Photoprotection from photoinhibition of symbiotic algae in corals by fluorescent pigments.

Abstract

Reef-building corals contain endosymbiotic dinoflagellates which contribute photosynthetic products to their hosts and corals and are therefore strongly dependent on sunlight for survival. However, shallow reefs are characterised by damagingly high levels of sunlight. Light also plays a role in temperature-induced damage of dinoflagellates causing their degradation and expulsion (coral bleaching). Our previous research showed that fluorescent pigments (FPs) in coral tissues form the first line of defence against solar damage. FPs, which fluoresce brightly under UV or visible light, are related to the well known green fluorescent protein (GFP) and although they are abundant in corals, their photoscreening function is still not fully understood. In the present study we investigated their photoprotective role in reducing the photoinhibitive effects of sunlight in a polymorphic coral species, by comparing dark-adapted maximal quantum yield (Fv/Fm) after exposure to high irradiances in fluorescent and non-fluorescent colonies by pulse amplitude modulation (PAM) fluorometry. Similarly, we compared the effects of elevated temperature on algal photosynthesis and used confocal laser scanning microscopy to compare structural degradation of dinoflagellate chloroplasts in these morphs. Our research demonstrated that FPs significantly reduced algal photoinhibition while the recovery of Fv/Fm was 12.5 % higher in fluorescent morphs. Experiments comparing photoinhibitory effects of elevated temperature revealed a striking difference, of 37%, in the Fv/Fm ratio between fluorescent and non-fluorescent corals. This study showed that photoprotection by FPs is an important factor even at temperature extremes, a result of great significance in the view of current trends for increases in ocean surface temperatures.