δ13C and δ15N in tissue of coral polyps and epilithic algae inhabiting damaged coral colonies under the influence of different light intensities

Studies were performed of the carbon and nitrogen stable isotope (δ13C and δ15N) composition (δ13C and δ15N) of the corals Porites cylindrica and P. lutea (5 years after damaging the colonies by the bleaching events) and of epilithic algae settled onto damaged areas of coral colonies. Coral polyps and three epilithic algal communities (‘red algal turf, green algal turf and red calcified crusts’) were sampled along the boundary between communities of coral polyps and algal colonizers from differently illuminated habitats from 2 to 90% of incident surface photosynthetically active radiation (PAR0). It was found that communities with a predominance of red algae significantly differed from communities with a predominance of green algae in δ13C but not in δ15N values. An influence of habitat irradiance was found only for communities of coral polyps for δ13C and δ15N values: under bright light (70–90% PAR0) polyp tissues of both coral species were significantly enriched in heavy carbon isotopes and insignificantly in nitrogen isotopes (δ13C values difference ~4‰) relative to tissues of corals under lower light 15–50% PAR0. On the basis of these results we assumed that differences in light intensities in the habitat ranging from 15 to 90% PAR0 do not influence on accessibility of the main carbon and nitrogen sources for corals and algae, and exchange by these elements between organisms. We also assumed that the relative enrichment in the heavy carbon isotopes of coral tissues in high light is a result of decreased isotope fractionation (or the absence of fractionation in photosynthesis of their zooxanthellae).