PHYSIOLOGICAL CHARACTERISTICS OF PHOTOSYNTHESIS IN <i>PORPHYRIDIUM CRUENTUM:</i> EVIDENCE FOR BICARBONATE TRANSPORT IN A UNICELLULAR RED ALGA¹

Abstract

ABSTRACT Various physiological characteristics of photosynthesis in the unicellular red alga Porphyridium cruentum Naegeli have been investigated. The rate of photosynthesis was optimal at 25° C and pH 7.5 and was not inhibited by 21% oxygen over a temperature range of 5 to 35° C. Kinetics of whole cell photosynthesis as a function of substrate concentration gave a K 1/2 , (CO 2 ) of 0.3 μM. CO 2 compensation point, measured in a closed system at pH 7.5, was a constant 6.7 &amp;#x006d;̈L · L −1 over the temperature range 15 to 30° C and was unaffected by O 2 concentration. Whole cell photosynthesis, measured in a closed system at alkaline pH, showed that the rates of oxygen evolution were greatly in excess of the rate of CO 2 supply from the spontaneous dehydration of HCO 3 − in the medium. This indicates that bicarbonate is utilized by the cell to support this photosynthetic rate. These physiological characteristics of Porphyridium cruentum are consistent with the hypothesis that this alga transports bicarbonate across the plasmalemma.