Methanogens are major contributors to nitrogen fixation in soils of the Florida Everglades

Abstract

The objective of this study was to investigate the interaction of the nitrogen (N) cycle with methane production in the Florida Everglades, a large freshwater wetland. This study provides an initial analysis of the distribution and expression of N-cycling genes in Water Conservation Area 2A (WCA-2A), a section of the marsh that underwent phosphorus (P) loading for many years due to runoff from upstream agricultural activities. The elevated P resulted in increased primary productivity and an N limitation in P-enriched areas. Results from quantitative real-time PCR (qPCR) analyses indicated that the N cycle in WCA-2A was dominated by nifH and nirK/S, with an increasing trend in copy numbers in P-impacted sites. Many nifH sequences (6 to 44% of the total) and nifH transcript sequences (2 to 49%) clustered with the methanogenic Euryarchaeota, in stark contrast to the proportion of core gene sequences representing Archaea (≤0.27% of SSU rRNA genes) for the WCA-2A microbiota. Notably, archaeal nifH gene transcripts were detected at all sites and comprised a significant proportion of total nifH transcripts obtained from the unimpacted site, indicating that methanogens are actively fixing N2. Laboratory incubations with soils taken from WCA-2A produced nifH transcripts with the production of methane from H2 plus CO2 and acetate as electron donors and carbon sources. Methanogenic N2 fixation is likely to be an important, although largely unrecognized, route through which fixed nitrogen enters the anoxic soils of the Everglades and may have significant relevance regarding methane production in wetlands.