Uptake of leucine byChlorellasymbionts of green hydra

Abstract

Cells of aChlorellasp. symbiotic with green hydra were able to take up leucine via a high-affinity transport mechanism after isolation from the symbiosis, and to incorporate sequestered amino acid into protein. The time course of uptake of leucine byChlorellacells in the intact symbiosis was followed after hydra were fed with nauplii of the brine shrimpArtemia salinathat had been labelled with radioactive leucine. Uptake proceeded in two stages, the first more rapid than the second, separated by a short interval in which there was a consistently observed decrease in the amount of radioactivity per cell. Although leucine fromArtemiafree amino acid pools was accumulated disproportionately byChlorellacells in symbiosis, this was not sufficient to explain the initial rapid phase of uptake, nor could changes in rate of uptake with time after feeding be explained by changes in properties of theChlorellacells. Rather, slower uptake in the second phase, and the decrease in amount of radioactivity per cell which preceded it, were probably due to changes in supply of amino acids to theChlorellacells. Amino acids transported by the same system as leucine caused efflux of accumulated leucine from isolatedChlorellacells when present in high external concentrations. Thus the observed accumulation of radioactivity in symbiosis may have been the difference between unidirectional influx and unidirectional efflux of leucine dependent upon changes in external concentrations of unlabelled amino acids fromArtemiaor from hydra pools. This is discussed with reference to host control of algal cell division, which has been shown to be dependent upon supply of a ‘division factor’ from host food.