Regulation and role of extracellular polymeric substances in the defensive responses of Dictyosphaerium sp. to enrofloxacin stress

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.

Mechanisms of hormetic effects of ofloxacin on Chlorella pyrenoidosa under environmental-relevant concentration and long-term exposure

Antibiotics are frequently detected in surface water and pose potential threats to organisms in aquatic ecosystem such as microalgae. The occurrence of biphasic dose responses raised the possibility of stimulation of microalgal biomass by antibiotics at environmental-relevant concentration and caused potential ecological risk such as algal bloom. However, the underlying mechanisms of low concentration-induced hormetic effects are not well understood. In this study, we evaluated the hormesis of ofloxacin on Chlorella pyrenoidosa under environmental-relevant concentration and long-term exposure. Results showed the hormetic effects of ofloxacin on cell density and carbon fixation rate (RC). The predicted maximum promotion was 17.45 % by 16.84 μg/L and 20.08 % by 15.78 μg/L at 21 d, respectively. The predicted maximum concentration of non-effect on cell density and RC at 21 d was 3.24 mg/L and 1.44 mg/L, respectively. Ofloxacin induced the mobilization of pigments and antioxidant enzymes to deal with oxidative stress. PCA analysis revealed Chl-a/Chl-b could act as a more sensitive biomarker under acute exposure while chlorophyll fluorescence parameters were in favor of monitoring long-term implication. The hormesis in increased secretion of extracellular organic matters was regarded as a defensive mechanism and accelerated indirect photodegradation of ofloxacin. Bioremoval was dominant and related to biomass accumulation in the total dissipation while abiotic removal appeared slight contributions. This study provided new insights into the understanding of hormesis of microalgae induced by antibiotics.

Effects of four antibiotics on the photosynthetic light reactions in the green alga Chlorella pyrenoidosa

Antibiotics are ubiquitously present in aquatic environments, posing a serious ecological risk to aquatic ecosystems. However, the effects of antibiotics on the photosynthetic light reactions of freshwater algae and the underlying mechanisms are relatively less understood. In this study, the effects of 4 representative antibiotics (clarithromycin, enrofloxacin, tetracycline, and sulfamethazine) on a freshwater alga (Chlorella pyrenoidosa) and the associated mechanisms, primarily focusing on key regulators of the photosynthetic light reactions, were evaluated. Algae were exposed to different concentrations of clarithromycin (0.0-0.3 mg/L), enrofloxacin (0.0-30.0 mg/L), tetracycline (0.0-10.0 mg/L), and sulfamethazine (0.0-50.0 mg/L) for 7 days. The results showed that the 4 antibiotics inhibited the growth, the photosynthetic pigment contents, and the activity of antioxidant enzymes. In addition, exposure to clarithromycin caused a 118.4 % increase in malondialdehyde (MDA) levels at 0.3 mg/L. Furthermore, the transcripts of genes for the adenosine triphosphate (ATP) - dependent chloroplast proteases (ftsH and clpP), genes in photosystem II (psbA, psbB, and psbC), genes related to ATP synthase (atpA, atpB, and atpH), and petA (related to cytochrome b6/f complex) were altered by clarithromycin. This study contributes to a better understanding of the risk of antibiotics on primary producers in aquatic environment.

Phytohormone gibberellins treatment enhances multiple antibiotics removal efficiency of different bacteria-microalgae-fungi symbionts

To develop and characterize novel antibiotics removal biomaterial technology, we constructed three different bacteria-microalgae-fungi consortiums containing Chlorella vulgaris (C. vulgaris), endophytic bacterium, Clonostachys rosea (C. rosea), Ganoderma lucidum, and Pleurotus pulmonarius. The results showed that under treatment with 50 mg/L of gibberellins (GAs), the three bacteria-microalgae-fungi symbionts had maximal growth rates (0.317 ± 0.030 d-1) and the highest removal efficiency for seven different antibiotics. Among them, C. vulgaris-endophytic bacterium-C. rosea symbiont had the best performance, with antibiotics removal efficiencies of 96.0 ± 1.4 %, 91.1 ± 7.9 %, 48.7 ± 5.1 %, 34.6 ± 2.9 %, 61.0 ± 5.5 %, 63.7 ± 5.6 %, and 54.3 ± 4.9 % for tetracycline hydrochloride, oxytetracycline hydrochloride, ciprofloxacin, norfloxacin, sulfadiazine, sulfamethazine, and sulfamethoxazole, respectively. Overall, the present study demonstrates that 50 mg/L GAs enhances biomass production and antibiotics removal efficiency of bacteria-microalgae-fungi symbionts, providing a framework for future antibiotics-containing wastewater treatment using three-phase symbionts.