The Concept Of Microbial Affinity For Limiting Nutrients In Steady State And Rhythmic Systems

Abstract

To evaluate the role of biological rhythms in competition for survival, rhythms in cell division and limiting nutrient transport ability induced by light/dark (LD) cycles were investigated for three species of pelagic, freshwater algae growing in phosphoruslimited continuous and serially diluted batch (SDB) cultures. Nutrient transport ability of nutrient-starved microbial populations was measured as the initial slope (affinity) of a plot of limiting nutrient transport rate (V) versus extracellular limiting nutrient concentration (S). A method was devised for the determination of the affinity in continuous culture (a(,T)) by monitoring S with time. Cell division was asynchronous for the green alga, Selenastrum capricornutum, grown in LD continuous cultures and a rhythm in a(,T) for soluble reactive phosphate (Pi) was greatly affected by choice of biomass parameter. Division was strongly phased in LD SDB culture and weakly phased in continuous light (LL) SDB culture, indicating that nutrient perturbations have a greater effect on phasing of division than LD cycles for S. capricornutum. A rhythm in Pi transport rate in LD SDB culture was similar to the rhythm in continuous culture a(,T) when expressed per cell volume or cell dry weight but not when expressed per cell number. Cell division was phased for the green alga, Scenedesmus quadricauda, grown in LD continuous culture. A rhythm in a(,T) for Pi was not greatly affected by choice of biomass parameter. Cell division was also rhythmic in LD for the blue-green alga, Synechococcus Nageli. Synechococcus was an extremely efficient Pi transporter at low Pi concentrations in LD continuous culture, indicating that it should be widespread in oligotrophic systems and has probably been overlooked in past floristic studies.