96 Chapter 3 because the sulfide was directly consumed by the microbial community and thus the in situ sulfide concentration remained below detection limit. Similar observations have been made for a sulfide and methane-oxidizing co-culture where a new Nitrobium species was enriched (Arshad et al., 2017; Dalcin Martins et al., 2022, Chapter 4). Another study also found that sulfur disproportionation could fuel DNRA, a reaction that might have occurred in our oligotrophic system (Shao et al., 2024). Aerobic methanotrophs of the Methylomonadaceae type I (Gammaproteobacteria) constituted a significant portion of the total bacterial reads in both systems, despite the supply of sulfide and nitrate (Figure 3A, Supplementary Figure 3 and Supplemental Table 1 and 2). Abundant persistent populations of gammaproteobacterial methanotrophs have been described in oxygen-depleted lakes and wetlands, and marine oxygen minimum zones (Oswald et al., 2016; Schorn et al., 2024b; Smith et al., 2018; Smith & Wrighton, 2019). In this regard, some of the physiological and genomic adaptations for survival under oxygenlimited conditions of MOB have been speculated to include: denitrification potential, fermentation pathways, high affinity oxidases, oxygen-carriers like bacteriohemrythrins, and gas vesicles (Reis et al., 2024). When further exploring the methanotrophic community of our two systems, the species belonging to the family Methylomonadaceae showed the potential for sulfide detoxification and partial denitrification (Figure 4 and Supplementary Figure 8). The relevance of these gammaproteobacterial MOBs in C, N, and S cycling has been also documented in freshwater canal biofilms, lake sediments, and in hypoxic eutrophic coastal system (Deng et al., 2024; Pelsma et al., 2023b; Venetz et al., 2023). Members of the genus Methylomonas seem to thrive very well under oxygen limitation as was first documented in 2015 (Kits et al., 2015), and later confirmed in other systems such as nitrate-reducing methane-oxidizing anoxic bioreactors systems (Guerrero Cruz et al., 2018). In addition to the use of oxidized nitrogen compounds, these Methylomonas seem well-equipped to deal with reduced S compound oxidation, and have the potential to disproportionate thiosulfate (S2O3 2−), tetrathionate (S4O6 2−), elemental sulfur (S0) or sulfide, as was likewise described in the pure culture of the alphaproteobacterial Mehylovirgula thiovorans HYI and
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