132 Chapter 4 nitrite from residual nitrate reduction or nitric oxide (Hu et al., 2019). Of these four MAGs, only MAG 65 Thiohalobacteraceae increased in abundance and had both nirK and norB transcription increased during the sulfide and NO toxicity experiment (Figure 5, Supplementary Table 2), while the other three suffered a degree of inhibition inferred from both decreases in genome coverage (Figure 3) and marker gene transcriptional activity (Figure 5, Supplementary Table 2). Given that sulfide remained below detection limit (0.15 μM) during all time points, we infer that this inhibition is attributed to nitrite (~ 100-400 μM) and NO (~ 10-13% of headspace). While nitrite and NO were likely toxic for several community members, MAG 65 Thiohalobacteraceae seemed to have taken advantage of these compounds as terminal electron acceptors. MAG 65 Thiohalobacteraceae had similar metabolic potential as the gammaproteobacterium Thiohalobacter thiocyanaticus, which has sulfur oxidation genes encoding FccAB, DsrABC, AprAB, Sat, SoeABC, and SoxXABYZ, as well as thiocyanate dehydrogenase and carbon fixation via the Calvin cycle (Tsallagov et al., 2019). Additionally, MAG 65 had three sqr copies and two sorA copies. Interestingly, MAG 39 N. versatile also had a sulfur oxidation pathway that included sqr, dsrABC, sorA or aprAB and sat. Umezawa et al. (2020) described the YTD gene cluster composed of genes yedE-like, tusA, dsrE-like, chp-1 and chp-2, which encoded proteins for sulfur disproportionation in Nitrospirota (Umezawa et al., 2020). As in this previous study, we also detected an incomplete YTD gene cluster in “Ca. N. versatile” (MAG 39), indicating that it might lack sulfur disproportionation potential via this cluster, contrasting to the Nitrospirota microorganism species 45J (Umezawa et al., 2020), which shares 64% average amino acid identity (AAI) to “Ca. N. versatile”. The lack of a complete YTD gene cluster is also described in the Nitrospirota microorganism “Ca. Sulfobium mesophilum” (Zecchin et al., 2018; Umezawa et al., 2020), which has sqr and dsrABCD as well as napAB, and nrfAH, and shares 57% average amino acid identity to MAG 39 N. versatile. These AAI values indicate that Ca. N. versatile and species 45J are likely part of the same “Ca. Nitrobium” genus but distinct species, and that together with “Ca. Sulfobium mesophilum” they form a family-level taxonomic group (Luo et al., 2014). While conducting our study, Umezawa et al. (2021) isolated a sulfur-disproportionating
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