Structural and functional changes in marine microbial communities associated with oxygen dynamics

Global warming and eutrophication promote hypoxia in aquatic systems, with projected decreases in ocean oxygenation and changes at all levels of biological organization. This PhD study investigated how spatial and temporal changes in oxygen availability affect the benthic community structure and organic matter degradation on the outer Western Crimean Shelf (Black Sea). With the onset of hypoxia the benthic community oxygen uptake rate decreased, and organic matter degradation pathways shifted from aerobic to anaerobic. Surface sediments accumulate more organic matter under hypoxic and anoxic conditions, accompanied by a decrease in faunal activity. However, microbial diversity increased towards anoxic conditions and was accompanied by an increase of microbial activity and a dominance of microbial organic matter degradation. Where the Black Sea chemocline meets the seabed, the seafloor morphology has led to the accumulation of labile organic matter. This environment harbored distinct thiotrophic mat-forming bacteria previously unnoticed in the Black Sea. The anaerobic microbial community has the potential to degrade the deposited material under anoxic conditions, increasing sulfide production that could eventually decrease the degradability of otherwise fresh organic matter. Accordingly, anoxic conditions presented three-fold more unique sulfur-bearing compounds, suggesting that sulfurization could protect organic matter from being degraded.