244 Chapter 8 allowing us to further study dynamic methane-oxidizing microbial communities under methane-saturated conditions, with varying ammonium, sulfide, and nitrate levels, mimicking different scenarios: oligotrophic and eutrophic. Although oxygen levels were below detection limits in the long-term monitoring bioreactors, under pressure during the sequencing batch reactor (SBR) settling phases and tubing leakage may have allowed trace amounts of oxygen to enter the systems. This trace oxygen could have supported the growth of cell containing copper-dependent membrane monooxygenases (CuMMOs), which are essential for both methane and ammonia oxidation to methanol and hydroxylamine, respectively. Similar (micro)oxic bioreactor systems have previously enriched for stable co-cultures of complete ammonia-oxidizing (comammox) bacteria and anaerobic ammonium oxidation (anammox) bacteria (van Kessel et al., 2015). Our study primarily enriched for methanotrophic bacteria from the Gammaproteobacteria and the family Methylomonadaceae, specifically Methylomonas species. These methanotrophs demonstrated high metabolic versatility, including the presence of both high- and low-affinity oxidases, partial denitrification capabilities, and sulfide detoxification through sulfide oxidoreductase (SQR), as detailed in Chapter 3. This adaptability to fluctuating oxygen conditions aligns with findings from other studies on ubiquitous and versatile Gammaproteobacteria in microxic/anoxic waters and sediments (Reis et al., 2024; Schorn et al., 2024a; Thamdrup et al., 2019) where the family Methylomonadaceae has been observed in coastal, seasonally hypoxic marine lakes (Venetz et al., 2023) or prevalent dutch canal wall biofilms (Pelsma et al., 2023b). Several strategies to overcome oxygen starvation have been described in literature: 1) scavenging oxygen via high-affinity oxidases or bacteriohemerythrin, intracytoplasmatic membranes to increase surface area, and gas vesicles to regulate position relative to oxygen gradients (Reis et al., 2024) cryptic oxygen production through nitric oxide dismutation (e.g. “Ca. Methylomirabilis”), perchlorate respiration, and associations with oxygenic photosynthetic organisms providing oxygen (Ettwig et al., 2010; Li et al., 2024; Milucka et al., 2015; Ruff et al., 2023) and, 3) shift to fermentation-based methanotrophy or, to use alternative electron acceptors respiration (nitrate, nitrite, iron-oxides) (Kits et al., 2015; Li et al., 2023).
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