Microbial Response in a Simulated Overturn of an Oxygen Minimum Zone; Devil’s Hole, Bermuda

Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand by living biota, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil’s Hole, Bermuda is a model ecosystem to study microbial processes and community succession within OMZs because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop and organic matter accumulates. The bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of anoxic waters was assessed using a simulated overturn experiment. A full factorial matrix remineralization experiment was conducted over 20 days where surface microbial assemblages were mixed with anoxic filtrate, anoxic microbial assemblages were incubated with oxic surface filtrate and surface microbial assemblages were incubated with oxic surface filtrate as a control. The surface microbial community responded to the anoxic DOC, increasing 2.5 times after 5 days while reducing the DOC concentration by >4 µM and the total dissolved amino acid concentration by 1 µM within 2 days. This quick response suggests that the DOC accumulation within the suboxic layer is bioavailable to the oxic surface community. The deep, originally anoxic microbial community, had an initial die-off but then responded to the surface DOC increasing 1.5 times and reducing DOC concentrations by 2µM after 5 days suggesting a reduced ability of this community to metabolize surface DOC. In contrast, the surface microbial community did not respond to the surface DOC with little growth in the control after 5 days and DOC concentrations actually increasing by <1µM after 2 days. Taxonomic profiling revealed rapid community shifts during the simulated overturn comparable to that measured during in situ overturn events. Carbon regeneration by the surface microbial community upon the simulated overturn suggests that the carbon within the suboxic layer of Devil’s Hole is labile and its sequestration is short lived.