Effects of perturbation on photosystem redox state in three species of octocorals

Abstract

Coral reefs are susceptible to climate change through bleaching, which is often caused by increases in ocean temperatures and light. Perturbation causes symbiotic dinoflagellates, Symbiodinium spp., that inhabit the coral to become stressed and lost. Bleaching begins with the perturbation of photosynthesis, but the actual site of damage is not clear. Chlorophyll fluorescence in living symbionts provides insight into the redox state of photosystem II and hence possible sites of damage. Using fluorescent microscopy, the relative chlorophyll fluorescence of individual symbionts was measured in control and thermally perturbed colonies of three species of alcyonacean octocoral, Phenganax parrini, Sarcothelia sp., and Sympodium sp. Individual colonies were cultured under standard conditions and images of three areas of coenenchyme per colony were obtained. The control colonies were then returned to standard culture conditions, while the thermally perturbed colonies were placed in an incubator. After 30 minutes, images of the same three areas of both colonies were acquired. For each area of coenenchyme, the fluorescence of the same symbionts was measured in images taken before and after experimental treatments. This allowed for changes in the fluorescence of individual symbionts to be followed over time. In all three species, control and thermally perturbed colonies generally showed increased chlorophyll fluorescence, suggesting greater reduction of photosystem II. Merely measuring colonies significantly increased the fluorescence of some symbionts, particularly in Sympodium sp., in which both control and thermally perturbed colonies showed significantly increased fluorescence. On the other hand, in Sarcothelia sp. and P. parrini, thermally perturbed colonies exhibited significantly increased fluorescence while controls did not. Phylogenetic analysis shows that Sarcothelia sp. and Sympodium sp. are closely related xeniids, but are only distantly related to P. parrini. Conversely, P. parrini and Sarcothelia sp. hosted Symbiodinium types D4-5 and D4-5-9, respectively, while Sympodium sp. had a type C1 variant. These results suggest that host genotype is less important and symbiont genotype more important in governing photosystem response to stress, with D type symbionts more resistant to thermal perturbation and C type more sensitive. The hypothesis that greater chlorophyll fluorescence corresponds to greater reduction of photosystem II (PSII) was supported by experiments with the PSII inhibitor dichorophenyl-dimethylurea (DCMU) carried out using similar methods. Measures of oxygen metabolism were used to determine the concentration of DCMU that largely inhibits photosynthesis. Since the effects of thermal perturbation on chlorophyll fluorescence are similar to DCMU, the results suggest that the water splitting reaction is initially unimpaired but, photochemistry is blocked down stream of photosystem II. It is possible that a coral's symbiont clade may help us to predict how coral in the field will react to future effects of climate change.