# Nitrogen Fixation Associated with Microcystis Colonies Promotes Harmful Cyanobacterial Blooms across North American Lakes

## TL;DR

This study shows that nitrogen fixation by Microcystis colonies helps fuel harmful algal blooms in lakes, even in low-nitrogen environments.

## Contribution

The study reveals that Microcystis colonies fix nitrogen more efficiently than free-living plankton, contributing to bloom formation.

## Key findings

- Microcystis colonies showed higher nitrogen fixation rates than free-living plankton in five of six lakes.
- Ammonium reduced nitrogen fixation in colonies by 50–85% in experiments.
- Colony-associated plankton preferentially used 14N2, indicated by lower δ15N in colonies.

## Abstract

Microcystis 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 Microcystis colonies, free-living plankton,
and the whole plankton community during dense Microcystis 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 (p < 0.05). N2 fixation rates
were inversely correlated with the rates of ammonium uptake by the
colony fraction (p < 0.05), and in experiments,
ammonium significantly reduced N2 fixation rates within
the colony fraction by 50–85% (p < 0.001).
In several lakes, the δ15N of isolated colonies was
lower (p < 0.05) than that of free-living plankton
presumably due to the preferential use of 14N2 by diazotrophic, colony-associated plankton. Cyanobacterial and
noncyanobacterial nifH sequences were associated
with the Microcystis colonies with the nifH-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 Microcystis colony fraction and
averaged 18% across systems, suggesting this process could support
the proliferation of Microcystis blooms.