PHOSPHATE-UPTAKE BY THE YEAST, RHODOTORULA-RUBRA, AND THE GREEN-ALGA,SELENASTRUM-CAPRICORNUTUM PRINTZ, AFTER PHOSPHATE ADDITIONS TO STEADY-STATE CONTINUOUS CULTURES

We examined phosphate (P-i) uptake by two well-characterized microorga nisms: a green alga (Selenastrum capricornutum) and a heterotrophic ye ast (Rhodotorula rubra). Phosphate uptake was measured in dual- and si ngle-species continuous cultures after perturbation of a phosphorus (P )-limited steady-state culture by additions of varying concentrations of P-i. We found that, under these conditions, bath organisms had very high transport rates for P-i. The yeast was able to attain higher int ernal P concentrations than predicted from either steady-state or from P-starved batch culture data. Because the yeast was able to sequester and store P-i more efficiently than the alga under dilute P-i continu ous culture conditions, co-existence of the two organisms was ultimate ly controlled by the concentration of carbon available for growth of t he yeast.