Is diazotrophy important in the Great Lakes: Quantifying who and how much

Session: Poster Session

Kaela Natwora, University of Minnesota- Duluth, [email protected]

Abstract

Nitrogen fixation (NF) or diazotrophy is an important microbially mediated process that converts dinitrogen (N₂) gas to ammonia (NH₃) using the nitrogenase protein complex (Nif). Nitrogen fixation is a linchpin of the nitrogen cycle that re-mobilizes N lost to denitrification processes. In Lake Superior, nitrogen fixation was considered negligible to non-existent in filtered waters < 64 microns. Recently, cryptic microbial diazotrophs, < 3.0 microns in size, were identified across oligotrophic portions of the oceans and suggest importance in other aquatic environments, like Large Lakes. Given, the nitrogen accumulation problems of Lake Superior, we sought to quantify whether diazotrophy 1) occurs in the Great Lakes and 2) the quantity of N fixed. Using L. Superior as a testing bed, we mined metagenomic sequencing data and quantified N-fixation rates using the acetylene reduction assay. Although previously determined insignificant, our preliminary findings indicate microorganisms containing nifH genes are present throughout the water column. Nitrogen fixation rates in cell fractions < 3.0 microns averaged 0.5 nmol N²day^-1 and were observed in nearshore and offshore waters in the western arm of Lake Superior. Together this suggests diazotrophy may be overlooked but significant a source of N in Superior and other large lakes.