Symbiodinium genotypic and environmental controls on lipids in reef building corals

Abstract

Background: Lipids in reef building corals can be divided into two classes; non-polar storage lipids, e.g. wax esters and triglycerides, and polar structural lipids, e.g. phospholipids and cholesterol. Differences among algal endosymbiont types are known to have important influences on processes including growth and the photobiology of scleractinian corals yet very little is known about the role of symbiont types on lipid energy reserves.

Methodology/principal findings: The ratio of storage lipid and structural lipid fractions of Scott Reef corals were determined by thin layer chromatography. The lipid fraction ratio varied with depth and depended on symbiont type harboured by two corals (Seriatopora hystrix and Pachyseris speciosa). S. hystrix colonies associated with Symbiodinium C1 or C1/C# at deep depths (>23 m) had lower lipid fraction ratios (i.e. approximately equal parts of storage and structural lipids) than those with Symbiodinium D1 in shallow depths (<23 m), which had higher lipid fraction ratios (i.e. approximately double amounts of storage relative to structural lipid). Further, there was a non-linear relationship between the lipid fraction ratio and depth for S. hystrix with a modal peak at ∼23 m coinciding with the same depth as the shift from clade D to C types. In contrast, the proportional relationship between the lipid fraction ratio and depth for P. speciosa, which exhibited high specificity for Symbiodinium C3 like across the depth gradient, was indicative of greater amounts of storage lipids contained in the deep colonies.

Conclusions/significance: This study has demonstrated that Symbiodinium exert significant controls over the quality of coral energy reserves over a large-scale depth gradient. We conclude that the competitive advantages and metabolic costs that arise from flexible associations with divergent symbiont types are offset by energetic trade-offs for the coral host.

Figure 1. Map of sampling sites of corals collected over a large-scale depth gradient at Scott Reef.

Figure 2. Influence of depth on the lipid fraction ratio of (a) P. speciosa and (b) S. hystrix.

Circles represent data points, smooth line indicates the fitted model and dashed lines are 95% confidence intervals.

Figure 3. Box and whisker plots of the influence of symbiont type on the lipid fraction ratio of (a) P. speciosa and (b) S. hystrix at South Scott Reef.

For the lines in a box and whisker plot: error bars are the 95% confidence interval, the bottom and top of the box are the 25th and 75th percentiles, the line inside the box is the 50th percentile (median), and any outliers are shown as open circles.

Figure 4. Depth-generalist corals occurring in shallow to deep habitats at Scott Reef.

(a) foliaceous Pachyseris speciosa (PS) and (b) branching Seriatopora hystrix (SH) at approximately 50 m in the central lagoon of South Scott Reef. Images: A. Heyward.