Contrasting the effects of nitrogen form and concentration on primary producer biomass, cyanotoxins, and microbiome structure in eutrophic and mesotrophic lake communities

Abstract

Cyanobacterial harmful algal blooms (cyanoHABs) are becoming more frequent and intense due to anthropogenic activities. CyanoHABs degrade water quality and impose threats to human and ecosystem health. Yet, we do not fully understand under which circumstances a bloom turns toxic. Recent research has shown that although increased phosphorus (P) stimulates algal biomass and shifts the community over to cyanobacteria, excessive nitrogen (N) may be responsible for the toxicity of a cyanoHAB. A cyanotoxin contains seven amino acids creating a N-rich molecule, and therefore, requires N. In this study, we asked: (1) What form of nitrogen fuels toxic cyanoHABs? (2) How does prior bloom history from a lake affect the propensity to stimulate blooms with added nitrogen? We used replicate 300 L aquatic mesocosms filled with lake water from a mesotrophic (low nutrient) lake and a eutrophic (high nutrient) lake, to which we added different N additions. We show the microbiome of the lake is the most responsible for determining a potential cyanoHAB. Furthermore, the eutrophic lake responded more strongly to additions of nitrogen compared to the mesotrophic lake. Cyanobacteria became more abundant in the ammonium mesocosms in the eutrophic lake relative to the other bacteria present, while chloroplasts become more abundant in the nitrate mesocosms indicating nitrate stimulate a green or yellow green bloom instead of a cyanoHAB. Despite the presence of cyanobacteria, toxin (microcystin-LR) concentrations were below detection limit in the water column and low in the sedimented microbe samples. The lack of toxin could be due to the methodology behind quantifying toxins, a lack of trigger for producing toxins by the cyanobacteria that were present, or an absence of toxin-producing bacteria from the microbial community. Management strategies should be aimed at alleviating eutrophication in lakes by managing for excess N.