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Toxic Microcystis bloom is a tough environment problem worldwide. Microcystin is highly toxic and is an easily accumulated secondary metabolite of toxic Microcystis that threatens water safety. Biodegradation of microcystin by protozoan grazing is a promising and efficient biological method, but the mechanism in this process is still unclear. The present study aimed to identify potential pathways involved in resisting and degrading microcystin in flagellates through transcriptomic analyses. A total of 999 unigenes were significantly differentially expressed between treatments with flagellates Ochromonas fed on microcystin-producing Microcystis and microcystin-free Microcystis. These dysregulated genes were strongly associated with translation, carbohydrate metabolism, phagosome, and energy metabolism. Upregulated genes encoding peroxiredoxin, serine/threonine-protein phosphatase, glutathione S-transferase (GST), HSP70, and O-GlcNAc transferase were involved in resisting microcystin. In addition, genes encoding cathepsin and GST and genes related to inducing reactive oxygen species (ROS) were all upregulated, which highly probably linked with degrading microcystin in flagellates. The results of this study provided a better understanding of transcriptomic responses of flagellates to toxic Microcystis as well as highlighted a potential mechanism of biodegrading microcystin by flagellate Ochromonas, which served as a strong theoretical support for control of toxic microalgae by protozoans.

Global warming and increased nutrient fluxes cause cyanobacterial blooms in freshwater ecosystems. These phenomena have increased the concern for human health and ecosystem services. The mass occurrences of toxic cyanobacteria strongly affect freshwater zooplankton communities, especially the unselective filter feeder Daphnia. However, the molecular mechanisms of cyanobacterial toxicity remain poorly understood. This study is the first to combine the established body growth rate (BGR), which is an indicator of life-history fitness, with differential peptide labeling (iTRAQ)-based proteomics in Daphnia magna influenced by microcystin-producing (MP) and microcystin-free (MF) Microcystis aeruginosa. A significant decrease in BGR was detected when D. magna was exposed to MP or MF M. aeruginosa. Conducting iTRAQ proteomic analyses, we successfully identified and quantified 211 proteins with significant changes in expression. A cluster of orthologous groups revealed that M. aeruginosa-affected differential proteins were strongly associated with lipid, carbohydrate, amino acid, and energy metabolism. These parameters could potentially explain the reduced fitness based on the cost of the substance metabolism.

The copper/zinc superoxide dismutase (Cu/Zn-SOD) is a representative antioxidant enzyme that is responsible for the conversion of superoxide to oxygen and hydrogen peroxide in aerobic organisms. Cu/Zn-SOD mRNAs have been cloned from many species and employed as useful biomarkers of oxidative stresses. In the present study, we cloned Cu/Zn-SOD cDNA from the cladoceran Daphnia magna, analyzed its catalytic properties, and investigated mRNA expression patterns after exposure to known oxidative stressors. The full-length Cu/Zn-SOD of the D. magna (Dm-Cu/Zn-SOD) sequence consisted of 703 bp nucleotides, encoding 178 amino acids, showing well-conserved domains that were required for metal binding and several common characteristics. The deduced amino acid sequence of Dm-Cu/Zn-SOD showed that it shared high identity with Daphnia pulex (88%), Alvinella pompejana (56%), and Cristaria plicata (56%). The phylogenetic analysis indicated that Dm-Cu/Zn-SOD was highly homologous to D. pulex. The variation of Dm-Cu/Zn-SOD mRNA expression was quantified by real-time PCR, and the results indicated that the expression was up-regulated after 48-h exposure to copper, un-ionized ammonia, and low dissolved oxygen. This study shows that the Dm-Cu/Zn-SOD mRNA could be successfully employed as a biomarker of oxidative stress, which is a common mode of toxicity for many other aquatic hazardous materials.