The physiological and transcriptional response of harmful algal blooms in nitrogen limited lakes

Nitrogen fixation (N_Fix), although potentially a significant source of nitrogen (N), is often overlooked or underrepresented in many ecological models. Given the cellular N-demand, and inherent N-rich toxin compounds, N_Fix may be an important microbial process in proliferating cyanobacterial harmful algal blooms (cHABs) in inland lakes. N_Fix is a lynchpin in the nitrogen cycle that converts inaccessible dinitrogen, N₂, to bioavailable ammonia, NH₃. To understand nutrient constraints, specifically N and P, and its effects on the physiological response and gene expression of cHABs we sampled a small, bloom prone lake every two weeks for five months. We monitored shifts and changes in community composition (using metagenomics), nutrients, and N_Fix rates. Peltier Lake, in Lino Lakes, MN routinely exhibits cHABs that include Dolichospermum, a cyanobacteria capable of producing toxins and fixing N. When Peltier became N-deficient, N_Fix rates increased from 0.089 nmol N hr^-1L^-1 to 2.5 nmol N hr^-1L^-1, roughly a 28x increase. This dramatic increase in N_Fix could offset the N-demand for the microbial community potentially supporting blooms and funding N to toxin production. Interestingly, Microcystsis, a non-N-fixer was also present in these blooms during N-limiting conditions. Together, this suggests that N_Fix although generally ignored may be important in cHABs.