Uric acid utilization inPlatymonas convolutaeand symbioticConvoluta roscoffensis

Abstract

Uric acid is an excellent nitrogen source for the growth of cultures ofPlatymonas convolutae, the symbiotic alga fromConvoluta roscoffensis, and relatedPlatymonasandTetraselmisspecies. Nitrate-grown cells ofP. convolutaeandT. tetrathelehave two uptake systems for uric acid, which conform to Michaelis–Menten kinetics; a high-affinity system operating in the concentration range 0·2–4·5 μM a nd a low-affinity system operating at higher concentrations of uric acid. Uric acid uptake byP. convolutaeis abolished by uncouplers of phosphorylation. In darkness, intact cells ofP. convolutaemetabolize [2-14C]uric acid to [14C]carbon dioxide. These results are consistent with the proposal that the algal symbionts ofC. roscoffensisutilize uric acid, received from the host, as a nitrogen source.Aposymbiotic juvenile and symbiotic adultC. roscoffensisunder standard culture conditions contain uric acid. The solid uric acid content of juveniles declines on establishment of symbiosis withP. convolutaeand the endogenous uric acid is utilized in the adult symbiosis under conditions of nitrogen demand. However, adultC. roscoffensisdo not utilize exogenous uric acid. The growth of adult but not juvenileC. roscoffensisis dependent on nitrogen enrichment of the medium, and it is proposed that uric acid utilization is of significance to the growth of the developing symbiosis in a nitrogen-poor environment.