Carbohydrate movement from autotrophs to heterotrophs in parasitic and mutualistic symbiosis.

Abstract

Summary1. The bulk of the fixed carbon which moves from autotroph to heterotroph in most symbiotic associations is in a single compound, a carbohydrate. Techniques employing 14C have been most valuable for investigating this movement.2. Most ‘zoochlorellae’ belong to the Chlorococcales, and they release carbohydrate to the animal tissue as either glucose or maltose. In some molluscs, the ‘zoochlorellae’ are actually chloroplasts, possibly derived from siphonaceous algae. Although it is known that these chloroplasts supply photosynthetically fixed carbon to the animal tissue, the form of the carbon compounds which move is not known. In Convoluta roscoffensis the ‘zoochlorellae’ belong to the Pyramimonadales, but carbohydrate movement has not yet been directly studied in this association.3. Most ‘zooxanthellae’ belong to the Dinophyceae. In associations involving co‐elenterates and molluscs, glycerol is the main carbohydrate moving to the animal. Homogenates of the host animal tissue stimulate excretion by isolated zooxanthellae.4. In lichens, symbiotic blue‐green algae release glucose to the fungus, but the various genera of green algae that have been studied all release polyols (either ery‐thritol, ribitol or sorbitol). Lichen fungi rapidly synthesize mannitol from all these compounds. When lichen algae are isolated into pure culture, they soon lose the ability to excrete carbohydrate, and intracellular production of the carbohydrate that is excreted either becomes much reduced, or ceases altogether.5. Mostly indirect evidence indicates that sucrose is the main carbohydrate moving from flowering plants to their associated symbiotic fungi. Diversion of the translocation stream towards the site of the association occurs. The fungi convert host sugars to their own carbohydrates, principally trehalose and polyols.6. ‘Saprophytic’ higher plants are all obligately mycotrophic and receive carbohydrate from their associated fungi. In at least some associations, the fungus is simultaneously associated with an autotrophic higher plant, which is the ultimate source of carbohydrate for the association.7. Some parasitic higher plants possess chlorophyll, but the extent to which they depend on their host for carbohydrate varies with different species. Green mistletoes evidently derive negligible carbon from their hosts, but other green parasites derive at least some. There is no evidence that any of the chlorophyll‐containing parasites export carbohydrate back to their hosts. Parasitic higher plants which lack chlorophyll presumably derive all their carbohydrates from their hosts, but experimental investigations of this are scarce.8. Comparison between different types of symbiotic association show that a number of common features emerge.9. The algal symbionts of both invertebrates and lichens have, in comparison to free‐living forms, reduced growth rates and greater incorporation of fixed carbon into soluble carbohydrates. They excrete a much greater proportion of their fixed carbon than free‐living forms, and most of it is usually as a single carbohydrate. Particularly striking is the fact that the excreted carbohydrate is one which is either not the major intracellular carbohydrate, or one which ceases or nearly ceases to be produced in culture.10. The translocation stream of autotrophic higher plants is diverted towards the site of association with either fungi or parasitic higher plants, but it is not known how this is achieved.11. In all associations, the cell walls of the autotroph become reduced or modified at the site of contact with the heterotroph, but it seems likely that this is not directly connected with the mechanism of carbohydrate transfer between the symbionts.12. In many associations, the heterotroph rapidly converts host sugars into other compounds (frequently into its own carbohydrates which are usually different from those of the host). This may serve to maintain a concentration gradient and so ensure a continued flow from the host.13. Polyols feature prominently in symbiotic and parasitic associations, not only as the carbohydrates of many plant heterotrophs, but also as the form of carbohydrate released by both zooxanthellae and the green algae of lichens to their heterotrophic partners.