Combined effects of sulfamethazine and sulfamethoxazole on a freshwater microalga, Scenedesmus obliquus: toxicity, biodegradation, and metabolic fate

This study investigated the environmental effects of two common emerging contaminants, sulfamethazine (SMZ) and sulfamethoxazole (SMX), and their mixture using a green microalga, Scenedesmus obliquus. The calculated EC₅₀ values of SMZ, SMX, and their mixture (11:1 wt/wt) after 96 h were 1.23, 0.12, and 0.89 mg L^-1, respectively. The toxicity of the mixture could be better predicted using a concentration addition model than an independent action model. The risk quotients of SMZ, SMX, and their mixture were >1 during the experiment, indicating their high potential risks on aquatic microorganisms. Despite their toxicity, S. obliquus exhibited 17.3% and 29.3% removal of 0.1 mg L^-1 and 0.2 mg L^-1 after 11 days of cultivation. The changes of SMZ and SMX removal were observed when combined, which showed a significantly improved removal of SMZ (up to 3.4 folds) with addition of SMX (0.2 mg L^-1). The metabolic pathways of SMZ and SMX were proposed according to mass spectroscopic analysis, which showed six metabolites of SMX and seven intermediates of SMZ, formed as a result of ring cleavage, hydroxylation, methylation, nitrosation, and deamination.

Influences of carbon and nitrogen sources and metal ions on the heterotrophic culture of Scenedesmus sp. LX1

In this study, the influences of organic carbon sources (OCS, including xylose, glucose, maltose, sucrose, and starch) and inorganic and organic nitrogen sources (INS, including ammonia chloride and sodium nitrate; ONS, including arginine, alanine, proline, and valine) and metal ions (including Na⁺, K⁺, Mn²⁺, Zn²⁺ and Cu²⁺) on the growth, lipid accumulation, and nitrogen and phosphorus (N&P) removal capabilities of oleaginous Scenedesmus sp. LX1 under heterotrophic conditions were investigated. The results showed that glucose was the only OCS for Scenedesmus sp. LX1 to grow well with specific growth rate of 0.935 days^-1, maximum biomass of 1.72 g L^-1, and largest removal rates of N&P and organic carbon reaching 72.228%, 93.034%, and 19.208%, respectively. After 11 days of cultivation, the maximal biomass reached in the group with starch or glucose while maximal lipid and triacylglycerol (TAG) yields reached in the groups with maltose and sucrose, respectively. Sodium nitrate was best nitrogen source as the largest algal density, maximal yields of lipids and TAGs, and highest N&P removal rates reached up to 1.105 × 10⁷ cells·mL^-1, 196.70 mg L^-1, 5.19 mg L^-1, 89.61% and 100%, respectively. Scenedesmus sp. LX1 was found to have great tolerance to Na⁺, K⁺, Mn²⁺, and Zn²⁺ while 0.5 mg L^-1 Cu²⁺ had a strong inhibition on growth and N&P removal rate of Scenedesmus sp. LX1. Concentration increasing of five metal ions all caused the yield increases of microalgal lipid and TAGs. Graphical abstract.

Evaluation of the novel nanoparticle material - CdSe quantum dots on Chlorella pyrenoidosa and Scenedesmus obliquus: Concentration-time-dependent responses

Quantum dots (QDs), as a kind of novel nanomaterial, have the extensive applications in various fields, inevitably leading to increasing risks for the ecological environment. The mobilization of cadmium including metal smelting and subsequent machining for multifarious applications has caused the release of cadmium element into the environment. In this study, we evaluated the potential toxicity of a novel nanoparticle material CdSe QDs, using two green algae Chlorella pyrenoidosa and Scenedesmus obliquus. The impact of CdSe QDs and cadmium ions on algae and the sensitivity of the two algae on target compounds were also considered and compared. Our results showed the algal growth rates and chlorophyll content decreased with increasing exposure concentrations and durations. Moreover, the glutathione levels were decreased while the activities of superoxide dismutase increased, exhibiting their pivotal functions in defeating toxic stress. The increment of malondialdehyde levels revealed that the stresses of CdSe QDs and cadmium ions were contributed to the occurrence of oxidative damage. Our study also indicated that the impact of CdSe QDs was stronger than that of cadmium nitrate and the algal response was also species-specific. In addition, the TEM photographs of the algal ultrastructure showed the presence of surface attachment and uptake of QDs.

