A community metabolism approach to examine the environmental regulation of coral growth

Coral reef diversity and productivity are directly dependent on the ecological and biogeochemical dynamics (e.g. calcification, growth and recruitment) of corals, the key ecosystem architects. These corals can be stressed by relatively small modifications in temperature, light availability (turbidity and sedimentation) and storm damage, which in turn can lead to alterations of the physical and biological structure of the entire ecosystem. Further anthropogenic-induced stresses, such as over-fishing and coastal land use changes, compound this stress; however, it is now generally recognised that two of the greatest environmental threats to corals are thermal induced bleaching events and ocean acidification. Thermal bleaching refers to the dipigmentation of corals as they experience higher than average temperatures whist ocean acidification refers to the lowering of pH as elevated atmospheric CO2 slowly diffuses into seawater. Under these lower pH condiitons coral calcification rates are reduced. Research has now begun to focus on how these factors act in concert to affect coral growth; unfortunately, these studies are still technologically limited to enable full control over the complex carbon chemistry of seawater required for acidification studies. Furthermore, light availability has been ignored as a co-regulatory factor, which is surprising since light not only exacerbates coral bleaching but also enhances the rate of calcification. We have recently identified that corals with certain morphologies (Types, our terminology) exhibit alternative bleaching responses, which in turn are a function of calcification patterns and rates. Type 1 species have fast growth and calcification rates, but are extremely sensitive to environmental change. Type 2 species are more robust, slow growing and have reduced rates of calcification. Thus, the extent to which a species will be affected by climate change appears to be associated with, and potentially dependant upon, the primary growth processes. This relative susceptibility of coral Types has profound implications for both the biological and physical structure of coral reefs. Work proposed here builds on several successful studies from our laboratory examining the effect of environmental change on calcifying organisms. Existing 'pH stat' technology from these studies will be modified to provide full control of the inorganic carbon system of seawater. A series of experiments will be performed to examine, using this new technology, how coral Types respond to simultaneous changes in light, temperature and CO2, the primary factors regulating coral growth. Specifically, we focus on coral community metabolism since this represents the net activity of respiration, by the coral host and associated bacterial flora, and photosynthesis, by the coral's symbiotic microalgae, that ultimately contribute to growth (calcification). We will apply this community metabolism approach in the laboratory to obtain high levels of control upon gas exchange, thus allowing physical/chemical environmental conditions to be controlled with high precision and metabolic rates measured with high temporal resolution.