133 Unraveling nitrogen, sulfur and carbon microbial bioreactor responses to stress microorganism that was named Dissulfurispira thermophila, matching the previously described genus “Ca. Nitrobium” (Arshad et al., 2017). “Ca. Sulfobium mesophilum” and “Ca. N. versatile” both have dsrA genes that affiliate with bacterial-type reductive sequences, and a dsrD gene, suggested as a potential marker for sulfate reduction, absent in sulfur oxidizers that utilize the reverse Dsr pathway (Rabus et al., 2015; Anantharaman et al., 2018; Dalcin Martins et al., 2018). However, our metatranscriptomic results suggest that “Ca. N. versatile” was performing sulfide oxidation. Therefore, it seems that “Ca. N. versatile”, similarly to the deltaproteobacterium Desulfurivibrio alkaliphilus, was yet another example of a microorganism disguised as a sulfate reducer (Thorup et al., 2017). Interestingly, “Ca. Sulfobium mesophilum” and Desulfurivibrio alkaliphilus could couple sulfide oxidation to DNRA, a metabolic potential also present in MAG 39, with transcripts of nrfA detected in our study (T0; TPM = 0.030 ± 0.031). Given that norBC had significantly higher transcriptional activity than nrfA and other denitrification genes (T0, TPM norB = 1.9 ± 0.7, TPM norC = 2.8 ± 1.1), it is more likely that “Ca. N. versatile” coupled sulfide oxidation to nitric oxide reduction. Given that “Ca. N. versatile” could not be enriched under sulfide and NO, we hypothesize that MAG 65 Thiohalobacteraceae represented a more competitive microorganism. However, the mechanism and substrate affinities remain to be elucidated. In our study, methanotrophs could withstand prolonged periods of ammonium, methane and nitrate deprivation as well as exposure to elevated concentrations of nitrite and nitric oxide. During these disturbances, “Ca. Methanoperedens nitroreducens” and “Ca. Methylomirabilis tolerans” did not thrive, as indicated by downregulation of mcrA and pmoA, but tolerated stresses and persisted as abundant community members. These results suggest that methane oxidation could be a relatively stable community function in coastal ecosystems under the stresses investigated in this study and as long as the system stays anoxic. Future investigations are needed to elucidate such dynamics. Interestingly, the “Ca. Methanoperedens nitroreducens” genome (MAG 36) encoded nitric oxide reductases (Figure 4), which had not been described before in this organism. The qNOR-encoding gene, present in the same contig as mcr, nar and nrf genes, had increased transcriptional activity by the end of the sulfide and NO stress 4
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