Seasonal shifts in fatty acids and sterols in sponges, corals, and bivalves, in a southern Gulf of Mexico coral reef under river influence

Tropical coral reefs influenced by rivers are exposed to changes in turbidity, sedimentation, and wastewater discharges that might affect the growth and survival of the corals and their associated community. However, riverine nutrients and fine particulate organic matter can constitute food resources for suspension-feeding animals within reef ecosystems. Organic biomarkers from sessile suspension feeders represent their surrounding environment more reliably and can assist in monitoring the health of ecosystems. Here we evaluated the seasonal (dry and rainy) effect of natural and anthropogenic sources of organic matter on fatty acid and sterol biomarkers of sessile suspension and filter-feeding animals such as the sponge Aplysina sp., the great star coral Montastraea cavernosa, and the amber pen shell Pinna carnea in a coral reef under river influence. Sponges did not show significant differences in their fatty acid profiles between dry and rainy seasons, except for increases in fatty acids associated with the consumption of bacteria in the rainy season. However, they shifted from higher proportions of episterol, brassicasterol, and poriferasterol, which are abundant in phytoplankton, red, and brown algae, respectively, in the dry season, to higher proportions of β-sitosterol abundant in higher plants such as seagrass and mangrove, in the rainy season. In contrast, the fatty acid profile of the whole coral M. cavernosa had an 80% multivariate similarity with that of its symbiont Symbiodiniaceae, a fraction which was separated only in the rainy season. The whole coral M. cavernosa also had a higher proportion of gorgosterol, in the dry season. However, during the rainy season, the whole coral shifted from higher proportions of gorgosterol and campesterol to higher proportions of cholesterol, brassicasterol from red algae and episterol, a distinctive sterol of phytoplankton. Bivalves reflected a significant shift in their fatty acids following seasonal phytoplankton changes, but did not show seasonal changes in their sterols. Sterol profiles in sponges, corals, and bivalves were significantly different from the sterol profile in sewage. However, in the rainy season, their similarity increased compared to the dry season. Still, coprostanol, a human feces biomarker indicative of sewage input, was detected in the suspended particulate matter at the surface, but not in any of the suspension-feeding animals studied. This suggests that marine currents dispersed urb