PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Abstract

ABSTRACT Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (12C/14C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. 14C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates 14C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of 14C02 from respiratory pathways at low relative growth rates. Metabolic channelling of 14C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO2 reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of 14C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.