Northern green algae have the capacity to remove active pharmaceutical ingredients

Eight recently isolated microalgal species from Northern Sweden and the culture collection strain Scenedesmus obliquus RISE (UTEX 417) were tested for their ability to remove 19 pharmaceuticals from growth medium upon cultivation in short light path, flat panel photobioreactors. While the growth of one algal species, Chlorella sorokiniana B1-1, was completely inhibited by the addition of pharmaceuticals, and the one of Scenedesmus sp. B2-2 was strongly inhibited, the other algal strains grew well and produced biomass. In general, lipophilic compounds were removed highly efficient from the culture medium by the microalgae (>70% in average within 2 days). The most lipophilic compounds Biperiden, Trihexyphenidyl, Clomipramine and Amitriptyline significantly accumulated in the biomass of most algal species, with a positive correlation between accumulation and their total biomass content. More persistent in the growth medium were hydrophilic compounds like Caffeine, Fluconazole, Trimetoprim, Codeine, Carbamazepin, Oxazepam and Tramadol, which were detected in amounts of above 60% in average after algal treatment. While Coelastrella sp. 3-4 and Coelastrum astroideum RW10 were most efficient to accumulate certain compounds in their biomass, two algae species, Chlorella vulgaris 13-1 and Chlorella saccharophila RNY, were not only highly efficient in removing all 19 pharmaceuticals from the growth medium within 12 days, at the same time only small amounts of these compounds accumulated in their biomass allowing its further use. Chlorella vulgaris 13-1 was able to remove most compounds within 6 days of growth, while Chlorella saccharophila RNY needed 8-10 days."Wild" Nordic microalgae therefore are able to remove active pharmaceutical ingredients, equally or more efficient than the investigated culture collection strain, thereby demonstrating their possible use in sustainable wastewater reclamation in Nordic conditions.

Influence of growth phase on the harvesting of Scenedesmus acuminatus using ultrafiltration

Cellular characteristics and algogenic organic matter (AOM) properties change with culture time. This study aims to understand the changes throughout the growth phase, and their effect on Scenedesmus acuminatus harvesting using ultrafiltration. The variations in cellular particle size distribution, cellular EPS content, and the biochemical composition and molecular weight of AOM were analyzed, followed by the membrane harvesting of the original S. acuminatus suspension, AOM-free cells and cell-free AOM. The results showed that the average flux for the original suspension increased with growth phase and reached an increase of 36.3% in the declining phase. AOM played a greater role than S. acuminatus cells in flux decline for all growth phases. Exponential-phase AOM contained a greater high-MW fraction and more carbohydrates, and the exponential cells were smaller cells and had a higher EPS content; these characteristics resulted in a reduced average flux.

Ecotoxicological effects on Scenedesmus obliquus and Danio rerio Co-exposed to polystyrene nano-plastic particles and natural acidic organic polymer

The importance of attention to unravel the interaction of nano-plastic particles (NPs) with natural acidic organic polymer (NAOP) in freshwater environment should not be neglected. However, toxicological data available for the interaction between NPs and NAOP remain limited. Here, we investigate the toxicological effects of three model polystyrene (PS) NPs with different functional groups (unmodified, amino- and carboxyl-modified PS NPs) on two freshwater organisms of different trophic levels (Scenedesmus obliquus and Danio rerio) in the absence and presence of two classes of NAOP, namely fulvic acid and humic acid. The NAOP interaction with the NPs is shown to alter oxidative stress and disturb membrane function in S. obliquus cells to a certain extent. Combined oxidative stress responses to the NPs and NAOP in D. rerio as a function of their mixture levels showed inhibition, alleviation, and reinforce. Changes in cellular oxidative stress and membrane function depended on the concentration and types of both NPs and NAOP. Furthermore, the characterization parameters of the NPs were important for the explanation of the ecotoxicological mechanism of the NPs in the presence of NAOP. Our findings emphasized the critical role of NAOP in the fate and toxicity of plastic particles in freshwater environment.

Optimising of Scenedesmus sp. biomass production in chicken slaughterhouse wastewater using response surface methodology and potential utilisation as fish feeds

