosa (M. aeruginosa, a bloom-forming cyanobacterium) and Chlorella pyrenoidosa (C. pyrenoidosa, a ubiquitous green alga) were analyzed in mono- and co-cultures. The two species were exposed to a series of phenol treatments (0, 2, 20, and 200 μg mL⁻¹). Population dynamics were measured by a flow cytometer and analyzed by the Lotka-Volterra model. The results showed that M. aeruginosa was more sensitive to phenol (EC₅₀ = 80.8 ± 0.16 μg mL⁻¹) compared to C. pyrenoidosa (EC₅₀ = 631.4 ± 0.41 μg mL⁻¹) in mono-cultures. M. aeruginosa won in the co-cultures when phenol was below or equal to 20 μg mL⁻¹, while C. pyrenoidosa became the dominant species in the 200 μg mL⁻¹ treatment. Photosynthetic activity was measured by a fluometer. Results showed phenol significantly impacted the photosynthetic activity of M. aeruginosa by inhibiting the acceptor side of its photosystem II (PSII), while such inhibition in C. pyrenoidosa was only observed in the highest phenol treatment (200 μg mL⁻¹). This study provides a better understanding for predicting the succession of algal community structure in water bodies susceptible to phenolic contamination. Moreover, it reveals the mechanism on photosynthetic responses of these two species under phenolic stress.

Interactions between Microcystis aeruginosa and coexisting bisphenol A at different nitrogen levels

Microcystis aeruginosa is known as a main contributor of cyanobacterial bloom. However, factors that drive its formation and dispersion remain poorly understood. The cellular-level responses to nutrient drivers of eutrophication were investigated. The results showed that growth rate of M. aeruginosa was significantly enhanced with the increasing bisphenol A (BPA) and nitrogen (N) level. Stress of BPA significantly inhibited cellular density, chlorophyll-a content across all the nutrient conditions, while Fv/Fm and rETRmax value were promoted by BPA. Responses of reactive oxygen species (ROS) value, superoxide dismutase (SOD) activity and malodialdehyde (MDA) content indicated that nitrogen deficiency and BPA caused oxidative stress to M. aeruginosa. Besides, nitrogen and BPA regulated the production and release of microcystins (MCs). M. aeruginosa exposed to BPA caused 95 up-regulated proteins, which was primarily associated with photosynthesis, nitrogen metabolism, glycolysis/glyconeogenesis and carbon fixation in photosynthetic organisms. The 91 down-regulated proteins were related to quorum sensing, longevity regulating and cell cycle-caulobacter, confirming that the driving force of regulating the change of cellular density and genes expression weakened. These findings provide important clues to elucidate the combined regulatory mechanisms of cyanobacterial blooms triggered by endocrine-disrupting compounds and environmental factors and help to effectively prevent and reduce cyanobacterial blooms.

Interactions between Microcystis aeruginosa and coexisting bisphenol A at different phosphorus levels

Microcystis aeruginosa is known as the main contributor to cyanobacterial bloom, which is prevalent globally and degrades freshwater systems worldwide. The argument that the introduction of anthropogenic contaminants in fresh water stimulates cyanobacterial growth and microcystin production has attracted widespread attention. Bisphenol A (BPA), one of the most abundant endocrine-disrupting compounds, is often detected in various water bodies due to its notably high annual levels of production and use. Research on the combined effects of endocrine-disrupting compounds and environmental factors on cyanobacteria remains limited. To investigate the mechanism of interactions between contaminants and cyanobacteria at the cellular and proteomic levels, the growth rate, chlorophyll-a content, photosynthetic activities, microcystin-LR (MC-LR) production and release, reactive oxygen species (ROS) content, superoxide dismutase (SOD) activities, malondialdehyde (MDA) content, and proteome expression of M. aeruginosa under 1 μM BPA stress at a standard phosphorus level were investigated. The results showed that stress responses to BPA included increases in the growth rate, chlorophyll-a content, and Fv/Fm and rETRmax values under the low phosphorus condition. Responses involving ROS, SOD, and MDA indicated that phosphorus sufficiency and BPA caused oxidative stress in M. aeruginosa. Moreover, phosphorus sufficiency and BPA stimulated the production and release of MCs. Compared to levels in the non-BPA-treated group, exposure of M. aeruginosa to BPA caused 72 up-regulated proteins, which were primarily associated with photosynthesis, ribosome, fatty acid biosynthesis, glycolysis/glyconeogenesis, and carbon fixation in photosynthetic organisms. The 105 down-regulated proteins were related to quorum sensing, base excision repair, ABC transporters, longevity regulating and cell cycle-caulobacter, suggesting that the cytotoxicity of cyanobacterial cells induced by BPA was significantly increased. These findings provide insights into the molecular mechanism of the effects of BPA and phosphorus on M. aeruginosa, suggesting that coexisting pollutants may cause greater harm to and health risks in the environment.

