Zooxanthellae providing assimilatory power for the incorporation of exogenous acetate in Heteroxenia fuscescens (Cnidaria: Alcyonaria)

Abstract

The soft coral Heteroxenia fuscescens (Ehrb.) and its isolated zooxanthellae (endosymbiotic dinoflagellates) were investigated with particular regard to uptake and utilization of exogenously supplied 14C-acetate in the light and in the dark. The incorporation of 14C from 14C-acetate into the host tissue and into the zooxanthellae was consistently much higher in the light than in the dark. The incorporated 14C-acetate was rapidly metabolized by the host and algae and was recovered from different assimilate fractions. The major proportion of radiocarbon from metabolized 14C-acetate was located in host tissue. The CHCl3-soluble fraction composed of diverse lipids showed the strongest 14C-labelling. Zooxanthellae isolated prior to incubation accounted for about 80% of the acetate incorporation recorded for zooxanthellae in situ (in vivo). It is concluded from a comparison of acetate incorporation and conversion under light and dark conditions that most of the lipid reserve of the host tissue originates from fatty acids, which are synthesized within the algal symbionts and are then translocated to the heterotrophic partner via extrusion. The acetate units needed for lipid synthesis are obtained by absorption of free acetate from dissolved organic matter (DOM) in the seawater as well as by photosynthetic assimilation of inorganic carbon. Thus, in H. fuscescens, lipogenesis is operated as a light-driven process to which the zooxanthellae considerably contribute assimilatory power by performing fatty acid synthesis and translocation of lipid compounds to their intracellular environment (host cell). A metabolic scheme is proposed to account for the different pathways of carbon conversion observed in H. fuscescens. The incubations took place in August 1980 and the analytical part from October 1980 to January 1984.