Production of Scenedesmus sp. biomass in chicken slaughterhouse wastewater (CSWW) is a promising alternative technique for commercial culture medium due to the high nutritional content of the generated biomass to be used as fish feeds. The current work deals with optimising of biomass production in CSWW using response surface methodology (RSM) as a function of two independent variables, namely temperature (10-30 °C) and photoperiod (6-24 h). The potential application of biomass yield as fish feeds was evaluated based on carbohydrate, protein and lipid contents. The results revealed that the best operating parameters for Scenedesmus sp. biomass production with high contents of carbohydrates, proteins and lipids were determined at 30 °C and after 24 h. The actual and predicted values were 2.47 vs. 3.09 g, 1.44 vs. 1.27 μg/mL, 29.9 vs. 31.60% and 25.75 vs. 28.44%, respectively. Moreover, the produced biomass has a high concentration of fatty acid methyl ester (FAME) as follows: 35.91% of C15:1; 17.58% of C24:1 and 14.11% of C18:1N9T. The biomass yields have 7.98% of eicosapentaenoic acid (EPA, C20:5N3) which is more appropriate as fish feeds. The Fourier transform infrared (FTIR) analysis of biomass revealed that the main functional groups included hydroxyl (OH), aldehyde (=C-H), alkanes and acyl chain groups. Scanning electron micrograph (SEM) and energy-dispersive X-ray spectroscopic analysis (EDS) indicated that the surface morphology and element distribution in biomass produced in BBM and CSWW were varied. The findings have indicated that the biomass produced in CSWW has high potential as fish feeds.

Toxicity of sulfamethazine and sulfamethoxazole and their removal by a green microalga, Scenedesmus obliquus

A comprehensive ecotoxicological evaluation of a sulfamethazine (SMZ) and sulfamethoxazole (SMX) mixture was conducted using an indicator microalga, Scenedesmus obliquus. The toxicological effects of this mixture were studied using microalgal growth patterns, biochemical characteristics (total chlorophyll, carotenoid, carbohydrate, fatty acid methyl ester), and elemental and Fourier-transform infrared spectroscopy analyses. The 96-h half maximal effective concentration (EC₅₀) of the SMZ and SMX mixture was calculated to be 0.15 mg L^-1 according to the dose-response curves obtained. The chlorophyll content decreased with elevated SMZ and SMX concentrations, while the carotenoid content initially increased and then decreased as concentration raised. The unsaturated fatty acid methyl esters (FAMEs) content was enhanced with higher SMZ and SMX concentrations, while that of saturated FAMEs simultaneously decreased due to SMZ and SMX stress. Elemental analyses showed an improved percentage of nitrogen and sulfur in the microalgal biomass as SMZ and SMX concentrations increased. The microalga S. obliquus was shown to biodegrade the chemicals tested and removed 31.4-62.3% of the 0.025-0.25 mg SMZ L^-1 and 27.7-46.8% of the 0.025-0.25 mg SMX L^-1 in the mixture after 12 days of cultivation. The greater biodegradation observed at higher SMZ and SMX concentrations indicates that microalgal degradation of SMZ and SMX could act as an efficient adaptive mechanism to antibiotics.

Growth inhibition and oxidative stress caused by four ionic liquids in Scenedesmus obliquus: Role of cations and anions

Ionic liquids (ILs) are widely used in various industrial applications. However, they are considered potential toxins in aquatic environments because of their physical stability and solubility. The growth inhibition and oxidative stress induced by four ionic liquids with different cations and anions on the green algae Scenedesmus obliquus was investigated in this study. The order of growth inhibition was 1‑hexyl‑3‑methylimidazolium nitrate ([HMIM]NO₃) > 1‑hexyl‑3‑methylimidazolium chloride ([HMIM]Cl) > N‑hexyl‑3‑metylpyridinium bromide ([HMPy]Br) > N‑hexyl‑3‑metylpyridinium chloride ([HMPy]Cl). Imidazolium IL had a higher growth inhibition effect than pyridinium IL, nitrate IL and bromide IL had a higher effect than chloride IL. Reactive oxygen species (ROS) level in S. obliquus increased with increasing IL concentrations. Green fluorescence in [HMIM]Cl treated algae showed increased brightness compared to the [HMPy]Cl treatment, and [HMIM]NO₃ treatment produced increased brightness compared to the [HMPy]Br treatment, suggesting that higher ROS levels were induced by [HMIM]Cl and [HMIM]NO₃. Soluble protein, catalase (CAT), and superoxide dismutase (SOD) activities were stimulated at lower concentrations but were inhibited at higher concentrations. Regression analysis suggested that ROS level is the main index responsible for oxidative stress induced by the four ILs. The ILs induced oxidative damage on S. obliquus, and ROS in high concentration treatments could not be effectively removed by the antioxidant system, leading to oxidative damage and ultimately resulting in growth inhibition and cell death.

