Total microcystins reduce the activity of genes involved in nitrogen fixation and denitrification in lakes of the south-central USA

Cyanotoxins impact microbes, potentially changing nutrient cycles. To examine this further, we studied 20 lakes in south-central United States to better understand links between the presence (metagenomics) and activity (metatranscriptomics) of cellular processes related to nitrogen and phosphorus, the concentration of total microcystins, and nutrient levels. We found that NOx concentrations were related to total microcystin concentrations. We discovered that microcystins were negatively associated with the presence and activity of several cellular pathways vital for nutrient cycles, including nitrogen fixation, denitrification, and phosphorus uptake and metabolism. The effects of total microcystins on the presence of genes controlling these pathways could not be separated from the effects of NOX and other environmental factors, but the effects on gene activity could be. Negative correlations were observed with nitrogen fixation, dissimilatory nitrite reductase, and other nitrogen metabolic processes. Typical concentrations of total microcystins in these lakes ranged from 0.15 to 0.50 µg L−1, with peaks between 1.6 and 2.2 µg L−1, still below levels of large cyanobacteria blooms in other systems. Nevertheless, impacts on the presence and activity of genes essential for nutrient cycles were detected. Studies like ours are needed during large blooms to better assess the effects of microcystins on geochemical cycles.