Response of bloom-forming cyanobacterium Microcystis aeruginosa to 17β-estradiol at different nitrogen levels

Co-existence of cyanobacterial harmful algal blooms (CyanoHABs) and steroid estrogens (SEs) has been an increasing concern in eutrophic waters. The cellular responses and biodegradation of 17β-estradiol (E2) in cyanobacterium Microcystis aeruginosa were investigated at different nitrogen levels. During the 10-d experiment, the growth of M. aeruginosa was stimulated by 10-100 μg L^-1 of E2 at the lowest nitrogen level of 0.5 mg L^-1, whereas the presence of E2 inhibited the cyanobacterial growth at 5 mg L^-1 of nitrogen. With nitrogen concentration increased to 50 mg L^-1, the impact of E2 on levels of growth rate and chlorophyll a (Chla) alleviated. Exposure to E2 also promoted the superoxide dismutase activity of M. aeruginosa, coupled with cellular oxidative damage as indicated by the increasing malondialdehyde content. A sufficient nitrogen supply mitigated the oxidative stress of E2 through enhancing the synthesis of detoxification-related enzymes. Simultaneously, the secretion of tryptophan-like substances in loosely- and tightly-bound extracellular polymeric substances was triggered for adapting to an E2 addition in the short term. Moreover, significant biodegradation of E2 was observed, and the process followed a first-order kinetic reaction. The obtained half-lives decreased with nitrogen levels and ranged from 2.47 to 2.81 and 3.39-5.04 d, respectively, at 10 and 100 μg L^-1 of E2. These results provide a better understanding of the potential effects of SEs on CyanoHABs formation, as well as the important role of CyanoHABs on SEs removal in aquatic ecosystems, which should be fully considered in the control of combined pollution.

Effect of elevated benzophenone-4 (BP4) concentration on Chlorella vulgaris growth and cellular metabolisms

Benzophenone-4 (BP4), as the raw material of common sunscreen products, usually shows strong eco-toxicity and endocrine-disrupting activity in aquatic animals. However, the potential adverse effect of BP4 on aquatic vegetation is still unclear. In order to evaluate the inhibitory effect of BP4 on phytoplankton, wild and acclimated Chlorella vulgaris was used as representative aquatic plant cells and experimental studies were conducted on the characteristics of its growth and cellular metabolisms upon exposure to elevated BP4 concentrations (1, 5, 10, 20, 50, and 100 mg L^-1). C. vulgaris basically appeared low sensitivity to BP4 exposure because the 96-h EC₅₀ was measured as 65.16 mg L^-1 for its wild type. The 96-h EC₅₀ of the acclimated type, which was pre-exposed to 10 mg L^-1 of BP4 and transferred twice, was 140.76 mg L^-1. By cellular response tests regarding non-enzymatic antioxidants carotenoid content, malondialdehyde (MDA), enzyme antioxidant superoxide dismutase (SOD) activity, and the photosynthetic efficiency, it was clarified that increasing exposure concentration elevated the hindrance to cellular metabolism. However, the rate of BP4 utilization as substrates for C. vulgaris growth showed a trend of decreasing with increasing BP4 concentration. The higher 96-h EC₅₀ value of the acclimated C. vulgaris to BP4 inhibition than the wild C. vulgaris showed the enhanced tolerance capability; however, the continuous stress response of acclimated type should be taken into account when using microalgae species for toxicity assessment.

