Transfer of Carbohydrate in Lichen Symbiosis

D.C. Smith and co-workers studied the nutrition of lichen thallus by incubating samples of intact lichen under light in solutions of NaH14CO3 and then dissecting the fungal medulla at intervals. Clean preparations of algal symbionts released glucose during photosynthesis. Fixed 14C was found immediately afterwards in fungal medulla, showing that photosynthate moves from alga to the fungus. An "inhibition technique" was developed to identify the mobile carbohydrate between the alga and the fungus (Drew and Smith, 1967). Portions of lichens were permitted to photosynthesize in the presence of NaH14CO3 and a high concentration (1 to 2%) of non-radioactive glucose was added on the basis that it would compete for entry of the photosynthetic product moving from alga into the fungus and this will diffuse out into the medium. Chromatographic analyses of inhibited and non-inhibited lichens showed that [14C]-glucose was detectable after 1 min but not detected after 2 min, being rapidly converted into [14C]-mannitol. Employing this technique with a range of lichens, it was concluded that mannitol, ribitol, erythritol or sorbitol were the mobile forms of carbohydrate; the conversion of fixed carbon into polyol occurred rapidly. Over half of fixed carbon moved from alga to fungus and the main form of carbon stored in the mycobiont is triacyglycerol (Bago et al., 2002).

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