Microbial Community Structure and Activity Changes in Response to the Development of Hypoxia in a Shallow Estuary

Abstract. We examined the effects of changing from oxic to anoxic conditions on microbial communities using both biogeochemical and molecular approaches in a semi-enclosed estuary (Jinhae Bay, Republic of Korea). Total microbial activity, represented by oxygen demand in the water column (WOD) or sediment (SOD), revealed that the respective microbial communities in the water and sediment responded differently to low dissolved-oxygen (DO) conditions. In the sediment, SOD and the total microbial abundance, as assessed by quantitative polymerase chain reaction (qPCR) analysis, decreased under low DO conditions, indicating that the microbial adaptation to anaerobic metabolism was not well established during hypoxia development. In the water column, however, neither the total abundance of microbes nor the WOD were affected by hypoxic conditions. Regardless of DO concentration, WOD showed a positive correlation with water temperature, implying that the aerobic metabolism was sustained even under hypoxic conditions, through the intermittent supply of oxygen. In addition to the spatially different responses of microorganisms, unique responses of specific groups were noted in sulfur (S) and nitrogen (N) cycling microbes. Sulfide-oxidizing prokaryotes (SOP) dominated in the water column, and no significant changes were evident in their abundance or diversity with hypoxia. However, in sediment, distinctive sulfate-reducing bacteria (SRB) were present at each sampling period during hypoxia development (a SRB succession), implying that each SRB group has varying sensitivity to DO and other electron acceptors. Our results illustrated similarities in composition and activity of N-cycling microbes between the seasonal hypoxia and permanent oxygen minimum zone (OMZ). Vertical profiles of dissolved inorganic nitrogen, including ammonium (NH₄⁺) and nitrate (NO₃⁻), and changes in archaeal abundance indicated that the NH₄⁺-oxidizing archaea (AOA) varied spatially and temporally, depending on NH₄⁺ and oxygen availability in the water column, under mature hypoxic conditions. The intriguing N dynamics recently discovered in the OMZ might also be important in the coastal hypoxic zone.