Light effects on the isotopic fractionation of skeletal oxygen and carbon in the cultured zooxanthellate coral, Acropora: implications for coral-growth rates

Skeletal isotopic and metabolic measurements of the branching coral Acropora cultured in constant conditions and subjected to two light intensities were revisited. We individually compared the data recorded at low light (LL) and high light (HL) for 24 colonies, all derived from the same parent colony. Metabolic and isotopic responses to the different light levels were highly variable. High light led to productivity enhancement, reduction of surface extension, doubling of aragonite deposited weight and increased delta O-18 levels in all nubbins; responses in respiration and delta C-13 were not clear. The partitioning of the colonies cultured at HL into two groups, one showing a delta C-13 enrichment and the other a delta C-13 decrease revealed common behaviors. Samples showing an increase in delta C-13 were associated with the co-variation of low surface extension and high productivity while samples showing a decrease in delta C-13 were associated with the co-variation of higher surface extension and limited productivity. This experiment, which allowed for the separation of temperature and light effects on the coral, highlighted the significant light influences on both skeletal delta O-18 and delta C-13. The high scattering of inter-colony delta O-18 observed at one site could be due to the differing photosynthetic responses of symbiotic algal assemblages. We compared our results with observations by Gladfelter on Acropora cervicornis (1982). Both set of results highlight the relationships between coral-growth rates, micro-structures and photosynthetic activity. It appears that extension growth and skeleton thickening are two separate growth modes, and thickening is light-enhanced while extension is light-suppressed. There are multiple consequences of these findings for paleoclimatic reconstructions involving corals.