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

Barbara J. MacGregor, Benjamin Van Mooy, Brett J. Baker, Mark Mellon, Pia H. Moisander, Hans W. Paerl, Jonathan Zehr, David Hollander, David A. Stahl

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)205-219
Number of pages15
JournalEnvironmental microbiology
Volume3
Issue number3
DOIs
StatePublished - Dec 27 2001

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