Inorganic polyphosphate involved in the symbiosis between chloroplasts of alga Codium fragile and mollusc Elysia viridis

Abstract

THE availability of inorganic phosphate (Pi) often limits plant growth, and in some cases a deficiency is avoided by the synthesis and degradation of phosphate storage compounds such as inorganic polyphosphates1. Such compounds have been detected in yeasts2, bacteria2, algae3, slime moulds3 and mosses3, but only occasionally in angiosperms4–7. In algae they are synthesised in the light8 at the expense of ATP generated by cyclic photophosphorylation9, and their function as an energy reservoir and phosphate store is well established1,2,10,11. The ability of algae to synthesise polyphosphate suggests that symbiotic forms may be important to the phosphate economy of their animal hosts in phosphate-deficient environments. The significance of symbiotic dinoflagellates in the ‘tight’ cycling of nutrients such as nitrogen and phosphorus has been suggested for coral reefs12,13, although the evidence is circumstantial. We report here direct evidence of the synthesis and breakdown of polyphosphate by Codium fragile chloroplasts symbiotic in the ophistobranch mollusc Elysia viridis, which lives in the intertidal zone, where ambient levels of Pi are extremely low.