Competition between two aquatic microorganisms for oscillating concentrations of phosphorus

Abstract

The availability of limiting nutrients is a critical factor regulating growth of aquatic microorganisms. In at least some aquatic systems the frequency of addition rather than the absolute concentration of nutrients controls community structure. Gnotobiotic continuous cultures were used to examine the growth characteristics of a green alga (Selenastrum capricornutum) and a heterotrophic yeast (Rhodotorula rubra) when phosphorus-limited steady-state populations were subjected to varying concentrations of pulsed phosphorus. The responses of these organisms to phosphorus additions were measured both in single and dual species continuous cultures. Both organisms exceeded the maximum transport rates for phosphorus predicted from batch and steady-state continuous cultures. Carbon limitation did not cause a decline in phosphorus accumulation in R. rubra. Carbon-limited yeast cultures perturbed with phosphorus attained the highest phosphorus per cell values seen in these studies. The phosphorus pool was not significantly diminished in these cultures only because the total yeast biomass was limited by carbon. These results suggest that carbon-limitation of heterotrophic populations may be essential to the existence of phytoplankton in low-nutrient aquatic environments.