Nutrient gradients simulate different adjustments of coral-algal symbiosis

Background: Eutrophication is one of the major causes of coral reef degradation but the effect of eutrophication on coral and its symbiont algae remains unclear, particularly for the larval stage of coral. In the present study, the physiological and transcriptomic responses of the larvae of an ecologically important scleractinian coral Pocillopora damicornis were analyzed after a 5-day exposure to elevated nitrate in order to assess the survival and adaptation of coral-algal symbiosis under elevated nutrients. Results: The results showed that multiple larval transcripts were significantly correlated with Symbiodiniaceae transcripts. The major differentially expressed transcripts in coral/Symbiodiniaceae included those responsible for energy synthesis/comsumption, nitrogen metabolism and stressor response. Slightly elevated nitrate concentration could in fact promote the health of coral meta-organism. With increase in nitrate concentrations, coral larvae showed significant stress response to maintain the coral-algal symbiosis and coral-algal symbiosis was impaired, while Symbiodiniaceae switched photosynthetic states for ATP synthesis, material transport and nitrogen metabolism for symbiosis maintenance under the control of the coral hosts.Conclusions: Our results suggest that adjustment of coral-algal symbiosis via coral control and a shift in Symbiodiniaceae photosynthetic states serves as the basis of coral meta-organism adaptation under eutrophication stresses. The larvae of P. damicornis and Symbiodiniaceae displayed different transcriptomic responses to nitrate enrichment. Coral larva meta-organism can adapt to moderately elevated nutrient concentration while extreme eutrophication can impair coral-algal symbiosis and affect coral larvae survival ultimately.

Coral recruitment: the critical role of early post-settlement survival

Martinez, S., and Abelson, A. 2013. Coral recruitment: the critical role of early post-settlement survival. – ICES Journal of Marine Science, 70: . Coral recruitment is a pivotal factor in coral reef stability and in recovery following substantial disturbances. Despite its immense importance, the study of coral recruitment has some major gaps, notably larval survival before and following settlement, mainly due to technical limitations, which stem from the difficulty in observing the minute larvae. To overcome the major limitation in coral recruitment studies, i.e. the in situ detection of recruits during their early stages, we designed a new detection set-up, composed of a fluorescence detection set-up, a grid-covered substrate, and a Geographic Information System tracking system. This set-up, enabling the identification of coral recruits soon after settlement, revealed that in the critical period of the first day, less than 45% of the settling corals may survive. The results also suggest that either coral larva select locations that may increase their survival chances or they experience dramatic mortality during the early hours of settlement, which induce a consistent pattern of spat distribution. Our study confirms an earlier speculation that the first 24 h post-settlement may determine the rates and spatial patterns of recruitment. The significant implications of these findings, and the implemented “detection set-up” for coral reef monitoring and management, are discussed.

Fluorescence of coral larvae predicts their settlement response to crustose coralline algae and reflects stress

Multi-coloured homologues of the green fluorescent protein generate some of the most striking visual phenomena in the ocean. Despite their natural prominence in reef-building corals and widespread use in biotechnology, their biological role remains obscure. Here, we experimented with larvae of Acropora millepora to determine what can be learned about a coral larva or recruit from its fluorescent colour. We performed 12 crosses between seven A. millepora colonies representing differing fluorescence phenotypes, the larvae of which were exposed to a natural settlement cue (crustose coralline algae) and heat–light stress. Parental effects explained 18 per cent of variation in colour and 47 per cent of variation in settlement. The colour of the larval family emerged as a predictor of the settlement success: redder families were significantly less responsive to the provided settlement cue ( p = 0.006). This relationship was owing to a correlation between parental effects on settlement and colour ( r 2 = 0.587, p = 0.045). We also observed pronounced (16%) decline in settlement rate, as well as subtle (2%), but a statistically significant decrease in red fluorescence, as a consequence of heat–light stress exposure. Variation in settlement propensity in A. millepora is largely owing to additive genetic effects, and is thought to reflect variation in dispersal potential. Our results suggest an optical signature to discriminate between long- and short-range dispersing genotypes, as well as to evaluate stress. Further research in this direction may lead to the development of field applications to trace changes in coral life history and physiology caused by global warming.