Abstract
Zooxanthellate corals have long been known to calcify faster in the light than in the dark, however the mechanism underlying this process has been uncertain. Here we tested the effects of oxygen under controlled pCO2 conditions and fixed carbon sources on calcification in zooxanthellate and bleached microcolonies of the branching coral Stylophora pistillata. In zooxanthellate microcolonies, oxygen increased dark calcification rates to levels comparable to those measured in the light. However in bleached microcolonies oxygen alone did not enhance calcification, but when combined with a fixed carbon source (glucose or glycerol), calcification increased. Respiration rates increased in response to oxygen with greater increases when oxygen is combined with fixed carbon. ATP content was largely unaffected by treatments, with the exception of glycerol which decreased ATP levels.
Highlights
Rising ocean temperatures and declining pH have received considerable attention in recent years (e.g., Hoegh-Guldberg et al, 2007; McCulloch et al, 2012)
Exposure to light or oxygen significantly (p < 0.05) increased calcification of zooxanthellate microcolonies relative to dark controls while 1% glycerol with oxygen led to a significant decline in dark calcification (Fig. 2A)
Treatment with glucose and oxygen or lower concentrations of glycerol with oxygen was not associated with a significant change in dark calcification for zooxanthellate microcolonies
Summary
Rising ocean temperatures and declining pH have received considerable attention in recent years (e.g., Hoegh-Guldberg et al, 2007; McCulloch et al, 2012). As temperatures rise, bleaching events (the loss of symbiotic zooxanthellae) become more frequent and severe, reducing the ability of the coral to cope with other stressors, leading to the loss of coral reefs (e.g., McWilliams et al, 2005). Rising CO2 levels further compound the problem by reducing aragonite saturation state and potentially calcification (e.g., Langdon & Atkinson, 2005; Holcomb, McCorkle & Cohen, 2010). Central to our understanding of the response of corals to changing environmental conditions is an understanding of the role the symbiotic zooxanthellae play in coral calcification. Comparisons of facultatively symbiotic corals with and without zooxanthellae show increased calcification in the presence of symbionts (Jacques, Marshall & Pilson, 1983). Comparisons of zooxanthellate corals to azooxanthellate corals show that some azooxanthellate corals can grow just as rapidly
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