Transcriptomics revealed the key molecular mechanisms of ofloxacin-induced hormesis in Chlorella pyrenoidosa at environmentally relevant concentration

Emerging pollutants such as antibiotics have aroused great concern in recent years. However, the knowledge of low concentration-induced hormesis was not well understood. This study evaluated and quantified hormetic effects of ofloxacin on Chlorella pyrenoidosa. LogNormal model predicted the maximal non-effect concentration was 0.13 mg/L and 2.96 mg/L at 3 and 21 d, respectively. The sensitive alterations in chlorophyll fluorescence suggested PSII was the main target. Transcriptomics revealed ofloxacin inhibited genes related to photosynthetic system while the cyclic electron around PSI decreased the pH value in stroma side and stimulated photoprotection via up-regulating psbS. The stimulation in citrate cycle pathway met the urgent requirements of energy for DNA replication and repair. In addition, the negative feedback of G3P in glycolysis pathway inhibited Calvin cycle. The degradation products illustrated the occurrence of multiple detoxification mechanisms such as demethylation and ring-opening. The mobilization of cytochrome P450 generated the constant detoxication of ofloxacin while glutathione was consumptively involved in biological binding. This study provided new insights into the molecular mechanisms of antibiotic-induced hormesis in microalgae.

Response of varying combined nutrients on biomass and biochemical composition of marine diatoms Chaetoceros gracilis and Thalassiosira weissflogii

Marine diatoms have high adaptability and are known to accumulate lipids under nutrient stress conditions. The present study involves determining the effect of varying macro and micronutrients on growth kinetics and metabolite production of oleaginous marine diatoms, Thalassiosira weissflogii and Chaetoceros gracilis. The results highlighted that C. gracilis and T. weissflogii showed maximum biomass yield of 0.86 ± 0.06 g/L and 0.76 ± 0.01 g/L in the 2f and f supplemented medium respectively. A 2.5-fold increase in cellular lipid content was recorded in the 2f culture setup of both strains ranging from 20 % to 26.7 % (w/w). The study also reveals that high eutrophic nutrient media (f, 2f and 4f) triggered biomass productivity as well as total protein and carbohydrate content in both strains. Thus, providing a reproducible insight of trophic flexibility of diatoms, concomitant with the increment in multiple commercially valuable products.

Removal of copper improves the lipid content in Nannochloropsis oculata culture

Mining is an important activity for the economic development of many countries. However, this activity produces toxic residues that pollute water and the environment. The heavy metal removal from effluents of acid mine water is crucial to avoid environmental pollution. The microalga Nannochloropsis oculata was cultured in algal medium, with the addition of 1.16, 1.74, 2.32, 3.48, and 4.64 mg Cu²⁺ L^-1 coming from acid mine water to assess its removal capacity and the effect of copper content on the cell density and lipid productivity. The results showed that N. oculata removed up to 94.88 ± 0.43% at copper concentration than 1.74 mg Cu²⁺ L^-1; additionally, a positive effect on the lipid content was found at copper concentration to be higher, 4.64 mg Cu²⁺ L^-1, yielding 77.04 ± 2.60% of lipid content, twice as high as that achieved in the control culture of 33.058 ± 5.398%, thus potentiating the biodiesel production. These findings are favorable because they indicate that microalgae can remove copper added in the culture and present in acid mine water and can yield high lipid content at the same time. The cell density and growth rate decreased with increased concentrations of copper in the culture medium.

Aquatic toxicity and mode of action of CdS and ZnS nanoparticles in four microalgae species

This study reports the differences in toxic action between cadmium sulfide (CdS) and zinc sulfide (ZnS) nanoparticles (NPs) prepared by recently developed xanthate-mediated method. The aquatic toxicity of the synthesized NPs on four marine microalgae species was explored. Growth rate, esterase activity, membrane potential, and morphological changes of microalgae cells were evaluated using flow cytometry and optical microscopy. CdS and ZnS NPs demonstrated similar level of general toxicity and growth-rate inhibition to all used microalgae species, except the red algae P. purpureum. More specifically, CdS NPs caused higher inhibition of growth rate for C. muelleri and P. purpureum, while ZnS NPs were more toxic for A. ussuriensis and H. akashiwo species. Our findings suggest that the sensitivity of different microalgae species to CdS and ZnS NPs depends on the chemical composition of NPs and their ability to interact with the components of microalgal cell-wall. The red microalga was highly resistant to ZnS NPs most likely due to the presence of phycoerythrin proteins in the outer membrane bound Zn²⁺ cations defending their cells from further toxic influence. The treatment with CdS NPs caused morphological changes and biochemical disorder in all tested microalgae species. The toxicity of CdS NPs is based on their higher photoactivity under visible light irradiation and lower dissociation in water, which allows them to generate more reactive oxygen species and create a higher risk of oxidative stress to aquatic organisms. The results of this study contribute to our understanding of the parameters affecting the aquatic toxicity of semiconductor NPs and provide a basis for further investigations.