Harsha Lake, like many other freshwater systems that serve as important recreation and drinking water sources, is suffering from frequent cyanobacterial harmful algal blooms (CyanoHABs). CyanoHABs in Lake Harsha start with Anabaena and Nostoc (N2-fixers) in early Summer and later transit into Microcystis (non-N2 fixers) when nitrogen supply is at low levels. To investigate the mechanisms that govern non-diazotrophic Microcystis bloom under low nitrogen level conditions, weekly transcriptional profiles of microbial communities were examined from Harsha Lake from June to September 2015.
Results/Conclusions
Gene transcripts confirmed the dominance of Anabaena and Nostoc expressing N2-fixing associated genes in June (N2-fixing stage), the early stages of CyanoHAB. Meanwhile, heterotrophic bacteria, Proteobacteria and Actinobacteria, were most abundant and mainly expressed genes involved in ATP synthesis. Microcystis and Planktothrix became dominant inJuly and early August (non-N2 fixing stage) and mainly expressed genes associated with organic nitrogen and phosphorus metabolism, DNA repair, and secondary metabolite (cyanotoxins) productions. During this non-N2 fixing stage, Proteobacteria and Actinobacteria significantly increased their relative abundance and expressed more genes associated with iron and phosphorus transport. Proteobacteria and Actinobacteria, which mainly upregulated genes for organic carbon degradation, continued to be the dominant heterotrophic bacteria during CyanoHAB senescence. Overall, our results suggest that the ability to process organic nutrients, to adapt high irradiance and warm environment within Microcystis and Planktothrix might explain their dominance in the bloom stage.