Metabolites change of Scenedesmus obliquus exerted by AgNPs

With increasing emission of silver nanoparticles (AgNPs) into the environment, it is important to understand the effects of ambient concentration of AgNPs. The biological effects of AgNPs on Scenedesmus obliquus, a ubiquitous freshwater microalgae, was evaluated. AgNPs exerted a minor inhibitory effect at low doses. Non-targeted metabolomic studies were conducted to understand and analyze the effect of AgNPs on algal cells from a molecular perspective. During the 48 hr of exposure to AgNPs, 30 metabolites were identified, of which nine had significant changes compared to the control group. These include d-galactose, sucrose, and d-fructose. These carbohydrates are involved in the synthesis and repair of cell walls. Glycine, an important constituent amino acid of glutathione, increased with AgNP exposure concentration increasing, likely to counteract an increased intracellular oxidative stress. These results provide a new understanding of the toxicity effects and mechanism of AgNPs. These metabolites could be useful biomarkers for future research, employed in the early detection of environmental risk from AgNPs.

High temperature promotes the inhibition effect of Zn2+ on inducible defense of Scenedesmus obliquus

Morphological defense is assumed to be an effective anti-grazer strategy in phytoplankton. Scenedesmus obliquus, a globally widespread freshwater chlorophyte, can form colonies in response to the infochemicals of herbivorous zooplankton and survive in coexistence with grazers. However, the inducible defense response is often disturbed by abiotic or biotic factors, especially under the increasing global warming and environmental pollution. In this study, two nonlethal environmental factors, namely, elevated temperature and environmentally relevant Zn²⁺ concentrations, decreased colony formation of S. obliquus induced by Daphnia grazing infochemicals. Elevated temperature (30 °C) reduced the inducible colony size and shortened the maintenance time of defensive colonies. Decreased colony size was detected with increased Zn²⁺ concentration. Colony formation was inhibited even at low Zn²⁺ concentration (0.131 μmol L^-1), which neither retarded growth nor affected photosynthesis. Warming promoted the inhibition effect of Zn²⁺ on inducible colony formation of S. obliquus. Warming also enhanced Zn²⁺ toxicity, which caused the growth rate of S. obliquus to be hindered by high Zn²⁺ concentrations at elevated temperature. Specially, S. obliquus which formed inducible colonies under the condition of Daphnia infochemicals had higher tolerance to Zn²⁺ toxicity and thus likely exerted protective effects against heavy metals. The results indicated the combined effects of global warming and heavy-metal pollution result in more severe impact on the inducible defense of S. obliquus.

Effects of aqueous extracts from the rhizome of Pontederia cordata on the growth and interspecific competition of two algal species

Single and co-culture systems of Microcystis aeruginosa and Scenedesmus obliquus were prepared with different initial algal densities and treated with different concentrations of aqueous extracts from the rhizome of Pontederia cordata to study its inhibitory effect on algal growth and the competitive relationship between these two algal species. The results showed that aqueous extracts could inhibit the growth of M. aeruginosa and S. obliquus, and the inhibition rate of aqueous extracts on the growth of M. aeruginosa was always higher than that of S. obliquus. A Lotka-Volterra competition model revealed that these two algal species can co-exist without the addition of aqueous extracts, and S. obliquus exhibited a stronger ability to compete than that of M. aeruginosa. Meanwhile, the dominant algal species changed with the addition of aqueous extracts regardless of the initial ratios of the two algae. The species ratio of mixed cultures had a strong effect on the interspecific interaction between the two algae. The higher proportion of S. obliquus in the initial proportion of two algae, the stronger competitive ability of S. obliquus when compared with that of M. aeruginosa.

Stereoselective bioactivity, acute toxicity and dissipation in typical paddy soils of the chiral fungicide propiconazole

Propiconazole is a widely used systemic agricultural triazole fungicide with two chiral centers. In the present study, systemic assessments of propiconazole stereoisomers are reported for the first time, including absolute configuration, stereoselective bioactivity toward pathogens (Ustilaginoidea virens, Magnaporthe oryzae, Fusarium moniliforme, Thanatephorus cucumeris, and Rhizoctonia solani), and stereoselective acute toxicity toward aquatic organisms (Scenedesmus obliquus, and Daphnia magna). Moreover, the stereoselective dissipation of propiconazole in three types of paddy soil under laboratory-controlled conditions (aerobic, anaerobic and sterile) was investigated. The degree of bioactivity and acute toxicity of the propiconazole stereoisomers differed depending on the type of target pathogens and non-target organisms. There were 2.43-23.47 and 1.48-2.13 fold differences between the best and worst stereoisomer in bioactivity and toxicity, respectively. Under aerobic conditions, (2S,4S)-propiconazole and (2S,4R)-propiconazole were preferentially degraded in the three types of soils. However, no significant stereoselectivity was observed under anaerobic and sterile conditions. Propiconazole was configurationally stable throughout the study. In comprehensive consideration of bioactivity, toxicity and environmental behavior, using stereoisomer mixture rather than pure stereoisomer may help to control more species of disease in practical application, and the stereoselectivity should be taken into consideration in risk assessment.

