Diverse nitrogen cycling pathways across a marine oxygen gradient indicate nitrogen loss coupled to chemoautotrophic activity

Genetic markers and geochemical assays of microbial nitrogen cycling processes, including autotrophic and heterotrophic denitrification, anammox, ammonia oxidation, and nitrite oxidation, were examined across the oxycline, suboxic, and anoxic zones of the Cariaco Basin, Venezuela. Ammonia and nitrite oxidation genes were expressed through the entire gradient. Transcripts associated with autotrophic and heterotrophic denitrifiers were mostly confined to the suboxic zone and below but were also present in particles in the oxycline. Anammox genes and transcripts were detected over a narrow depth range near the bottom of the suboxic zone and coincided with secondary NO(2)(-)maxima and available NH4+. Dissolved inorganic nitrogen (DIN) amendment incubations and comparisons between our sampling campaigns suggested that denitrifier activity may be closely coupled with NO(3)(-)availability. Expression of denitrification genes at depths of high rates of chemoautotrophic carbon fixation and phylogenetic analyses of nitrogen cycling genes and transcripts indicated a diverse array of denitrifiers, including chemoautotrophs capable of using NO(3)(-)to oxidize reduced sulfur species. Thus, results suggest that the Cariaco Basin nitrogen cycle is influenced by autotrophic carbon cycling in addition to organic matter oxidation and anammox.

Automated summary

Genetic markers and geochemical assays were used to study microbial nitrogen cycling processes in the Cariaco Basin, Venezuela. The study found that denitrifier activity is closely related to nitrate availability, and identified a diverse array of denitrifiers, including chemoautotrophs capable of using nitrate to oxidize reduced sulfur species. The results suggest that autotrophic carbon cycling also plays a significant role in the nitrogen cycle of the Cariaco Basin, in addition to organic matter oxidation and anammox processes.