Going with the flow: corals in high-flow environments can beat the heat

Coral reefs are experiencing unprecedented declines in health on a global
scale leading to severe reductions in coral cover. One major cause of this
decline is increasing sea surface temperature. However, conspecific
colonies separated by even small spatial distances appear to show varying
responses to this global stressor. One factor contributing to differential
responses to heat stress is variability in the coral’s micro-environment,
such as the amount of water flow a coral experiences. High flow provides
corals with a variety of health benefits, including heat stress
mitigation. Here, we investigate how water flow affects coral gene
expression and provides resilience to increasing temperatures. We examined
host and photosymbiont gene expression of Acropora cf. pulchra colonies in
discrete in situ flow environments during a natural bleaching event. In
addition, we conducted controlled ex situ tank experiments where we
exposed A. cf. pulchra to different flow regimes and acute heat stress.
Notably, we observed distinct flow-driven transcriptomic signatures
related to energy expenditure, growth, heterotrophy and a healthy coral
host-photosymbiont relationship. We also observed disparate transcriptomic
responses during bleaching recovery between the high- and low-flow sites.
Additionally, corals exposed to high flow showed “frontloading” of
specific heat-stress related genes such as heat shock proteins,
antioxidant enzymes, genes involved in apoptosis regulation, innate
immunity, and cell adhesion. We posit that frontloading is a result of
increased oxidative metabolism generated by the increased water movement.
Gene frontloading may at least partially explain the observation that
colonies in high-flow environments show higher survival and/or faster
recovery in response to bleaching events.