Microalgal cultivation for biofertilization in rice plants using a vertical semi-closed airlift photobioreactor

Nitrogen (N) is one of the most important limiting factors in conventional rice (Oryza sativa) production, which heavily relies on synthetic fertilizers. In this study, we researched on the development and use of a vertical semi-closed airlift photobioreactor (PBR) for microalgal cultivation and subsequently determined the efficacy of microalgae-based fertilizers to rice plant growth. The PBR system was developed to produce two strains of N2-fixing cyanobacteria (Anabaena sp. UTEX 2576, Nostoc muscorum UTEX 2209S), and a polyculture of Chlorella vulgaris (UTEX 2714) and Scenedesmus dimorphus (UTEX 1237). When these biofertilizers were evaluated for rice under the greenhouse conditions, results showed that the rice plant heights treated with polyculture-based microalgal biomass were similar to or better than the urea treatment. The effects of the inoculation of the N2-fixing cyanobacterial inoculation on seedling growth was not statistically significant. In conclusion, the vertical semi-closed system PBR cultivation method developed in this study proved to be a simple and effective method for cultivating microalgae. Demonstration of the reliable production system for N2-fixing cyanobacteria and chlorophytes at a medium scale could potentially open the future application of microalgal biofertilizers in rice production.

Removal, biotransformation and toxicity variations of climbazole by freshwater algae Scenedesmus obliquus

Climbazole (CBZ) is an antibacterial and antifungal agent widely used in personal care products. In this study, we investigated the interactions between climbazole (CBZ) and freshwater microalgae Scenedesmus obliquus (S. obliquus). Dose-effect relationships between CBZ concentrations and growth inhibitions or chlorophyll a content were observed. After 12 days of incubation, the algae density and chlorophyll a content in 2 mg/L treatment group was 56.6% and 15.8% of those in the control group, respectively. Biotransformation was the predominant way to remove CBZ in the culture solution, whereas the contribution of bioaccumulation and bioadsorption were negligible. More than 88% of CBZ was removed by S. obliquus across all treatments after 12 days of incubation, and the biotransformation of CBZ followed the first order kinetic model with half-lives of approximately 4.5 days at different treatments. CBZ-alcohol (CBZ-OH) was the only biotransformation product identified in algal solution. Moreover, the toxicity of biotransformation products was much lower than its corresponding precursor compound (CBZ). The results of this study revealed that S. obliquus might have a great impact on the environmental fates of CBZ and could be further applied to remove organic pollutants in aquatic environment.

Carbon-dioxide biofixation and phycoremediation of municipal wastewater using Chlorella vulgaris and Scenedesmus obliquus

The pure cultures of microalgae Chlorella vulgaris ATCC 13482 and Scenedesmus obliquus FACHB 417 were grown in municipal wastewater in 7-L airlift bubble column photobioreactor supplied with 5% CO₂/air (v/v). Batch experiments were conducted at 25 °C with 14-h light/10-h dark cycle for a period of 10 days. The CO₂ capture efficiencies for both the microalgae were monitored in terms of their respective biomass productivities, carbon contents, and CO₂ consumption rates. In the present study, the initial concentration of ammonia (43.7 mg L^-1) was decreased to 2.9 and 3.7 mg L^-1 by C. vulgaris and S. obliquus, respectively. And, the initial concentration of phosphate (18.5 mg L^-1) was decreased to 1.1 and 1.6 mg L^-1 by C. vulgaris and S. obliquus, respectively. CO₂ biofixation rates by C. vulgaris and S. obliquus, cultivated in municipal wastewater, were calculated to be 140.91 and 129.82 mg L^-1 day^-1, respectively. The findings from the present study highlight the use of microalgae for wastewater treatment along with CO₂ uptake and biomass utilization for pilot scale production of biodiesel, biogas, feed supplements for animals, etc., thus minimizing the production costs.

