The role of plankton in coral trophodynamics

Historically, reef-building corals have been considered to be photoautotrophs due to their symbiosis with dinoflagellates that transfer photosynthetically fixed carbon to the animal tissue. Nevertheless, corals also obtain carbon heterotrophically through capture of plankton, ingestion of suspended particulate matter, and uptake of dissolved organic compounds. This review assesses the effects of heterotrophy on coral physiology, and how strongly feeding on all of these food sources contributes to coral energy budgets. Evidence in the literature demonstrates that feeding has a positive effect on coral tissue, enhancing the growth of both partners of the symbiosis. Nevertheless, the effects of feeding are light dependent: in general, tissue quality (lipid and protein composition) is enhanced in the presence of an adequate food source only under low-light conditions or in bleached corals. On the other hand, growth rates are typically highest under conditions of high light and food availability. However, under low-light conditions, feeding can provide a mechanism to maintain skeletal growth rates even though photosynthesis is reduced. Overall, a strong interaction between autotrophy and heterotrophy is apparent for scleractinian corals. Feeding can play a central role in maintaining physiological function when autotrophy is reduced. Moreover, taking all food sources into account, heterotrophy contributes more strongly to coral energy budgets than was previously thought. Nevertheless, not all symbiotic corals can sufficiently upregulate heterotrophic feeding to compensate for reduced photosynthesis, and identifying which coral species are facultative heterotrophs should be a focus of future research