106 Chapter 4 on eutrophication (Howarth and Marino, 2006) and sulfide toxicity can lead to mortality of marine life (Grieshaber and Völkel, 1998). Such studies are needed to unravel key microbial interdependencies, competitive interactions and functional shifts, as well as to comprehend their environmental distribution and resilience under current and future disturbances. Carbon-, nitrogen- and sulfur-cycling microbial communities harbor potential for biotechnological applications such as the improvement of wastewater treatment systems. For instance, Deng et al. proposed that sulfide-driven partial denitrification could be coupled to anaerobic ammonium oxidation (anammox) in future applications, given that rapid oxidation of sulfide to elemental sulfur can prevent toxicity and inhibition of anammox activity (Deng et al., 2021). On the other hand, sulfide addition in a controlled aerated bioreactor setting promoted undesirable production of nitrous oxide and nitrite via partial denitrification and dissimilatory nitrite reduction to ammonium (DNRA), respectively (Delgado Vela et al., 2021). These observations indicate that further studies are necessary to understand complex microbial community interactions and to evaluate how they can be best employed in biotechnological applications. In this research project, we investigated transcriptional stress responses of a complex microbial community enriched in an anoxic bioreactor mimicking dynamic, brackish sediment conditions, where periodic ammonium deprivation, and sulfide and nitric oxide (NO) toxicity stresses, the chosen stressors in this study, might occur. The culture performed sulfide, ammonium and methane oxidation at the expense of nitrate via sulfide-oxidizing denitrifiers, anammox bacteria, and nitrite/nitrate-dependent anaerobic methane oxidizers (Arshad et al., 2017). This study’s aims were (i) to understand the effect of periodic ammonium removal on DNRA as a source of ammonium for anammox activity, as well as on general microbial community structure and transcriptional activity, (ii) to enrich sulfideoxidizing NO reducers (potentially represented by “Ca. Nitrobium versatile”) while characterizing microbial community structure and transcriptional responses to prolonged sulfide and NO toxicity stresses, and (iii) to unravel potential metabolic reactions utilized by key community members while pursuing the first two aims.
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