The growth of Scenedesmus quadricauda in RO concentrate and the impacts on refractory organic matter, Escherichia coli, and trace organic compounds

This study achieves a better operational simplicity for the phycoremediation of reverse osmosis (RO) concentrate using Scenedesmus quadricauda microalgae. Under continuous illumination with CO₂ supplementation, algal growth in the RO concentrate resulted in a conversion of polymeric organic matter (a mixture of humic substances and polysaccharides) to biodegradable fractions and their prompt removal along with inorganic nutrients (NO₃^- and PO₄^3-). The algal-induced degradation of humic-like substances which are typically refractory to microbial decomposition was demonstrated in an indirect manner. In this study, we also investigated the effects of algal treatment on the growth of Escherichia coli and removal of trace organic compounds (TOrCs) from the RO concentrate. Our results indicate that algal treatment of the RO concentrate using aeration with 10% (v/v) CO₂ under continuous illumination is highly feasible as a safe and inexpensive technology to remove non- or slowly-biodegradable organic matter, reduce enteric bacteria, and attenuate TOrCs in wastewater. However, the results should not be generalized, but critically discussed, due to limitations of using the synthetic RO concentrate in evaluating the performance of wastewater remediation with microalgae.

The toxicity of ionic liquid 1-decylpyridinium bromide to the algae Scenedesmus obliquus: Growth inhibition, phototoxicity, and oxidative stress

Although ionic liquids (ILs) are unlikely to act as air contaminants, their high solubility and slow degradation make them a potential threat to the aquatic environment. The IL 1-decylpyridinium bromide ([DPy]Br) is a common type of pyridine IL, which has varied applications such as in extraction, separation, and catalytic synthesis. Herein, the toxicity of [DPy]Br to S. obliquus is determined. Growth was inhibited by high-concentration [DPy]Br, whereas it had a hormetic effect at low concentrations. The IC_50-96h was approximately 0.06mg/L. The cell membrane permeability of S. obliquus increased with [DPy]Br concentration, indicating that [DPy]Br can cause damage to the algae cell structure. Chlorophyll content decreased at high [DPy]Br concentration; chlorophyll fluorescence parameters, such as the maximum effective quantum yield of PSII (F_v/F_m), potential activity of PSII (F_v/F₀), yield of the photochemical quantum [Y(II)], and the non-photochemical quenching coefficient (NPQ) were affected, suggesting that [DPy]Br can damage PSII. The ROS fluorescent images revealed that the morphology of cells changed gradually from fusiform to round. High ROS levels were observed with high concentrations of [DPy]Br, indicating that [DPy]Br induced oxidative stress on S. obliquus. The SOD and CAT activities increased when the concentration was lower than IC₅₀, whereas they decreased when the concentration was higher than IC₅₀. The relative ROS content was significantly correlated with growth inhibition rate, cell membrane permeability, chlorophyll content, and SOD and CAT activities. The increase of ROS content in algal cells is an important toxicological mechanism of [DPy]Br to S. obliquus.

The Effect of Bisphenol A on Growth, Morphology, Lipid Peroxidation, Antioxidant Enzyme Activity, and PS II in Cylindrospermopsis raciborskii and Scenedesmus quadricauda

To investigate the effect of bisphenol A (BPA) on Cylindrospermopsis raciborskii (Cyanobacteria) and Scenedesmus quadricauda (Chlorophyta), we grew the two species at BPA concentrations of 0, 0.1, 1, 2, 5, 10, and 20 mg/L and examined their growth, lipid peroxidation, antioxidant enzyme activity, and chlorophyll a fluorescence. The 96-h EC₅₀ values (effective concentration causing 50% growth inhibition) for BPA in C. raciborskii and S. quadricauda were 9.663 ± 0.047, and 13.233 ± 0.069 mg/L, respectively. A significant reduction in chlorophyll a concentration was found in C. raciborskii and S. quadricauda when BPA concentrations were greater than 1 and 2 mg/L, respectively. Furthermore, F _v/F _m, ΔF/F _m', and qP decreased significantly at 10 mg/L BPA in C. raciborskii but started to decrease at 10 mg/L in S. quadricauda. The changes in chlorophyll fluorescence parameters (α, rETR_max) that were obtained from the rapid light response curves of both algae species showed similar responses to F _v/F _m, ΔF/F _m', and qP under BPA-induced stress. Values for all of the chlorophyll fluorescence parameters in S. quadricauda were higher than in C. raciborskii; however, the nonphotochemical quenching measured in C. raciborskii was considerably higher than it was in S. quadricauda. In addition, lipid peroxidation (determined as MDA content) and antioxidant enzyme activities (SOD and CAT) increased in both species as the BPA concentration increased. These results suggest that C. raciborskii is more sensitive to the effects of BPA than S. quadricauda and that photosystem II might be a target for the activity of BPA in vivo.