Modelling the effects of PSII inhibitor pulse exposure on two algae in co-culture

A weakness of standard testing procedures is that they do not consider interactions between organisms, and they focus only on single species. Furthermore, these procedures do not take into account pulse exposure. However, pulse exposure is of particular importance because in streams, after crop application and during and after precipitation, herbicide concentrations fluctuate widely and can exceed the Annual Average Environmental Quality Standards (AA-EQS), which aim to protect the aquatic environment. The sensitivity of the algae Scenedesmus vacuolatus and Pseudokirchneriella subcapitata in a co-culture exposed to pulses is thus analysed in this study. As a first step, the growths of the algae in co-culture are investigated. For initial cell densities fixed, respectively, to 100,000 and 50,000 cells/mL, the growth of each alga is exponential over at least 48 h. S. vacuolatus seems to influence the growth of P. subcapitata negatively. Allelopathy is a possible explanation for this growth inhibition. The toxicity of the herbicide isoproturon is later tested on the algae S. vacuolatus and P. subcapitata cultured alone and in the co-culture. Despite the supplementary stress on the algae in the co-culture competing for nutrients, the toxicity of the herbicide is lower for the two algae when they are in the co-culture than when they are in separated culture. A model is adapted and used to predict the cell-density inhibition on the alga S. vacuolatus in the co-culture with the alga P. subcapitata exposed to a pulse concentration of isoproturon. Four laboratory experiments are performed to validate the model. The comparison between the laboratory and the modelled effects shows good agreement. The differences can be considered minor most of time. For future studies, it is important to ensure that the cell count is precise, as it is used to determine the parameters of the model. The differences can be also induced by the fact that the cell number of the alga P. subcapitata re-suspended in a new OECD medium after the centrifugation process cannot be fixed.

Effects of benzophenone-3 on the green alga Chlamydomonas reinhardtii and the cyanobacterium Microcystis aeruginosa

Effects of benzophenone-3 (BP-3) on the green alga, Chlamydomonas reinhardtii, and the cyanobacterium, Microcystis aeruginosa, were investigated. The tested organisms were exposed to environmental levels of BP-3 for 10 days, at nominal concentrations from 0.01 to 5000μgL^-1. Specific growth rate and photosynthetic pigments were employed to evaluate the toxic responses. The two tested algae had distinct toxic responses towards BP-3 stress, with the green alga C. reinhardtii being more sensitive than the cyanobacterium M. aeriginosa, based on EC₂₀ and EC₅₀ values. Uptake of BP-3 from the medium occurred in both species, with M. aeruginosa showing greater overall uptake (27.2-77.4%) compared to C. reinhardtii (1.1-58.4%). The effects of BP-3 on C. reinhardtii were variable at concentrations lower than 100μgL^-1. At higher concentrations, the specific growth rate of C. reinhardtii decreased following a reduction in chlorophyll a (chl-a) content. Further experiments showed that BP-3 regulated the growth of C. reinhardtii by affecting the production of chl-a, chlorophyll b and carotenoids. In M. aeruginosa, specific growth rate was only moderately affected by BP-3. Additionally, the production of chl-a was significantly inhibited over the different exposure concentrations, while the production of carotenoids was stimulated. These results indicate a potential detrimental effect on prokaryotes and eukaryotes and that the mechanism of action varies with species.

Interactions between Microcystis aeruginosa and coexisting amoxicillin contaminant at different phosphorus levels

