Nitrogen Fixation Associated with <i>Microcystis</i> Colonies Promotes Harmful Cyanobacterial Blooms across North American Lakes

<i>Microcystis</i> forms harmful cyanobacterial algal blooms around the world, including regions with low inorganic nitrogen (N) concentrations. Here, we measured the N₂ fixation rates of isolated <i>Microcystis</i> colonies, free-living plankton, and the whole plankton community during dense <i>Microcystis</i> blooms over a three year period in six diverse lakes across eastern North America including Lake Erie. For five of six lakes, rates within the colony fraction were greater than the rates measured for free-living plankton (<i>p</i> < 0.05). N₂ fixation rates were inversely correlated with the rates of ammonium uptake by the colony fraction (<i>p</i> < 0.05), and in experiments, ammonium significantly reduced N₂ fixation rates within the colony fraction by 50-85% (<i>p</i> < 0.001). In several lakes, the δ¹⁵N of isolated colonies was lower (<i>p</i> < 0.05) than that of free-living plankton presumably due to the preferential use of ¹⁴N₂ by diazotrophic, colony-associated plankton. Cyanobacterial and noncyanobacterial <i>nifH</i> sequences were associated with the <i>Microcystis</i> colonies with the <i>nifH</i>-based diversity of both groups being lower in colonies compared to free-living plankton. Compared to nitrate, ammonium, and urea uptake, N₂ fixation rates accounted for 1-76% of total N assimilation by the <i>Microcystis</i> colony fraction and averaged 18% across systems, suggesting this process could support the proliferation of <i>Microcystis</i> blooms.