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This geomicrobiological study integrates pore-fluid chemistry and high-throughput bacterial 16S rRNA sequencing across eight sediment cores spanning the Arabian Sea oxygen minimum zone (OMZ). The research reveals that sediments at the OMZ center (530–580 mbsl) exhibit enhanced total organic carbon (TOC) content, elevated sulfate reduction rates (JSO₄²⁻ up to 0.0113 mmol cm⁻² yr⁻¹), and maximum bacterial diversity (3,374 OTUs), driven by greater organic matter preservation and fermentative production of bioavailable carbon compounds. These findings demonstrate intensified carbon-sulfur cycling in hypoxic sediments with implications for understanding H₂S and CH₄ dynamics in expanding OMZs.

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Abstract

Biogeochemistry of oxygen minimum zone (OMZ) sediments, which are characterized by high input of labile organic matter, have crucial bearings on the benthic biota, gas and metal fluxes across the sediment-water interface, and carbon-sulfur cycling. Here we couple pore-fluid chemistry and comprehensive microbial diversity data to reveal the sedimentary carbon-sulfur cycle across a water-depth transect covering the entire thickness of eastern Arabian Sea OMZ, off the west coast of India. Geochemical data show remarkable increase in average total organic carbon content and aerial sulfate reduction rate (J SO4 2- ) in the sediments of the OMZ center coupled with shallowing of sulfate methane transition zone and hydrogen sulfide and ammonium build-up. Total bacterial diversity, including those of complex organic matter degraders, fermentative and exoelectrogenic bacteria, and sulfate-reducers (that utilize only simple carbon compounds) were also found to be highest in the same region. The above findings indicate that higher organic carbon sequestration from the water-columns (apparently due to lower benthic consumption, biodegradation and biotransformation) and greater bioavailability of simple organic carbon compounds (apparently produced by fermetative microflora of the sediments) are instrumental in intensifying the carbon-sulfur cycle in the sediments of the OMZ center.

Keywords

Oxygen minimum zoneEnvironmental chemistryTotal organic carbonBenthic zoneOrganic matterSulfate

Identifiers

DOI
10.1038/s41598-018-27002-2
PubMed
29875466
Journal
Scientific Reports
Year
2018