Photosynthesis and Calcification at Cellular, Organismal and Community Levels in Coral Reefs: A Review on Interactions and Control by Carbonate Chemistry

Abstract

SYNOPSIS. Photosynthesis and calcification in zooxanthellate scleractinian corals and coral reefs are reviewed at several scales: cellular (pathways and transport mechanisms of inorganic carbon and calcium), organismal (interaction between photosynthesis and calcification, effect of light) and ecosystemic (community primary production and calcification, and air-sea CO 2 exchanges).The coral host plays a major role in supplying carbon for the photosynthesis by the algal symbionts through a system similar to the carbon-concentrating mechanism described in free living algal cells. The details of carbon supply to the calcification process are almost unknown, but metabolic CO 2 seems to be a significant source. Calcium supply for calcification is diffusional through oral layers, and active membrane transport only occurs between the calicoblastic cells and the site of calcification. Photosynthesis and calcification are tightly coupled in zooxanthellate scleractinian corals and coral reef communities. Calcification is, on average, three times higher in light than in darkness. The recent suggestion that calcification is dark-repressed rather than light-enhanced is not supported by the literature. There is a very strong correlation between photosynthesis and calcification at both the organism and community levels, but the ratios of calcification to gross photosynthesis (0.6 in corals and 0.2 in reef communities) differ from unity, and from each other as a function of level.The potential effect of global climatic changes (pCO 2 and temperature) on the rate of calcification is also reviewed. In various calcifying photosynthetic organisms and communities, the rate of calcification decreases as a function of increasing pCO 2 and decreasing calcium carbonate saturation state. The calculated decrease in CaCO, production, estimated using the scenarios considered by the International Panel on Climate Change (IPCC), is 10% between 1880 and 1990, and 9-30% (mid estimate: 22%) from 1990 to 2100. Inadequate understanding of the mechanism of calcification and its interaction with photosynthesis severely limits the ability to provide an accurate prediction of future changes in the rate of calcification.

Nutritional status and metabolism of the coral Stylophora subseriata along a eutrophication gradient in Spermonde Archipelago (Indonesia)

“…At the same time, elevated inorganic nutrient input and sedimentation in near-shore reefs hamper G L , explaining the overall low increase in G L with respect to P g . Reasons for decreased G L under high nutrient levels are found in a decoupling of symbiont and host (Dubinsky et al 1990;Ferrier-Pagès et al 2000;Allemand et al 2004) or a direct inhibition of carbonate crystallization by inorganic nutrients (Simkiss 1964;Gattuso et al 1999). A support of G (light and dark) by either R or elevated nutritional status of the coral host (Houlbrèque et al 2003;Houlbrèque and Ferrier-Pagès 2008) was not evident in this study, except possibly at KAY, where highest R, tissue protein and biomass occurred together with highest G.…”

“…The influence of photosynthesis on calcification was recognized early (Goreau and Goreau 1959), and the theory of lightenhanced calcification is widely accepted; the underlying mechanism, however, is still strongly debated (Gattuso et al 1999;Furla et al 2000;Allemand et al 2004;Colombo-Pallotta et al 2010). Calcification is not only dependent on the metabolic behavior of the holobiont but also directly affected by environmental factors.…”

Coral reefs and changing seawater carbonate chemistry

“…This misperception is based on two assumptions about the coral/algal symbiotic relationship: a) that increased photosynthesis increases calcification rates, and b) that zooxanthellar photosynthesis will increase with rising CO 2 . The first assumption is based on the fact that zooxanthellate corals calcify faster in the light than in the dark (estimated three times as fast by Gattuso et al [1999]), and also that calcification in bleached corals slows or stops [Leder et al, 1991]. The inference is that photosynthesis enhances calcification.…”

Long-term effects of nutrient and CO2 enrichment on the temperate coral Astrangia poculata (Ellis and Solander, 1786)

“…The median coral calcification rate in the light is three times that in the dark, and symbiotic corals calcify at higher rates than non-symbiotic corals (e.g. Gattuso et al, 1999). When corals bleach and lose their zooxanthellae, calcification rates decline (e.g.…”