Chesapeake Bay is one of many estuaries that experience an annual shift from oxic to hypoxic or anoxic bottom waters.  This plunge of oxygen concentration is driven by phytoplankton production when nutrient levels are elevated which causes a massive increase in heterotrophic bacterial respiration. Despite the common name given to these waters “Dead Zones” are teeming with microbial life which is able to thrive using alternative terminal electron acceptors (e.g. NO3-, Mn(IV), Fe(III), and SO42-) in a succession of redox reactions with decreasing energy yield.

Along with collaborators at UMCES Horn Point Laboratory, we aim to characterize the bacterial communities and roles they play in these variable chemical conditions. The three major hypotheses are:

• Dominant sub-pycnocline respiratory processes undergo a succession from aerobic respiration to nitrate respiration and metal reduction to sulfate reduction. 
• Bacterial growth efficiency decreases with this respiratory succession, but bacterial production remains high, resulting in very high carbon respiration rates. 
• Bacterial community composition changes little during respiratory succession until sulfate respiration dominates (i.e., the sulfide threshold), but gene expression closely tracks changes in redox conditions in order to support the most energetic respiratory processes.

The Hewson lab is focusing on the functional capabilities of these microbial communities by gene transcript (metatranscriptome) analysis of water at varying depths and chemical gradients within the bay. The Crump and Cornwell labs are analyzing genomic and geochemical data and together we hope to understand how the biogeochemistry of Chesapeake Bay is shaped by, and shapes, its diverse microbial community.