Coral lipid biomarkers and stable isotope values in the Anthropocene

Abstract

Coral reefs generally flourish under nutrient-poor (oligotrophic) conditions, providing significant ecosystem functioning services to human coastal populations. However, recent anthropogenic global change (i.e., climate change, eutrophication) are severely affecting coral biodiversity. Nonetheless, corals have shown species-specific tolerance ranges to these various stressors. Some species are able to survive and reproduce in chronically polluted environments. The main research questions here include what is more detrimental (climate change vs. human activities) to coral reefs and how corals are able to thrive in the Anthropocene. Reef-building corals have a mutualistic relationship with endosymbiotic dinoflagellates of Symbiodiniaceae (hereafter “symbionts”) within their gastrodermal cells. These symbionts contribute up to 90 % to the nutrient intake of their coral host through photosynthesis (autotrophy). In addition, corals can obtain nutrients through external feeding (heterotrophy) as well. To investigate the human impacts on the nutritional physiology and biochemistry in the field of coral reef ecology, a variety of sophisticated biogeochemical proxies, namely stable isotope ratios, compound-specific isotope analysis (CSIA), and fatty acid (FA) profiles were applied to corals. Two coral cores were used to trace the history of human impact on nearshore reefs in the Western Pacific Ocean. It was found that variations in CS-δ15N were the results of coupling of human activities and precipitation during the two different phases of the El Niño Southern Oscillation (ENSO) climate pattern. Decreased values of CS-δ15N after the Pacific climate shift was due to the construction of a sewage treatment plant which lowered the influx of nutrients into the ocean. As another proxy, FA profiles were applied to examine the influence of water quality. FA baselines for five symbiotic coral species (Acropora samoensis, Pavona decussata, Turbinaria peltata, Favites abdita, Platygyra carnosa) were established. It was observed that all five species had statistically discrete FA profiles. In addition, species-specific feeding strategies were defined by FA biomarkers. The two most distinct species, A. samoensis, a relatively autotrophic, and P. carnosa, a relatively heterotrophic species, showed that differences in FA profiles of each species of coral along a water quality gradient were attributed to the specific feeding strategies. Finally, FA-CSIA was applied to investigate the contribution of autotrophy to the carbon metabolism in these two trophically distinct coral species. The results revealed that carbon translocation from symbionts to coral host was related with photosynthesis efficiency, occurring rapidly and immediately. These results provide new insights into the mechanisms and limitations of FA metabolism and anabolism in coral, allowing for assessment of the overall health of a coral reef ecosystem in the Anthropocene. This further enables the determination of general health of coral reefs under environmental changes. In conclusion, both climate change and human activities negatively affect nearshore coral reefs. However, when both occur together, their individual impact is enhanced. Regardless, this study found that especially heterotrophic species of corals are more resilient to moderate eutrophication. Their primary feeding mode comes as an advantage under such conditions, initiating accelerated external feeding to balance out the shortage of sunlight reducing the photosynthesis activity of their symbionts.