71 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments higher preference for nitrite (apparent Km of 2.5 µM) over sulfite (apparent Km of 15.6 µM) (Jespersen et al., 2023). F420-dependent sulfite reductases have been found to be highly abundant sulfur metabolism proteins in an ANME-2 metaproteomics study (Yu et al., 2018), which also reported the inhibition of AOM activity by ANME-2a/2c in methane seep sediments incubated with 1 mmol L-1 sulfite, 1 mmol L-1 polythionate and 0.25 mmol L-1 polysulfide. The authors concluded that the role of F420-dependent sulfite reductases in ANME-2 is more likely sulfite detoxification rather than sulfur assimilation, which could occur via several other ANME-2 enzymes (Yu et al., 2018). Furthermore, while methanogens are known to withstand 3-5 mmol L-1 sulfide levels and assimilate sulfur from sulfide (Liu et al., 2012b), ANME archaea have been reported to be inhibited by 3-4 mmol L-1 sulfide under the low sulfate concentrations that we measured in our study (4 mmol L-1 range), but not under high (21 mmol L-1) sulfate concentrations (Timmers et al., 2015). Our experimental results (Figure 6) provide further evidence for a dose-dependent, sulfide-driven inhibition of AOM activity under ~ 4 mmol L-1 sulfate. MAG 11 ANME-2 had several genes encoding multiheme c-type cytochromes (Figure 5) including S-layer proteins, lowly expressed in Fe-AOM-performing ANME-2d (Cai et al., 2018), and OmcZ-like proteins suggested as the ANME mechanism for extracellular electron transfer (Chadwick et al., 2022), which could be used for electron transfer to a sulfate-reducing partner or to metal oxides. These multiheme c-type cytochromes identified in ANME might be targets of sulfite toxicity (Liu et al., 2012a). These previous studies suggest that a threshold sulfide concentration, potentially dependent on sulfate concentrations, might exert thermodynamic and toxicity controls on AOM activity. Our results match these previous observations, but also indicate that sulfide exposure (total sulfide in mmol year-1), which differed more between Sites 5 and 7 than sulfide concentrations (mmol L-1 at the time of sampling), could play a role in the lower putative ANME-2 abundances at Site 7 (Figure 4). Overall, these results support our experimental, biogeochemical and metagenomic evidence for the proposed mechanism of sulfide toxicity as a key control on putative ANME-2 abundances and activity in coastal sediments, suggesting that the expansion of euxinia in coastal areas (Breitburg et al., 2018) 2
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