NITROGEN CYCLING NETWORKS OF LAKE SUPERIOR SEDIMENTS

Lake Superior is a valuable natural and cultural resource: it holds 10% of the world's freshwater and is the headwaters for the Laurentian Great Lakes. Over the last century, Lake Superior’s nitrate concentration has increased ~0.2 μM/year leading to an extreme stoichiometric imbalance relative to phosphorus (N:P +10,000). In Lake Superior sediments, nitrate production depends on the proximity to shore but could account for nearly 50% of the annual accumulation of nitrate in the water column. To date little is known of the microorganisms that drive the cycling of nitrogen in these sediments. Here we assess the potential and active transformation of nitrogen species by individual organisms, through metagenomics and metatranscriptomics, in order to reconstruct hypothetical nitrogen cycling networks. We uncovered novel organizational structures for nitrogen recycling across both spatial and biochemical gradients. We find that regardless of oxygen penetration, there's potential for nitrate reduction and that taxonomic diversity for nitrate reducers doesn’t correlate with oxygen concentration (e.g., sites with deeper oxygen penetration have greater diversity). Ultimately, our findings provide insight into the uniqueness of nitrogen cycling networks across a single system and provide further evidence of anaerobic processes in oxic environments and aerobic processes in anoxic environments.