Microcystis aeruginosa was cultured with 0.05-5 mg L(-1) of phosphorus and exposed to 200-500 ng L(-1) of amoxicillin for seven days. Amoxicillin presented no significant effect (p>0.05) on the growth of M. aeruginosa at phosphorus levels of 0.05 and 0.2 mg L(-1), but stimulated algal growth as a hormesis effect at phosphorus levels of 1 and 5 mg L(-1). Phosphorus and amoxicillin affected the contents of chlorophyll-a, adenosine triphosphate (ATP) and malondialdehyde, the expression of psbA and rbcL, as well as the activities of adenosinetriphosphatase and glutathione S-transferase in similar manners, but regulated the production and release of microcystins and the activities of superoxide dismutase and peroxidase in different ways. Increased photosynthesis activity was related with the ATP consumption for the stress response to amoxicillin, and the stress response was enhanced as the phosphorus concentration increased. The biodegradation of amoxicillin by M. aeruginosa increased from 11.5% to 28.2% as the phosphorus concentration increased. Coexisting amoxicillin aggravated M. aeruginosa pollution by increasing cell density and concentration of microcystins, while M. aeruginosa alleviated amoxicillin pollution via biodegradation. The interactions between M. aeruginosa and amoxicillin were significantly regulated by phosphorus (p<0.05) and led to a complicated situation of combined pollution.

Antioxidant enzymes are induced by phenol in the marine microalga Lingulodinium polyedrum

Knowing the impacts of different anthropogenic activities on ecosystems promotes preservation of aquatic organisms. Aiming to facilitate the identification of polluted or contaminated areas, the study of microalga Lingulodinium polyedrum in phenol-containing medium comprises the determination of toxic and metabolic phenol effects, featuring a possible use of this microorganism as bioindicator for this pollutant. Marine microalga L. polyedrum exposure to phenol increases superoxide dismutase (SOD) and catalase (CAT) activities. The 20% and 50% inhibitory concentrations (IC20 and IC50) of cells exposed to phenol were 40 μmol L(-1) and 120 μmol L(-1), respectively. Phenol biodegradation by L. polyedrum was 0.02 μmol h(-1)cell(-1), and its biotransformation was catalyzed by glutathione S-transferase (GST), phenol hydroxylase and catechol 2,3-dihydroxygenase metabolic pathways. Phenol exposure produced the metabolites 2-hydroxymuconic semialdehyde acid, 1,2-dihydroxybenzene (catechol), and 2-oxo-4-pentenoic acid; also, it induced the activity of key antioxidant biomarker enzymes SOD and CAT by three folds compared to that in the controls. Further, phenol decreased the glutathione/oxidized glutathione ratio (GSH/GSSG), highlighting the effective glutathione oxidation in L. polyedrum. Overall, our results suggest that phenol alters microalga growth conditions and microalgae are sensitive bioindicators to pollution by phenol in marine environments.

Rapid fluorescence-based measurement of toxicity in anaerobic digestion

A rapid fluorescence measurement based on resazurin reduction was developed and applied for the detection of toxicants/inhibitors to anaerobic digestion metabolism. By initially using a pure facultative anaerobic strain, Enterococcus faecalis as a model organism, this technique proved to be fast and sensitive when detecting the model toxicant, pentachlorophenol (PCP). The technique revealed significant metabolic changes in Enterococcus faecalis with a PCP spike ranging from 0.05 to 100 mg/L, and could detect PCP's toxicity to E. faecalis at a concentration of only 0.05 mg/L in 8 min. Furthermore, by extending this technique to a mixed anaerobic sludge, not only could the effect of 0.05-100 mg/L PCP be determined on anaerobic digestion metabolism within 10 min, but also its rate of biogas production. These results suggest that a resazurin-based fluorescence measurement can potentially be incorporated into a microfluidic system to develop a biosensor for the real-time monitoring, control and early warning of toxicant/inhibitor loads in the influent to an anaerobic digestion system.

Hormesis effects of amoxicillin on growth and cellular biosynthesis of Microcystis aeruginosa at different nitrogen levels

