130 Chapter 4 then shared among three main organisms: “Ca. Methylomirabilis lanthanidiphila”, “Ca. Methylomirabilis tolerans”, and “Ca. Nitrobium versatile”. However, under ammonium limitation, “Ca. Methanoperedens nitroreducens” further reduced nitrite to ammonium via DNRA, and this had a cascade effect that changed microbial community structure, resulting that “Ca. Methylomirabilis tolerans” dominated and “Ca. Methylomirabilis lanthanidiphila” and “Ca. Nitrobium versatile” concomitantly decreased in abundance and activity. These results suggest that “Ca. Methylomirabilis tolerans” was a more competitive organism in scavenging nitric oxide. However, the mechanisms remain to be elucidated. Both “Ca. Methylomirabilis” species encoded only a lanthanide-dependent methanol dehydrogenase and could be enriched in the bioreactor due to cerium being provided as part of the medium (Versantvoort et al., 2018, 2019; Guerrero-Cruz et al., 2019). Anammox bacteria were more strongly limited by ammonium removal than “Ca. Methylomirabilis” species, presumably due to the dual competition for both ammonium and nitrite, the key substrates for anammox energy metabolism. Although “Ca. Methylomirabilis” organisms have a lower affinity for nitrite (Ks = 7 μM; “Ca. M. oxyfera” and “Ca. M. lanthanidiphila” (Guerrero-Cruz et al., 2019) than anammox bacteria (Ks =0.2-3 μM; “Ca. K. stuttgartiensis” and “Ca. Scalindua sp.” (Oshiki et al., 2016), additional competition for ammonium could have caused anammox to be less fit. Yet, “Ca. K. stuttgartiensis” (Ks < 5 μM for ammonium (Strous et al., 1999) seemed to better withstand ammonium starvation than “Ca. Scalindua rubra” (Ks = 3 μM for ammonium in several “Ca. Scalindua” species (Oshiki et al., 2016), indicating that “Ca. K. stuttgartiensis” affinity for ammonium could be higher than that of “Ca. Scalindua”. “Ca. Nitrobium versatile”, on the other hand, seemed to rely on nitric oxide for sulfide oxidation, decreasing in abundance as “Ca. Methanoperedens nitroreducens” (Ks = 2.1 ± 0.4 mg N L-1 for nitrate (Lu et al., 2019) and “Ca. Methylomirabilis tolerans” increased (Figure 3). All three organisms had potential to generate ammonium (Figure 4), but “Ca. Methanoperedens nitroreducens” dominated DNRA activity (Figure 5). Therefore, we conclude that DNRA from “Ca. M. nitroreducens” alone could not sustain anammox activity under ammonium, nitrite and organic carbon (except methane) limitation, favoring “Ca. Methylomirabilis tolerans”, which likely outcompeted “Ca.
RkJQdWJsaXNoZXIy MTk4NDMw