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.

Insights into the transcriptional responses of a microbial community to silver nanoparticles in a freshwater microcosm.

Silver nanoparticles (AgNPs) are widely used because of their excellent antibacterial properties. They are, however, easily discharged into the water environment, causing potential adverse environmental effects. Meta-transcriptomic analyses are helpful to study the transcriptional response of prokaryotic and eukaryotic aquatic microorganisms to AgNPs. In the present study, microcosms were used to investigate the toxicity of AgNPs to a natural aquatic microbial community. It was found that a 7-day exposure to 10 µg L-1 silver nanoparticles (AgNPs) dramatically affected the structure of the microbial community. Aquatic micro eukaryota (including eukaryotic algae, fungi, and zooplankton) and bacteria (i.e., heterotrophic bacteria and cyanobacteria) responded differently to the AgNPs stress. Meta-transcriptomic analyses demonstrated that eukaryota could use multiple cellular strategies to cope with AgNPs stress, such as enhancing nitrogen and sulfur metabolism, over-expressing genes related to translation, amino acids biosynthesis, and promoting bacterial-eukaryotic algae interactions. By contrast, bacteria were negatively affected by AgNPs with less signs of detoxification than in case of eukaryota; various pathways related to energy metabolism, DNA replication and genetic repair were seriously inhibited by AgNPs. As a result, eukaryotic algae (mainly Chlorophyta) dominated over cyanobacteria in the AgNPs treated microcosms over the 7-d exposure. The present study helps to understand the effects of AgNPs on aquatic microorganisms and provides insights into the contrasting AgNPs toxicity in eukaryota and bacteria.

The effects of hydrogen peroxide on the circadian rhythms of Microcystis aeruginosa.

BACKGROUND: The cyanobacterium Microcystis aeruginosa is one of the principal bloom-forming cyanobacteria present in a wide range of freshwater ecosystems. M. aeruginosa produces cyanotoxins, which can harm human and animal health. Many metabolic pathways in M. aeruginosa, including photosynthesis and microcystin synthesis, are controlled by its circadian rhythms. However, whether xenobiotics affect the cyanobacterial circadian system and change its growth, physiology and biochemistry is unknown. We used real-time PCR to study the effect of hydrogen peroxide (H(2)O(2)) on the expression of clock genes and some circadian genes in M. aeruginosa during the light/dark (LD) cycle. RESULTS: The results revealed that H(2)O(2) changes the expression patterns of clock genes (kaiA, kaiB, kaiC and sasA) and significantly decreases the transcript levels of kaiB, kaiC and sasA. H(2)O(2) treatment also decreased the transcription of circadian genes, such as photosynthesis-related genes (psaB, psbD1 and rbcL) and microcystin-related genes (mcyA, mcyD and mcyH), and changed their circadian expression patterns. Moreover, the physiological functions of M. aeruginosa, including its growth and microcystin synthesis, were greatly influenced by H(2)O(2) treatment during LD. These results indicate that changes in the cyanobacterial circadian system can affect its physiological and metabolic pathways. CONCLUSION: Our findings show that a xenobiotic can change the circadian expression patterns of its clock genes to influence clock-controlled gene regulation, and these influences are evident at the level of cellular physiology.

Analyses of gene expression and physiological changes in Microcystis aeruginosa reveal the phytotoxicities of three environmental pollutants.

When the concentrations of ampicillin (Amp), atrazine (Atr) and cadmium chloride (Cd) reach excessive quantities, they become toxic to aquatic organisms. Due to the acceleration of the industrialization and the intensification of human activities, the incidence and concentrations of these types of pollutants in aquatic systems are increasing. The primary purpose of this study was to evaluate the short-term effects of Amp, Atr and Cd on the physiological indices and gene expression levels in Microcystis aeruginosa. These three pollutants significantly induced antioxidant activity but continuously accelerated the cellular oxidative damage in microalgae, which suggests an imbalance between the oxidant and the antioxidant systems. Amp, Atr and Cd also decreased the transcription of psaB, psbD1 and rbcL; the lowest transcription of these genes was only 38.1, 23.7 and 7% of the control, respectively. These three pollutants affected nitrogen (N) and phosphorous (P) uptake by inhibiting the transcription of N or P absorbing and transporting related genes, and they down regulated the transcription of microcystin-related genes, which caused a decrease of microcystin levels; and the lowest level of microcystin was only 42.4% of the control. Our results suggest that these pollutants may cause pleiotropic effects on algal growth and physiological and biochemical reactions, and they may even affect secondary metabolic processes.

Effects of streptomycin on growth of algae Chlorella vulgaris and Microcystis aeruginosa.

