Nitrate use by sulfur bacteria in a stratified lake

Nitrate (NO₃^‑) disappearance is assumed to be from denitrification, which couples NO₃^- reduction to carbon oxidation, an important process for improving water quality.  Our previous work in freshwater sediments demonstrated that NO₃^- removal can be coupled to sulfur (sulfide, H₂S) oxidation, a reaction driven in part by Sulfurimonas denitrificans.  Sulfur oxidizers do not necessarily denitrify to dinitrogen (N₂), but may instead conduct dissimilatory nitrate reduction to ammonium (DNRA).  We examined the relative importance of NO₃^- reduction to N₂ vs. NH₄⁺ and the role of S. denitrificans using a combination of ¹⁵N experiments and quantitative-PCR.  We added ¹⁵NO₃^- to water from three depths across a suboxic-anoxic gradient with an H₂S range of 0-100 micromolar.  Over 4 days of in-situ incubation most of the added ¹⁵NO₃^- was reduced to ¹⁵N₂, though a smaller fraction (4-10%) was reduced to NH₄⁺.  An equimolar amount of H₂S was oxidized to sulfate in the anoxic water.  Copies of S. denitrificans were below detection (400 cells L^-1) in the live controls, whereas addition of NO₃^- stimulated its growth up to 15-fold (6,700 cells L^-1).  Stratified lakes and reservoirs may support planktonic sulfur bacteria that intercept nitrate by processes akin to those in benthic environments.