Abstract
Regular oxygen depletion is occurring every summer in the depression area of the Bohai Sea. The community structure and potential functions of microbes in expanding marine hypoxic area are of great importance due to their roles in biogeochemical cycling. In this study, the diversity and distribution of bacteria based on 16S rRNA gene in sediment along an inshore-offshore transect across the oxygen-depletion area in the Bohai Sea was investigated in June, July and August of 2018 by employing high-throughput sequencing. Results revealed that the bacteria community was dominated by Proteobacteria (42.67%), Actinobacteria (14.13%), Chloroflexi (13.02%), Acidobacteria (8.01%), and Bacteroidetes (6.30%). During oxygen depletion, the bacteria community from inshore site A3 subjected to dramatic variation from June to August, but the composition tended to be stable in sites from the depression area along the transect. Distinct switch of bacteria from aerobic to anaerobic group was observed when the DO concentration <4.2 mg/L, typically represented by dominance of Anaerolineaceae in August sample. Further, KEGG prediction by PICRUSt confirmed the variations by showing significant difference in functional pathways, especially the nitrogen metabolisms, before and after DO depletion (p < 0.05). These variations could be influenced by depth, NO2– concentration and DO availability based on RDA analysis. The details in diversity and composition of bacteria under continually observation provide insights into both instant and long-term response of bacteria community to oxygen depletion, and the distinct functional switch under this process expands our knowledge on the metabolic character of bacteria in worldwide hypoxia areas.
Highlights
As a vital part of marine ecosystem, sediments are impacted by geological, hydrodynamical, physicochemical and biological processes (Köster and Meyer-Reil, 2001) and acting as both source and sink of adsorbed nutrients (Zhou et al, 2017), heavy metals (Dou et al, 2013), and persistent hydrophobic organics (Lofrano et al, 2017)
The ocean oxygen minimum zones (OMZ) were found extending globally, typical OMZ had been reported from the Indian Ocean (Arabian Sea, the Bay of Bengal), the Pacific Ocean and the Atlantic (Namibian upwelling) (Stramma et al, 2008; Wright et al, 2012; Schmidtko et al, 2017)
Under the increasing pressure of anthropogenic activity and climate change, oxygen depletion zone began to occur in central Bohai Sea (Zhai et al, 2012, 2020; Wei et al, 2019), with a regular occurrence every August ranging from 1.07 to 3.60 mg/L in the past 10 years
Summary
As a vital part of marine ecosystem, sediments are impacted by geological, hydrodynamical, physicochemical and biological processes (Köster and Meyer-Reil, 2001) and acting as both source and sink of adsorbed nutrients (Zhou et al, 2017), heavy metals (Dou et al, 2013), and persistent hydrophobic organics (Lofrano et al, 2017). Bacteria in Oxygen-Depleted Sediment nitrogen inputs resulted from application of synthetic nitrogenous fertilizers and industry activity, nitrogen pollution in coastal ecosystem has led to deleterious ecological changes (Canfield et al, 2010; Cui et al, 2013). As a key environmental factor, dissolved oxygen (DO) affects biochemical processes of marine organisms and acts as one of the vital drivers in the global carbon and nitrogen cycling (Voss et al, 2013; Mahaffey et al, 2020). Some autotrophic denitrifying bacteria and anaerobic ammonium oxidizing (anammox) bacteria are involved in the fixation of carbon dioxide (CO2) by activating electron acceptors: the former fixes CO2 while reducing sulfur, forming compounds with nitrate; the latter fixes CO2 while oxidizing ammonium plus nitrite to dinitrogen, both of which are under anaerobic environment (Koeve and Kähler, 2010)
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