Abstract
Carbonate rocks at marine methane seeps are commonly colonized by sulfur-oxidizing bacteria that co-occur with etch pits that suggest active dissolution. We show that sulfur-oxidizing bacteria are abundant on the surface of an exemplar seep carbonate collected from Del Mar East Methane Seep Field, USA. We then used bioreactors containing aragonite mineral coupons that simulate certain seep conditions to investigate plausible in situ rates of carbonate dissolution associated with sulfur-oxidizing bacteria. Bioreactors inoculated with a sulfur-oxidizing bacterial strain, Celeribacter baekdonensis LH4, growing on aragonite coupons induced dissolution rates in sulfidic, heterotrophic, and abiotic conditions of 1773.97 (±324.35), 152.81 (±123.27), and 272.99 (±249.96) μmol CaCO3 • cm−2 • yr−1, respectively. Steep gradients in pH were also measured within carbonate-attached biofilms using pH-sensitive fluorophores. Together, these results show that the production of acidic microenvironments in biofilms of sulfur-oxidizing bacteria are capable of dissolving carbonate rocks, even under well-buffered marine conditions. Our results support the hypothesis that authigenic carbonate rock dissolution driven by lithotrophic sulfur-oxidation constitutes a previously unknown carbon flux from the rock reservoir to the ocean and atmosphere.
Highlights
Steep gradients in pH were measured within carbonate-attached biofilms using pH-sensitive fluorophores. These results show that the production of acidic microenvironments in biofilms of sulfur-oxidizing bacteria are capable of dissolving carbonate rocks, even under well-buffered marine conditions
The atmospheric flux of oceanic methane is mitigated though the sulfate-dependent anaerobic oxidation of methane (AOM), which serves as the dominant sink for methane in the oceans [1]
Mats of sulfur-oxidizing bacteria have been recorded at many localities worldwide including on the surface of authigenic carbonates at methane seeps [37,38,39]
Summary
The atmospheric flux of oceanic methane is mitigated though the sulfate-dependent anaerobic oxidation of methane (AOM), which serves as the dominant sink for methane in the oceans [1]. The potential for sulfur oxidation to induce dissolution of carbonates in marine systems that are wellbuffered against bulk acidification has not been investigated. Through rRNA gene sequencing of mat samples collected from the Del Mar East Methane Seep Field, we show that abundant and diverse sulfur-oxidizing bacteria colonize carbonate surfaces. PH gradients within mineral-attached biofilms were measured using laser scanning confocal microscopy to assess a potential mechanism of dissolution in which acid generated by sulfur oxidization overcomes buffering within the biofilm microenvironment. We use our experimentallyderived dissolution rates to estimate the potential impact of seafloor weathering of carbonates by sulfur-oxidizing bacteria on global carbonate rock reservoirs. Nuclease-free water from the kit was processed alongside the samples as a negative control for iTag sequence analyses. The samples and negative control were sequenced on 1⁄4 of a lane of MiSeq for paired end 2×300 bp reads. ASVs with statistically different abundances between the top and bottom surfaces were detected with DESeq v. 1.28.0
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