The effect of depth on the composition and saturation of total fatty acids present within the tissues and skeletons of two reef-building corals

Coral lipids and their fatty acid (FA) constituents fulfil a variety of biological functions. Here we report the FA compositions of the total lipids in the tissues and skeletons of Acropora retusa and Pocillopora meandrina at two water depths (5 and 25 m) in Mo'orea, French Polynesia. Saturated FAs C16:0 and C18:0, commonly associated with storage lipids and cell membrane phospholipids, are the dominant FAs within both the tissues and skeletons of both coral species. Polyunsaturated FAs in this study are usually only detected in coral tissues at 5 m depth. Unsaturated FAs contribute significantly more to the total FA pool at 5 m than at 25 m in the tissues of both coral species and in A. retusa compared to P. meandrina at 5 m depth. Unsaturated FAs are involved in regulating membrane structure fluidity and stress resistance. Their increased presence in shallow corals may be a response to the more variable seawater temperatures recorded at this depth, and/or may be attributed to different feeding strategies, possibly due to varying light intensities between depths. Previous research indicates that shallow water corals at the study site are more prone to bleaching, and that Acropora spp. may be more sensitive than Pocillopora spp. Variations in tissue FA composition may relate to this bleaching sensitivity, as unsaturated FAs are susceptible to lipid peroxidation caused by the presence of reactive oxygen species (ROS). Overall, the contributions of FAs to the total FA pool are similar between the tissues and skeletons suggesting a lack of active regulation of FAs within the coral skeleton.