Combined yeast and microalgal cultivation in a pilot-scale raceway pond for urban wastewater treatment and potential biodiesel production

A mixed culture of oleaginous yeast Lipomyces starkeyi and wastewater native microalgae (mostly Scenedesmus sp. and Chlorella sp.) was performed to enhance lipid and biomass production from urban wastewaters. A 400 L raceway pond, operating outdoors, was designed and used for biomass cultivation. Microalgae and yeast were inoculated into the cultivation pond with a 2:1 inoculum ratio. Their concentrations were monitored for 14 continuous days of batch cultivation. Microalgal growth presented a 3-day initial lag-phase, while yeast growth occurred in the first few days. Yeast activity during the microalgal lag-phase enhanced microalgal biomass productivity, corresponding to 31.4 mgTSS m^-2 d^-1. Yeast growth was limited by low concentrations in wastewater of easily assimilated organic substrates. Organic carbon was absorbed in the first 3 days with a 3.7 mgC L^-1 d^-1 removal rate. Complete nutrient removal occurred during microalgal linear growth with 2.9 mgN L^-1 d^-1 and 0.96 mgP L^-1 d^-1 removal rates. Microalgal photosynthetic activity induced high pH and dissolved oxygen values resulted in natural bactericidal and antifungal activity. A 15% lipid/dry weight was measured at the end of the cultivation time. Fatty acid methyl ester (FAME) analysis indicated that the lipids were mainly composed of arachidic acid.

High temperature and pH favor Microcystis aeruginosa to outcompete Scenedesmus obliquus

Competition between cyanobacteria and green algae affects phytoplankton succession and the well-known cyanobacteria blooms. Climate warming and water acidification are two concerned environmental issues changing the freshwater ecosystems. To investigate the competitive responses of phytoplankton to warming and acidification, we co-cultured Microcystis aeruginosa and Scenedesmus obliquus at a temperature range of 15-35 °C and a pH range of 5-9. Results showed that S. obliquus was superior competitor at 15 °C. At 20-30 °C, the populations of both Scenedesmus and Microcystis were inhibited by the presence of each other. S. obliquus was in competitive domination at the initial phase of cultivation, but was finally replaced by M. aeruginosa. Microcystis kept competition advantage at 35 °C, whereas Scenedesmus outcompeted Microcystis at acidic conditions (pH ≤ 6). Neutral and weakly alkaline conditions (pH 7-9) supported the replacement of competition domination from Scenedesmus to Microcystis. The present study revealed that climate warming may accelerate the phytoplankton succession from green algae to cyanobacteria, with the predicted promoted cyanobacteria blooms. Nonetheless, water acidification causes Microcystis to be a weak competitor with green algae, suggesting that the advantageous effect of Microcystis toward green algae at high temperatures was controlled by other variables like the water pH.

Enrichment of highly settleable microalgal consortia in mixed cultures for effluent polishing and low-cost biomass production

Microalgae cultivation is a promising technology for integrated effluent polishing and biofuel production, but poor separability of microalgal cells hinders its industrial application. This study intended to selectively enrich settleable microalgal consortia in mixed culture by applying "wash-out" pressure, which was realized by controlling settling time (S_T) and volume exchange ratio (VER) in photo-SBRs. The results demonstrated that highly settleable microalgal consortia (settling efficiency>97%; SVI = 17-50 mL/g) could be enriched from indigenous algal cultures developed in WWTP's effluent. High VER was the key factor for the fast development of settleable microalgae. VER was also a controlling factor of the algal community structure. High VERs (0.5 and 0.7) resulted in the dominance of diatom, while low VER (0.2) facilitated the dominance of cyanobacteria. The settleable microalgal consortia were very efficient in phosphorus removal (effluent PO₄^3--P<0.1 mg/L; removal efficiency>99%), which was largely attributed to intensive chemical precipitation of phosphate induced by high pH (8.5-10). However, the high pH decreased the bioavailable inorganic carbon, resulting in incomplete nitrate removal (effluent NO₃^--N = 2.2-4 mg/L; removal efficiency = 61-79%) under high VERs and low lipid content (up to 10%) in the settleable microalgae. This problem could be resolved by sparging CO₂ or controlling pH. Overall, this study demonstrated a simple and effective method to overcome the separation challenge in scale-up of microalgae biotechnology for advanced wastewater purification and biofuel production.

