A method to disentangle and quantify host anabolic turnover in photosymbiotic holobionts with subcellular resolution

A wide range of organisms host photosynthesizing symbionts. In these animals the metabolic exchange between host and symbionts has prevented in situ host anabolic turnover to be studied without the confounding effect of translocated photosynthates. Using the symbiotic coral Stylophora pistillata as a model organism and [1-¹³C]-pyruvate and [2,3-¹³C]-pyruvate in different incubation conditions (light, light + DCMU, and darkness), we employed NanoSIMS isotopic imaging to quantify host anabolism, with and without translocated metabolites from their photosynthesizing dinoflagellate symbionts. Under our experimental conditions, host de novo lipid synthesis accounted for ~40% of the total holobiont lipid reserve, and dinoflagellate recycling of metabolic ¹³CO₂ enhanced host tissue ¹³C-enrichment by 13-22% in the epidermis, 40-58% in the gastrodermis, and 135-169% in host lipid bodies. Furthermore, we show that host anabolic turnover in different tissue structures differs, in a manner consistent with the localisation, function and cellular composition of these structures.