PNNL

Abstract

Increasing anthropogenic inputs of fixed nitrogen are leading to greater eutrophication of aquatic environments, but it is unclear how this impacts the flux and fate of carbon in lacustrine and riverine systems. Here, we present evidence that nitrogen form governs the partitioning of carbon among members in a genome-sequenced, model phototrophic biofilm of 20 members. Consumption of NO3- as a sole nitrogen source unexpectedly resulted in more rapid transfer of carbon to heterotrophs than when NH4+ was also provided, suggesting alterations in the form of carbon exchanged. Nitrogen form dramatically impacted net community nitrogen, but not carbon, uptake rates. Furthermore, this alteration in nitrogen form caused very large but focused alterations to community structure, strongly impacting abundances of only two species within the biofilm and modestly impacting a third member species. The nitrogen effect on carbon partitioning was accompanied by differential cyanobacterial iron acquisition gene expression, signifying likely variations in metal availability. Our data suggest that nitrogen metabolism may coordinate coupled carbon-nitrogen-iron biogeochemical cycling in benthic biofilms and, potentially, in phototroph-heterotroph consortia more broadly. It further indicates that the form of nitrogen inputs may significantly impact the contribution of these communities to carbon partitioning across the terrestrial-aquatic interface.