Mechanisms of carbon acquisition for endosymbiont photosynthesis in Anthozoa

Abstract

In contrast to free-living photoautotrophs, endosymbiontic dinoflagellates of the genus Symbiodinium must absorb their inorganic carbon from the cytoplasm of their host anthozoan cell rather then from seawater. The purpose of this paper is to review the present knowledge on the source of dissolved inorganic carbon supply for endosymbiont photosynthesis and the transport mechanisms involved. Symbiodinium spp., generally known as zooxanthellae, live within the endodermal cells of their hosts, corals and sea anemones. They are separated from the surrounding seawater by the host tissues (oral ectodermal cell layer, collagenous basal membrane, endodermal cell, and perisymbiotic vesicles). The symbiotic association is therefore faced with the problem of delivering dissolved inorganic carbon to an endodermal site of consumption from an, essentially, ectodermal site of availability. Studies using original methods demonstrated that neither the internal medium (coelenteric fluid) nor paracellular diffusion could supply enough dissolved inorganic carbon for endosymbiont photosynthesis. A transepithelial active mechanism must be present in the host tissues to maintain the photosynthetic rate under saturating irradiance. A pharmacological approach led to propose a working model of dissolved inorganic carbon transport from seawater to zooxanthellae. This vectorial transport generates a pH gradient across the epithelium. The role of this gradient as well as the physiological adaptation of Symbiodinium spp. to symbiotic life are discussed.Key words: carbon concentrating mechanism, anthozoan, dinoflagellates, anion transport, symbiosis, transepithelial transport.

Carbonic Anhydrase in the Scleractinian Coral Stylophora pistillata

“…Similarly, in the gorgonian Leptogorgia virgulata, the CA activity was preferentially localized in the scleroblasts and axial epithelium, precisely on the membrane of the spicule-forming vacuole (13), again suggesting an important role of CA in the calcification process. This does not preclude, however, the presence of at least another CA isoform for CO 2 supply to symbiont as previously suggested (42,44,46).…”

Intra-Specific Variation Reveals Potential for Adaptation to Ocean Acidification in a Cold-Water Coral from the Gulf of Mexico

“…However, metabolic CO 2 is not the main source of DIC for calcification in deep-water corals (Adkins et al, 2003;Mueller et al, 2013). An increase in CO 2 will cause an increase in DIC, specifically by producing HCO − 3 and H + , which ultimately will alter the internal chemistry and lower the pH inside the cells (Allemand et al, 1998). It is plausible that the role of the isoform of αCA found in corals (Bertucci et al, 2013), is to regulate the build-up of H + ions that result from the hydration of CO 2 to HCO 3− in order to avoid internal acidosis.…”

Marine Invertebrate Larvae Associated with Symbiodinium: A Mutualism from the Start?

“…The host supplies the symbiont with CO 2 and other compounds for cellular synthesis such as nitrogen and phosphorus (Trench, 1979;Allemand et al, 1998;Leggat et al, 2003;Weis et al, 2008). In turn, the symbiont supplies the host with more than 90% of its metabolic requirement, in the form of organic compounds including glucose, glycerol, fatty acids, and amino acids (Muscatine, 1990;Grant et al, 1997;Papina et al, 2003;Burriesci et al, 2012).…”