Microbiome signature of different stages of hypoxia event in Wonmun Bay.

We investigated how microbial communities associated with different hypoxic stages respond to environmental changes across three water depths in Wonmun Bay, South Korea. Analysis of temperature, salinity, dissolved oxygen (DO), and nutrient concentrations revealed prominent seasonal shifts and strong stratification during summer hypoxia. Metabarcoding of prokaryotic 16 S rRNA genes and phototrophic eukaryotic chloroplasts along with quantitative PCR (qPCR) revealed variations in the abundance and composition of these communities. Chloroplast 16 S sequences in May were dominated by land plants (93% of Embryophyta), contrasting with the diverse phytoplankton taxa detected in other months. The water communities in May also had higher total microbial abundance than other months but significantly lower alpha diversity. These results suggest a major influence of freshwater discharge on water communities, pre-conditioning for hypoxia events by promoting organic matter decomposition coupled with DO consumption in bottom water. Subsequently, distinct microbial communities were observed across depths during hypoxia in June and July, while less variability was detected among different depths in September and later months when hypoxia events disappeared. Principal Coordinate analysis (PCoA) demonstrated the distinct patterns of microbial communities in May, June, and July from other months. Both sulfur-oxidizing and sulfate-reducing bacteria (SRB) were prevalent in June while the increase of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) was observed in mid and bottom water in July. This data suggests the intricate interaction between sulfur and nitrogen-cycling microbes during the hypoxia events in Wonmun Bay. In conclusion, this study provides valuable insights into the microbial community responses to the varying environmental conditions at different stages of hypoxia events in eutrophic coastal ecosystems.