Influence of pathogenic bacterial activity on growth of Scenedesmus sp. and removal of nutrients from public market wastewater

The present study aims to investigate the influence of Staphylococcus aureus, Escherichia coli and Enterococcus faecalis in public market wastewater on the removal of nutrients in terms of ammonium (NH₄^-) and orthophosphate (PO₄³) using Scenedesmus sp. The removal rates of NH₄^- and orthophosphate PO₄^3- and batch kinetic coefficient of Scenedesmus sp. were investigated. The phycoremediation process was carried out at ambient temperature for 6 days. The results revealed that the pathogenic bacteria exhibited survival potential in the presence of microalgae but they were reduced by 3-4 log at the end of the treatment process. The specific removal rates of NH₄^- and PO₄^3- have a strong relationship with initial concentration in the public market wastewater (R² = 0.86 and 0.80, respectively). The kinetic coefficient of NH₄^- removal by Scenedesmus sp. was determined as k = 4.28 mg NH₄^- 1 log₁₀ cell mL^-1 d^-1 and k_m = 52.01 mg L^-1 (R² = 0.94) while the coefficient of PO₄^3- removal was noted as k = 1.09 mg NH₄^- 1 log₁₀ cell mL^-1 d^-1 and k_m = 85.56 mg L^-1 (R² = 0.92). It can be concluded that Scenedesmus sp. has high competition from indigenous bacteria in the public market wastewater to remove nutrients, with a higher coefficient of removal of NH₄^- than PO₄³.

Cylindrospermopsin induced changes in growth, toxin production and antioxidant response of Acutodesmus acuminatus and Microcystis aeruginosa under differing light and nitrogen conditions

Growing evidence suggests that some bioactive metabolites (e.g. cyanotoxins) produced by cyanobacteria have allelopathic potential, due to their inhibitory or stimulatory effects on competing species. Although a number of studies have shown that the cyanotoxin cylindrospermopsin (CYN) has variable effects on phytoplankton species, the impact of changing physicochemical conditions on its allelopathic potential is yet to be investigated. We investigated the physiological response of Microcystis aeruginosa (Cyanobacteria) and Acutodesmus acuminatus (Chlorophyta) to CYN under varying nitrogen and light conditions. At 24h, higher microcystins content of M. aeruginosa was recorded under limited light in the presence of CYN, while at 120h the lower levels of the toxins were observed in the presence of CYN under optimum light. Total MCs concentration was significantly (p<0.05) lowered by CYN after 120h of exposure under limited and optimum nitrogen conditions. On the other hand, there were no significant (p>0.05) changes in total MCs concentrations after exposure to CYN under high nitrogen conditions. As expected, limited light and limited nitrogen conditions resulted in lower cell density of both species, while CYN only significantly (p<0.05) inhibited the growth of M. aeruginosa. Regardless of the light or nitrogen condition, the presence of CYN increased internal H₂O₂ content of both species, which resulted in significant (p<0.05) changes in antioxidant enzyme (catalase, peroxidase, superoxide dismutase and glutathione S-transferase) activities. The oxidative stress caused by CYN was higher under limited light and limited nitrogen. These results showed that M. aeruginosa and A. acuminatus have variable response to CYN under changing light and nitrogen conditions, and demonstrate that need to consider changes in physicochemical conditions during ecotoxicological and ecophysiological investigations.

Enantioselective oxidative stress and oxidative damage caused by Rac- and S-metolachlor to Scenedesmus obliquus

The rational use and environmental security of chiral pesticides has gained the interest of many researchers. The enantioselective effects of Rac- and S-metolachlor on oxidative stress in Scenedesmus obliquus were determined in this study. Stronger green fluorescence was observed in response to S-metolachlor treatment than to Rac-metolachlor treatment, suggesting that more reactive oxygen species (ROS) were stimulated by S-metolachlor. ROS levels following S-metolachlor treatment were 1.92-, 8.31-, and 1.08-times higher than those observed following Rac-metolachlor treatment at 0.1, 0.2, and 0.3 mg/L, respectively. Superoxide dismutase (SOD) and catalase (CAT) were stimulated with increasing herbicide concentrations, with S-metolachlor exhibiting a greater effect. Oxidative damage in terms of chlorophyll (Chl) content, cellular membrane permeability, and cellular ultrastructures of S. obliquus were investigated. Chla and Chlb contents in algae treated with Rac-metolachlor were 2-6-fold higher than those in algae treated with S-metolachlor at 0.1, 0.2, and 0.3 mg/L. The cellular membrane permeability of algae exposed to 0.3 mg/L Rac- and S-metolachlor was 6.19- and 42.5-times that of the control. Correlation analysis implied that ROS are the major factor responsible for the oxidative damage caused by Rac- and S-metolachlor. Damage to the chloroplasts and cell membrane of S. obliquus, low production of starch granules, and an increased number of vacuoles were observed upon ultrastructural morphology analysis by transmission electron microscope. These results indicate that S-metolachlor has a greater effect on S. obliquus than Rac-metolachlor.