Dissimilatory nitrate reduction by a freshwater cable bacterium

Ugo Marzocchi*, Casper Thorup, Ann Sofie Dam, Andreas Schramm, Nils Risgaard-Petersen

*Corresponding author for this work

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review

Abstract

Cable bacteria (CB) are filamentous Desulfobulbaceae that split the energy-conserving reaction of sulfide oxidation into two half reactions occurring in distinct cells. CB can use nitrate, but the reduction pathway is unknown, making it difficult to assess their direct impact on the N-cycle. Here we show that the freshwater cable bacterium Ca. Electronema sp. GS performs dissimilatory nitrate reduction to ammonium (DNRA). 15NO3-amended sediment with Ca. Electronema sp. GS showed higher rates of DNRA and nitrite production than sediment without Ca. Electronema sp. GS. Electron flux from sulfide oxidation, inferred from electric potential (EP) measurements, matched the electron flux needed to drive CB-mediated nitrate reduction to nitrite and ammonium. Ca. Electronema sp. GS expressed a complete nap operon for periplasmic nitrate reduction to nitrite, and a putative octaheme cytochrome c (pOCC), whose involvement in nitrite reduction to ammonium remains to be verified. Phylogenetic analysis suggests that the capacity for DNRA was acquired in multiple events through horizontal gene transfer from different organisms, before CB split into different salinity niches. The architecture of the nitrate reduction system suggests absence of energy conservation through oxidative phosphorylation, indicating that CB primarily conserve energy through the half reaction of sulfide oxidation.

Original languageEnglish
JournalISME Journal
Volume16
Issue1
Pages (from-to)50-57
Number of pages8
ISSN1751-7362
DOIs
Publication statusPublished - Jan 2022

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