No abstract

The symbiosis between corals and dinoflagellates promotes the rapid growth of corals in shallow tropical oceans, and the high overall productivity of coral reefs. The aim of this study was to quantify and understand variation in carbon acquisition and allocation among coral species. We measured multiple physiological traits (including symbiont density, calcification, photosynthesis and tissue composition) for the same coral fragments to facilitate direct comparisons between species (Stylophora pistillata, Pocillopora damicornis, Galaxea fascicularis, Turbinaria reniformis and Acropora sp.). Tissue protein content was highly sensitive to the availability of particulate food, increasing in fed colonies of all species. Despite among-species variation in physiology, and consistent effects of feeding on some traits, overall energy allocation to tissue compared with skeleton growth did not depend on food availability. Extrapolating from our results, estimated whole-assemblage carbon uptake varied >20-fold across different coral assemblages, but this variation was largely driven by differences in the tissue surface area of different colony morphologies, rather than by differences in surface-area-specific physiological rates. Our results caution against drawing conclusions about reef productivity based solely on physiological rates measured per unit tissue surface area. Understanding the causes and consequences of among-species variation in physiological energetics provides insight into the mechanisms that underlie the fluxes of organic matter within reefs, and between reefs and the open ocean.

Extracellular alkaline phosphatase activity (APA) was measured in four tropical (Stylophora pistillata, Pocillopora damicornis, Pavona cactus, Galaxea fascicularis) and two Mediterranean (Oculina patagonica, Cladocora caespitosa) symbiotic coral species, both in the coral host and associated symbionts, as well as in one tropical non-symbiotic coral (Tubastraea sp.). The effects of light, feeding, and bleaching (loss of symbionts) were also tested in S. pistillata. Host APA increased with long-term starvation, irradiance, and bleaching, suggesting that APA is linked to the metabolic activity of the host and symbionts, and to their phosphate limitation or repletion status. The comparison of APA between coral species containing different symbiont clades suggests that clade C is less efficient than clades A and B, but this result remains to be confirmed. At environmental phosphate concentrations, if the total amount of phosphate generated by APA is taken up by the coral colony, it can supply 0.3-1.6 mmol P m 22 d 21 . In comparison, dissolved inorganic phosphorus and particulate organic phosphorus have been shown to supply 0.2-1.6 and 0.9 mmol P m 22 d 21 , respectively, highlighting the importance of APA in symbiotic corals.

Dissolved organic matter, which contains many compounds such as lipids, sugars and amino acids, is an important source of carbon and nitrogen for several symbiotic and asymbiotic tropical coral species. However, there is still no information on its possible uptake by cold-water coral species. In this study, we demonstrated that dissolved organic matter, in the form of dissolved free amino acids (DFAA), is actively absorbed by four cold-water coral species from the Mediterranean Sea. Although the uptake rates observed with 3 mM DFAA concentration were one order of magnitude lower than those observed in tropical species, they corresponded to 12-50% of the daily excreted-nitrogen, and 16-89% of the daily respired-carbon of the cold-water corals. Consequently, DFAA, even at in situ concentrations lower than those tested in this study, can supply a significant amount of carbon and nitrogen to the corals, especially during periods when particulate food is scarce.