Streptomycin is a common contaminant in a variety of industrial and agricultural wastewaters. The available information on the potential toxicity of streptomycin of fresh algae implicated in the treatment of biological wastewater is extremely limited. The objective of this study was to evaluate the effects of streptomycin on physiological indices and photosynthesis-related gene transcription. The results of short-term batch bioassays indicated that streptomycin was more sensitive to cyanobacteria than to green algae. The EC50 of streptomycin in Microcystis aeruginosa and Chlorella vulgaris were 0.28 and 20.08 mg L(-1) , respectively. These selected streptomycin concentrations inhibited algal cell growth and decreased chlorophyll or phycocyanobilin content. Streptomycin also destroyed the overall membrane system, which was speculated from malondialdehyde (MDA) content and electrolyte leakage increasing after streptomycin exposure. Two algae were induced to increase their antioxidant enzyme activities to withstand streptomycin. However, the balance between oxidant substance and antioxidant enzyme was broken, because reactive oxygen species (ROS) content simultaneously increased. Streptomycin inhibited photosynthesis-related gene transcription in C. vulgaris and M. aeruginosa. Transcript levels of psaB, psbA, and rbcL in C. vulgaris decreased to only 14.5%, 32.2%, and 9.3% of the control, respectively. Similarly, the transcript levels of psaB, psbD, and rbcL in M. aeruginosa decreased markedly in the present of streptomycin. The transcription of these genes was 12.4%, 26.1%, and 28.4% of the control after 0.1 mg L(-1) streptomycin exposure, respectively. Our results demonstrate that streptomycin is toxic to fresh algae, affects photosynthesis-related gene transcription, and blocks electron transport and ROS overproduction.

Combined effect of copper and cadmium on heavy metal ion bioaccumulation and antioxidant enzymes induction in Chlorella vulgaris.

The relationships between metal uptake and antioxidant enzyme activities or a response to membrane lipid peroxidation (i.e., malondialdehyde production) in Chlorella vulgaris exposed to Cu and Cd compounds singly and in combination were investigated. The results showed that bioaccumulation of a single metal was influenced by the presence of the other metal. The activities of superoxide dismutase and peroxidase increased to more than fivefold of the control after exposure to Cu(1.5 µM) alone or to Cu(1.5 µM) with Cd mixtures. Malondialdehyde levels in C. vulgaris also increased to approximately twofold of the control after exposure to high concentration of Cu(1.5 µM) alone or to Cu and Cd mixtures. However, Cd alone did not significantly increase the levels of antioxidant enzymes or malondialdehyde.

Effect of nonylphenol on response of physiology and photosynthesis-related gene transcription of Chlorella vulgaris.

Nonylphenol (NP) is regarded as a kind of persistent organic pollutant which exists ubiquitously in the environment. The objective of this study was to evaluate the effects of NP on Chlorella vulgaris physiological indices and gene transcription. The results showed that NP stress inhibited algal growth in short-term bioassay. NP also decreased chlorophyll content, including chl a, chl b, and total chlorophyll. NP caused oxidant hurt by overproducing reactive oxygen species (ROS), which might destroy the overall membrane system to cause malondialdehyde content increase. NP inhibited photosynthesis-related gene transcription in C. vulgaris after 24 to 48 h exposure. The lowest transcript levels of psaB, psbA, and rbcL in C. vulgaris decreased to only 18.5%, 7%, and 4% of the control, respectively. Taken together, our results demonstrate that NP is toxic to fresh algae growth by affecting the photosynthesis-related genes transcription and overproducing ROS to disrupt cell structure in a short period.

Photoperiod and temperature influence cadmium's effects on photosynthesis-related gene transcription in Chlorella vulgaris.

Routine metal compound toxicity tests are performed at one constant photoperiod and temperature. There is little knowledge about the interactions between metal compound toxicity and photoperiod or temperature. The purpose of this study was to analyze the effects of photoperiod and temperature on cadmium (Cd) toxicity in the fresh alga, Chlorella vulgaris, both singly and in combination. Exposure to 2 or 4 microM Cd alone significantly decreased the transcription of the photosynthesis-related genes psbA, psaB and rbcL. Three-way ANOVA analysis showed that both temperature and photoperiod interacted with the dosage of Cd to influence the abundance of psbA and psaB, but not rbcL. Specifically, psbA transcription was more sensitive to Cd under long photoperiods or high temperatures than under short photoperiods or low temperatures. Because photoperiod and temperature have certain impacts on the toxicity of metal compounds, these two environmental factors should be given more attention in laboratory research.