Coexisting antibiotic contaminants have potential to regulate cyanobacterial bloom, and the regulation is likely affected by nitrogen supply. A typical cyanobaterium Microcystis aeruginosa was cultured with 0.05-50 mg L(-1) of nitrogen and exposed to 100-600 ng L(-1) of amoxicillin for 7 days. Algal growth was not significantly (p > 0.05) affected by amoxicillin at the lowest nitrogen level of 0.05 mg L(-1), stimulated by 600 ng L(-1) of amoxicillin at a moderate nitrogen level of 0.5 mg L(-1) and enhanced by 100-600 ng L(-1) of amoxicillin at higher nitrogen levels of 5-50 mg L(-1). Amoxicillin affected chlorophyll-a, psbA gene, and rbcL gene in a similar manner as algal growth, suggesting a regulation of algal growth via the photosynthesis system. At each nitrogen level, synthesis of protein and polysaccharides as well as production and release of microcystins (MCs) increased in response to environmental stress caused by amoxicillin. Expression of ntcA and mcyB showed a positive correlation with the total content of MCs under exposure to amoxicillin at nitrogen levels of 0.05-50 mg L(-1). Nitrogen and amoxicillin significantly (p < 0.05) interact with each other on the regulation of algal growth, synthesis of chlorophyll-a, production and release of MCs, and expression of ntcA and mcyB. The nitrogen-dependent stimulation effect of coexisting amoxicillin contaminant on M. aeruginosa bloom should be fully considered during the combined pollution control of M. aeruginosa and amoxicillin.

Cellular responses and biodegradation of amoxicillin in Microcystis aeruginosa at different nitrogen levels

The influence of nitrogen on the interactions between amoxicillin and Microcystis aeruginosa was investigated using a 7-day exposure test. Growth of M. aeruginosa was not significantly (p>0.05) affected by amoxicillin at the lowest nitrogen level of 0.05 mg L(-1), stimulated by 500 ng L(-1) of amoxicillin at a moderate nitrogen level of 0.5 mg L(-1) and enhanced by 200-500 ng L(-1) of amoxicillin at the highest nitrogen level of 5 mg L(-1). The generation of reactive oxygen species (ROS) and the synthesis of glutathione S-transferases (GST) and glutathione (GSH) were more sensitive to amoxicillin and were stimulated at all nitrogen levels. At the lowest nitrogen level of 0.05 mg L(-1), superoxide dismutase and peroxidase were not effective at eliminating amoxicillin-induced ROS, resulting in the highest malondialdehyde content in M. aeruginosa. The biodegradation of 18.5-30.5% of amoxicillin by M. aeruginosa was coupled to increasing GST activity and GSH content. Elevated nitrogen concentrations significantly enhanced (p<0.05) the stimulation effect of amoxicillin on the growth of M. aeruginosa, the antioxidant responses to amoxicillin and the biodegradation of amoxicillin in M. aeruginosa. The nitrogen-dependent hormesis effect of the coexisting amoxicillin contaminant on the M. aeruginosa bloom should be fully considered during the control of M. aeruginosa bloom.

Pentachlorophenol toxicity to a mixture of Microcystis aeruginosa and Chlorella vulgaris cultures

Pentachlorophenol (PCP) is a priority pollutant due to its persistence and high toxicity. For the first time, PCP effects were investigated at laboratory scale on co-cultures of two ubiquitous freshwater phytoplankton species: the cyanobacterium Microcystis aeruginosa and the microalgae Chlorella vulgaris. The cells were exposed to environmental levels of PCP for 10 days in Fraquil culture medium, at nominal concentrations from 0.1 to 10,000 μg L(-1). Growth was assessed by area under growth curve (cell count vs. time). The phytoplankton community structure can be changed as a consequence of a PCP contamination. Low μg L(-1) levels of PCP are advantageous to M. aeruginosa. This is the first report of the promoting effect of PCP on the growth of aquatic cyanobacteria, using mixtures with microalgae. As a result of the direct toxic effects of high PCP concentrations on M. aeruginosa, C. vulgaris cell count increased given that in biological controls M. aeruginosa inhibited the C. vulgaris growth. At 16.7 mg L(-1), PCP already had direct toxic effects also on the microalga. The pH of culture medium tended to decrease with increasing PCP concentrations, which was mostly related to the growth inhibition of cyanobacterium caused by PCP. The PCP concentration was stable in the co-cultures, which differed from what has been observed in monocultures of the same two species. Short-term laboratory assays with two phytoplankton species gives important information on the species interactions, namely possible direct and indirect effects of a toxicant, and must be considered in ecotoxicity studies regarding environmental extrapolations.