95 Contrasting methane, sulfide and nitrogen regimes in coastal sediment bioreactors DISCUSSION Our study investigated the effects of distinct substrate loading regimes of nitrate, sulfide, and ammonium on a microbial community under methane saturation in two bioreactors inoculated with sediment from a coastal anoxic sediment from the Stockholm Archipelago. The microbial communities in both systems were quite different from each other after two months of operation and remained relatively stable for the remainder of the incubation period (15.5 months; Figure 2A-B and Supplementary Figure 4). The drop in diversity and richness from eutrophic to oligotrophic systems observed aligned with the general ecological principle of fast-growing species outcompeting the abundant and diverse rare biosphere. The (gamma)proteobacteria were the most abundant group under all conditions and growth types (bioreactor and biofilm) (Figure 2C and Supplementary Figure 5). These observations are in line with reports of the uncharacterized metabolic ability of proteobacteria for the utilization of sulfur and C1 compounds in deep-sea hydrothermal sediments (Zhou et al., 2020). Sulfide detoxification was dominated by nitrate-reducing sulfide-oxidizing bacteria followed by MOB. In the eutrophic bioreactor, Campylobacterales ultimately became enriched over Gammaproteobacteria (Figure 2C). Several members of the Campylobacterales phylum have been described as opportunistic denitrifiers and seem to be stimulated under low sulfide and high nitrate concentrations (Murphy et al., 2020). In accordance, our two recovered Campylobacterales MAGs, Sulfurimonas and Sulfurovum (Supplementary Table 1), contained two copies of sqr and exhibited either partially complete denitrification potential (napA, nirS, norB, and nosZ) or only norB, respectively (Supplementary Table 5). The oligotrophic system sustained DNRA while denitrification dominated in the eutrophic bioreactor (Figure 1A-B). Nitrate limitation as key factor for DNRA was also observed in other studies (Arshad et al., 2017; Dalcin Martins et al., 2022, Chapter 4; van den Berg et al., 2016). Although previous studies reported inhibition of denitrification and DNRA by sulfide (Delgado Vela et al., 2021; Russ et al., 2014; Zhang et al., 2024), in our systems we did not observe this inhibition, possibly 3
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