Active Degradation-Nitrification Microbial Assemblages in the Hypoxic Zone in a Subtropical Estuary

In 2017 summer, we observed a widespread bottom hypoxic zone at the lower estuary of the Pearl River estuary (PRE). We observed i) bacterial and AOA biomass accumulation, ii) oxygen consumption rates (respiration and nitrification) in the hypoxic zone. The potential microbial interaction of both oxygen-consuming microbes in the hypoxia remained unclear. To investigate the potential microbial interaction between AOA and heterotrophic bacteria in the hypoxic zone, we used bacterial and AOA communities (inferred from 16S rRNA gene and amoA gene, respectively) and multi-comparisons between the co-occurrence networks retrieved from DNA and RNA levels to reflect the pattern. We observed a highly diverse and active bacterial community in the hypoxia zone. The network topology revealed that Gammaproteobacteria, Bacteroidetes (Flavobacteriales), Alphaproteobacteria (Rhodobacterales and Rhodospirillales), Marinimicrobia, Cyanobacteria (Synechococcales), and AOA sublineages were module hubs and connectors, indicating that they were the keystone taxa of the microbial communities. The hub-subnetwork further showed robust co-occurrence between Gammaproteobacteria, Bacteroidetes (Flavobacteriales), Alphaproteobacteria (Rhodobacterales and Rhodospirillales), Marinimicrobia with AOA sublineages, and Nitrospinae (presumably NOB)) reflecting the formation of Degradation-Nitrification microbial assemblage in the hypoxia zone. The subnetworks revealed AOA ecotype-specific modularization and niche partitioning of different AOA sublineages. Interestingly, the recurring co-occurrence of SCM1-like II sublineages with Nitrospinae OTUs formed nitrifiers assemblage in the RNA subnetworks suggested an interaction via nitrite exchange. The Degradation-Nitrification microbial assemblage may contribute substantially to the oxygen consumption in the hypoxia formation in PRE. Our results provide new insight into the nitrifier-associated microbial assemblage that should be further explored in the marine ecosystem