Microbiological, molecular biological and stable isotopic evidence for nitrogen fixation in the open waters of Lake Michigan

We have used a combination of microbiological, molecular biological and stable isotope methods to relate specific microbial populations to elemental cycling at an offshore site in Lake Michigan. Several lines of evidence suggest that atmospheric N₂ may be a significant source of nitrogen to the lake. Particulate organic nitrogen (PON) at ≈ 10–15 m depth in July and October had a δ¹⁵N of 0.5–1.5‰. These values closely reflect the ¹⁵N composition of atmospheric N₂, suggesting biological nitrogen fixation. Historical data show a developing late-summer N:P minimum at ≈ 15 m; low abundance of inorganic nitrogen relative to phosphorus favours species able to acquire atmospheric nitrogen. Microscopic examination of October water samples revealed abundant heterocystous cyanobacteria, including Nodularia sp. Potentially nitrogen-fixing Anabaena spp. have been found in Lake Michigan before but, to our knowledge, this is the first report of Nodularia . Finally, we have amplified both cyanobacterial and non-cyanobacterial nifH sequences (encoding the nitrogenase iron protein) from lakewater samples, evidence for the presence of bacteria capable of nitrogen fixation. The surface waters of Lake Michigan are considered to be phosphate limited in the stratified season and, under these conditions, energetically expensive nitrogen fixation is expected to be uncompetitive with assimilation of combined nitrogen. Our results suggest that, from both microbiological and biogeochemical perspectives, this may be an oversimplification.