Coral responses to temperature, irradiance and acidification stress: linking physiology to satellite remote sensing

Abstract

The success of the symbiosis of scleractinian corals with dinoflagellates of the genus Symbiodinium is highly dependent on the availability of sufficient, but not excess, light for photosynthesis. After decades of fundamental research into the effect of light on the coral-dinoflagellate symbiosis, an important practical application is emerging in remote monitoring of bleaching at coral reefs. Coral bleaching that originates with the dysfunction of photosynthesis can be either photoacclimatory, a controlled adjustment in response to environmental change, or it can be associated with photodamage, an uncontrolled response to environmental change. It is the latter that tends to lead to severe bleaching events that decrease the rate of carbon fixation, generate excessive oxygen radicals and may ultimately lead to coral death if unfavourable conditions persist. Current best practice methods for the prediction of coral bleaching use water temperature as detected via satellite, and predict the onset of coral bleaching accurately, but not the percent of corals bleached at a reef or the extent of the ensuing mortality.Due to its central role in causing photodamage, the use of light level (in addition to temperature) as a predictive variable may improve the accuracy of predictions of coral bleaching severity. The rate of photoacclimation affects the duration of elevated stress following an increase or decrease in incident light level and so it is potentially important for predicting the severity of coral bleaching.As coral reef management practitioners aim to predict bleaching and bleaching-induced mortality at entire reef locales, which are typically composed of a multi-species coral community, differences in coral physiological responses are of importance in designing accurate prediction methods. The interaction of high light and high temperature with a third stressor, ocean acidification, may influence coral bleaching, as ocean acidification changes oceanic chemical parameters that are key to cellular mechanisms of homeostasis and to dinoflagellate photosynthesis.Using multiple-stressor aquarium experiments, I investigated the photoacclimation rate in corals, differences in response to light and temperature stress among coral species, and the interaction of light, temperature and ocean acidification on coral bleaching onset and severity (at the colony scale). My investigation of photoacclimation (via 24 days of exposure to a large increase, a moderate increase, and a large decrease in light) in Acropora muricata and mounding Porites spp.indicated that the direction of light change and the water temperature influenced photoacclimation duration. However, the exact patterns were specific to the variable used to measure photoacclimation (for instance, changes in net areal photosynthesis, or changes in quantum 2 efficiency of photosystem II), with important ramifications for the development of a combined light and temperature bleaching prediction method.I investigated species-specific physiological responses in A. mu...