Microbial Community Dynamics Provide Evidence for Hypoxia During a Coral Reef Mortality Event

Abstract

In July 2016, a severe coral reef invertebrate mortality event occurred approximately 200km southeast of Galveston, Texas at the East Flower Garden Bank wherein upwards of 80% of corals in a 0.06 km2 area died. Based on surveys of dead corals and other invertebrates shortly after this mortality event, responders hypothesized that localized hypoxia was the most likely direct cause. However, no dissolved oxygen data were available to test this hypothesis because oxygen is not continuously monitored within the Flower Garden Banks sanctuary. Here we quantify microbial plankton community diversity based on four cruises over two years at the Flower Garden Banks, including a cruise just 5-8 days after the mortality event was first observed. In contrast with observations collected during baseline conditions, microbial plankton communities in the thermocline were differentially enriched with taxa known to be active and abundant in oxygen minimum zones or that have known adaptations to oxygen limitation shortly after the mortality event (e.g. SAR324, Thioglobaceae, Nitrosopelagicus, and Thermoplasmata MGII). Unexpectedly, these enrichments were not localized to the East Bank, but were instead prevalent across the entire study area, suggesting there was a widespread depletion of dissolved oxygen concentrations in the thermocline around the time of the mortality event. Hydrographic analysis revealed the southern East Bank coral reef (where the localized mortality event occurred) was uniquely within the thermocline at this time. Our results demonstrate how temporal monitoring of microbial communities can be a useful tool to address questions related to past environmental events.