Microbial dissimilatory sulfate reduction to sulfide is a predominant terminal pathway of organic matter mineralization in the anoxic seabed. Chemical or microbial oxidation of the produced sulfide establishes a complex network of pathways in the sulfur cycle, leading to intermediate sulfur species and partly back to sulfate. The intermediates include elemental sulfur, polysulfides, thiosulfate, and sulfite, which are all substrates for further microbial oxidation, reduction or disproportionation. New microbiological discoveries, such as long-distance electron transfer through sulfide oxidizing cable bacteria, add to the complexity. Isotope exchange reactions play an important role for the stable isotope geochemistry and for the experimental study of sulfur transformations using radiotracers. Microbially catalyzed processes are partly reversible whereby the back-reaction affects our interpretation of radiotracer experiments and provides a mechanism for isotope fractionation. We here review the progress and current status in our understanding of the sulfur cycle in the seabed with respect to its microbial ecology, biogeochemistry, and isotope geochemistry.

The thermodynamics for the first electron transfer step for sulfide and oxygen indicates that the reaction is unfavorable as unstable superoxide and bisulfide radical ions would need to be produced. However, a two-electron transfer is favorable as stable S(0) and peroxide would be formed, but the partially filled orbitals in oxygen that accept electrons prevent rapid kinetics. Abiotic sulfide oxidation kinetics improve when reduced iron and/or manganese are oxidized by oxygen to form oxidized metals which in turn oxidize sulfide. Biological sulfur oxidation relies on enzymes that have evolved to overcome these kinetic constraints to affect rapid sulfide oxidation. Here we review the available thermodynamic and kinetic data for H 2 S and HS• as well as O 2 , reactive oxygen species, nitrate, nitrite, and NO x species. We also present new kinetic data for abiotic sulfide oxidation with oxygen in trace metal clean solutions that constrain abiotic rates of sulfide oxidation in metal free solution and agree with the kinetic and thermodynamic calculations. Moreover, we present experimental data that give insight on rates of chemolithotrophic and photolithotrophic sulfide oxidation in the environment. We demonstrate that both anaerobic photolithotrophic and aerobic chemolithotrophic sulfide oxidation rates are three or more orders of magnitude higher than abiotic rates suggesting that in most environments biotic sulfide oxidation rates will far exceed abiotic rates due to the thermodynamic and kinetic constraints discussed in the first section of the paper. Such data reshape our thinking about the biotic and abiotic contributions to sulfide oxidation in the environment.

This study demonstrates that the deltaproteobacterium Desulfurivibrio alkaliphilus can grow chemolithotrophically by coupling sulfide oxidation to the dissimilatory reduction of nitrate and nitrite to ammonium. Key genes of known sulfide oxidation pathways are absent from the genome of D. alkaliphilus. Instead, the genome contains all of the genes necessary for sulfate reduction, including a gene for a reductive-type dissimilatory bisulfite reductase (DSR). Despite this, growth by sulfate reduction was not observed. Transcriptomic analysis revealed a very high expression level of sulfate-reduction genes during growth by sulfide oxidation, while inhibition experiments with molybdate pointed to elemental sulfur/polysulfides as intermediates. Consequently, we propose that D. alkaliphilus initially oxidizes sulfide to elemental sulfur, which is then either disproportionated, or oxidized by a reversal of the sulfate reduction pathway. This is the first study providing evidence that a reductive-type DSR is involved in a sulfide oxidation pathway. Transcriptome sequencing further suggests that nitrate reduction to ammonium is performed by a novel type of periplasmic nitrate reductase and an unusual membrane-anchored nitrite reductase.

Intertidal aquifers host a reactive zone comprised of Fe mineral‐coated sands where fresh and saline groundwaters mix. This zone may significantly influence the export of C, N, P, Fe, and other metals in submarine groundwater discharge (SGD). Toward determining the roles of microbes in Fe and S mineralization, and the interplay of microbiology with geochemistry and physical hydrology, we conducted a biogeochemical study of pore waters at Cape Shores, Delaware. Here, fresh groundwater provides Fe(II), which precipitates as FeIIIOOH predominantly through microbial Fe(II) oxidation. Candidate division OP3 was the dominant microbial group associated with Fe(II)‐ and Fe(III)‐rich regions of the aquifer, suggesting that this uncharacterized phylum may be involved in Fe(II) oxidation. Saline water brings O2, sulfate, and organic C into the intertidal mixing zone. Microbial reduction of sulfate produces sulfide that is transported to the Fe‐mineralized zone leading to the transformation of FeOOH to Fe(II) sulfides. Microbial populations are structured by the availability of chemical species supplied along groundwater flow paths. Seasonal changes in the relative supply of fresh and saline groundwater affect solute fluxes, and therefore, microbial controls on the location and composition of the Fe‐mineralized zone. Ultimately, the composition, extent, and dynamics of the Fe‐mineralized zone will affect the sequestration, affinity, and residence time of solutes bound for export to coastal oceans through SGD.