Active degradation-nitrification microbial assemblages in the hypoxic zone in a subtropical estuary

Area of Science:

• Estuarine microbial ecology
• Biogeochemical cycling
• Molecular microbial ecology

Background:

• Widespread bottom hypoxia was observed in the Pearl River estuary (PRE) in the summer of 2017.
• Previous studies indicated high abundance and activity of ammonia-oxidizing archaea (AOA) and bacteria within these hypoxic zones.
• Elevated nitrification and respiration rates were noted in hypoxic waters, suggesting a role for these microorganisms.

Purpose of the Study:

• To investigate potential symbiotic relationships between AOA and bacteria in hypoxic estuarine environments.
• To identify specific microbial groups that consistently coexist and form ecologically significant associations.
• To understand the functional microbial assemblages driving biogeochemical processes in hypoxia zones.

Main Methods:

• Network analysis was employed to study both the presence (DNA) and active (RNA) microbial communities.
• Bacterial and AOA communities were sequenced using 16S rRNA and amoA genes, respectively.
• Analysis focused on network topology, module hubs, connectors, and subnetwork co-occurrence patterns.

Main Results:

• A diverse and active bacterial community was identified in the hypoxia zone, with RNA networks showing greater modularization than DNA networks.
• Key microbial taxa (keystone taxa) including Gammaproteobacteria, Bacteroidetes, Alphaproteobacteria, Marinimicrobia, Cyanobacteria, and AOA sublineages were identified as module hubs and connectors.
• Robust co-occurrence patterns revealed a 'Degradation-Nitrification' microbial assemblage involving bacteria, AOA, and Nitrospinae (nitrite-oxidizing bacteria), suggesting active interactions and potential nitrite exchange.

Conclusions:

• The study identified a functional 'Degradation-Nitrification' microbial assemblage in the Pearl River estuary's hypoxia zone, with specific AOA ecotypes showing modularization and niche partitioning.
• Active interactions, possibly via nitrite exchange between AOA and nitrite-oxidizing bacteria, were suggested by recurring co-occurrence in active (RNA) networks.
• This microbial assemblage likely contributes significantly to oxygen consumption, playing a crucial role in hypoxia formation in estuarine waters.