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 (JSO42−) 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.
Highlights
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
Consistency of the (TOC/TN)molar ratios (13.6 ± 3.4) and δ13CTOC (−20.6 ± 1‰) values measured across the water-depth transect (Supplementary Fig. 1 and Supplementary Table 2) suggests that marine productivity[42] is the dominant source of organic matter in the OMZ sediments
In the same way as that observed for the average Total organic carbon (TOC) contents of the cores, average operational taxonomic units (OTUs) count of the sediment cores exhibited remarkable variation with bottom water oxygen concentration: across the water-depth transect, average OTU count is highest in the sediments of the OMZ center and relatively lower in the two extremities (Fig. 2a). These findings indicate that dissolved oxygen in the bottom water, and consequentially the TOC content of the sediments, are the key determinants of microbial species richness in the OMZ-underlain sediments
Summary
High organic carbon sequestration boosts microbial diversity in sediments underlying the OMZ center. While the abundance of labile organic matter (simple short chain fatty acids, such as acetate, lactate, succinate and formate) and H2 in sediments are known to determine the extent of OSR, methanogenesis[58,59] and the depth of sulfate-methane transition zones (SMTZs), it is noteworthy that the frequencies of OTUs affiliated to fermentative[60] and exoelectrogenic[61] bacteria, which are potent sources of in situ lactate, acetate, CO2 and/or H2, are highest along the sediment-depth of SSK42/6, as compared to the other cores (Supplementary Tables 10–15). Greater preservation of labile organic matter within the OMZ center has been insinuated in previous studies that measured chlorophyll degradation products in OMZ sediments across the eastern Arabian Sea[82] In this context it is further noteworthy that TOC and SRB diversity reach their maxima within the zone of high JSO4, water-depth dependent variation profiles of the first two parameters do not exactly overlap with that of the third (Fig. 2b). Solid phase analyses of these sediments might offer more insights into the past variations in AOM intensity, SMTZ depth, and overall expanse of the OMZ84,85
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