Evaluating coral trophic strategies using fatty acid composition and indices

Abstract

The ecological success of shallow water reef-building corals has been linked to the symbiosis between the coral host and its dinoflagellate symbionts (herein 'symbionts'). As mixotrophs, symbiotic corals depend on nutrients 1) transferred from their photosynthetic symbionts (autotrophy) and 2) acquired by host feeding on particulate organic resources (heterotrophy). However, coral species differ in the extent to which they depend on heterotrophy for nutrition and these differences are typically poorly defined. Here, a multi-tracer fatty acid approach was used to evaluate the trophic strategies of three species of common reef-building coral (Galaxea fascicularis, Pachyseris speciosa, and Pocillopora verrucosa) whose trophic strategies had previously been identified using carbon stable isotopes. The composition and various indices of fatty acids were compared to examine the relative contribution of symbiont autotrophy and host heterotrophy in coral energy acquisition. A linear discriminant analysis (LDA) was used to estimate the contribution of polyunsaturated fatty acids (PUFA) derived from various potential sources to the coral hosts. The total fatty acid composition and fatty acid indices revealed differences between the more heterotrophic (P. verrucosa) and more autotrophic (P. speciosa) coral hosts, with the coral host G. fascicularis showing overlap with the other two species and greater variability overall. For the more heterotrophic P. verrucosa, the fatty acid indices and LDA results both indicated a greater proportion of copepod-derived fatty acids compared to the other coral species. Overall, the LDA estimated that PUFA derived from particulate resources (e.g., copepods and diatoms) comprised a greater proportion of coral host PUFA in contrast to the lower proportion of symbiont-derived PUFA. These estimates provide insight into the importance of heterotrophy in coral nutrition, especially in productive reef systems. The study supports carbon stable isotope results and demonstrates the utility of fatty acid analyses for exploring the trophic strategies of reef-building corals.

Fig 1. Total fatty acid composition by tissue type.

Principal component analysis of the total fatty acid compositions of coral host species (Galaxea fascicularis, Pachyseris speciosa, Pocillopora verrucosa) and their symbionts with 95% confidence ellipses.

Fig 2. Comparison of polyunsaturated fatty acid (PUFA) composition among coral host species and associated symbionts.

Principal component analysis based on the PUFA composition of Galaxea fascicularis, Pachyseris speciosa, and Pocillopora verrucosa a) coral hosts and b) their associated symbionts with 95% concentration ellipses.

Fig 3. Fatty acid indices in the context of coral trophic strategies.

Established fatty acid indices are applied in a novel context as potential indicators of coral trophic strategies (autotrophy versus heterotrophy). The ratio of fatty acids 18:1n-7 to 18:1n-9 shows the relative proportion of photosynthesis-derived versus animal-derived nutrition for corals Galaxea fascicularis, Pachyseris speciosa, and Pocillopora verrucosa in their a) host tissues and b) symbiont tissues. An additional index of photosynthesis- versus animal-derived nutrition considers the input of typically photosynthesis-derived fatty acids (16:1n-7 and 18:1n-7) relative to typically animal-derived fatty acids (18:1n-9, 20:1n-9, and 22:1n-11) for c) coral host tissues and d) symbiont tissues. Statistical differences are designated within each boxplot with capital letters.

Fig 4. Sum of long-chain monounsaturated fatty acids (LC-MUFA).

The fatty acid index of the sum of LC-MUFA (Σ20:1 and Σ22:1, as percent of total fatty acids) was used to evaluate the relative proportion of nutrition derived from copepods for the corals Galaxea fascicularis, Pachyseris speciosa, and Pocillopora verrucosa in their a) host tissues and b) symbiont tissues. Statistical differences are designated within each boxplot with capital letters.

Fig 5. Linear discriminant analysis of polyunsaturated fatty acids (PUFA).

Linear discriminant analysis based on PUFA composition (18:2n-6, 18:3n-3, 18:3n-6, 18:4n-3, 20:4n-6, 20:5n-3, 22:6n-3) was used to estimate the proportional contribution of different sources of PUFA to the host PUFA composition of corals a) Galaxea fascicularis, b) Pachyseris speciosa, and c) Pocillopora verrucosa (95% confidence ellipses).