Gene expression of endangered coral (Orbicella spp.) in the Flower Garden Banks National Marine Sanctuary after Hurricane Harvey

Abstract

About 160 km south of the Texas-Louisiana border, the East and West Flower Garden Banks (FGB) have maintained >50% coral cover with infrequent and minor incidents of disease or bleaching since monitoring began in the 1970s. However, a storm that generated coastal flooding, which ultimately interacted with the reef system, triggered a mortality event in 2016 that killed 2.6% of the East FGB.To capture the immediate effects of storm-driven freshwater runoff on coral and symbiont physiology, we leveraged the heavy rainfall associated with Hurricane Harvey in late August 2017 by sampling FGB corals at two times: September 2017, when salinity was reduced; and one month later when salinity had returned to typical levels (~36 ppt in October 2017). Tissue samples (N = 47) collected midday were immediately preserved for gene expression profiling from two congeneric coral species (Orbicella faveolata and Orbicella franksi) from the East and West FGB to determine the physiological consequences of storm-derived runoff. In the coral, differences between host species and sampling time points accounted for the majority of differentially expressed genes. Gene ontology enrichment for genes differentially expressed immediately after Hurricane Harvey indicated increases in cellular oxidative stress responses. Although tissue loss was not observed on FGB reefs following Hurricane Harvey, our results suggest that poor water quality following this storm caused FGB corals to experience sub-lethal stress. We also found dramatic expression differences across sampling time points in the coral's algal symbiont, Breviolum minutum. Some of these differentially expressed genes may be involved in the symbionts' response to changing environments, whereas a group of differentially expressed post-transcriptional RNA modification genes also suggest a critical role of post-transcriptional processing in symbiont acclimatization. In this study, we cannot disentangle the effects of reduced salinity from the collection time point, so these expression patterns may also be related to seasonality. These findings highlight the urgent need for continued monitoring of these reef systems to establish a baseline for gene expression of healthy corals in the FGB system across seasons, as well as the need for integrated solutions to manage stormwater runoff in the Gulf of Mexico.