Rare cyanobacteria drive nitrogen-fixation and cyanotoxin production in an Aphanizomenon-dominated bloom

Late summer, recurring cyanobacterial blooms in Lake of the Woods (LOW) are polycyanobacterial and typically dominated by Aphanizomenon flos-aquae. LOW waters are typically nitrogen limited, relative to phosphorus. As such, the dominance of Aphanizomenon flos-aquae, a putative nitrogen-fixing cyanobacterium, suggests that its ability to fix nitrogen may be advantageous and aid in its ability to bloom. This study sought to quantify nitrogen fixation rates and identify cyanotoxin-producing species during the blooms. Throughout the 2021 season, we quantified nutrients, N-fixation rates, microbial community composition, and gene expression to determine who is responsible for cyanotoxin synthesis and nitrogen fixation. We found nitrogen fixation rates increased throughout the season, coincided with the bloom, but likely cannot fully support the bloom's nitrogen demand. However, the transcription of nitrogenase genes was solely done by less abundant Dolichospermum spp. and not by A. flos-aquae. Genome analysis suggests this population of A. flos-aquae cannot create a functioning nitrogenase, but they do still express the genes to initiate heterocyst differentiation. Microcystin gene transcripts were primarily from Microcystis spp. and Planktothrix spp. and coincided with microcystin concentrations. Interestingly, Planktothrix highly expressed anabaenopeptin genes, suggesting the presence of additional bioactive compounds in LOW. This work suggests that rare cyanobacterial members drive nitrogen fixation, and may be necessary for the seasonal bloom's function, toxicity, and longevity.