Regulation of inorganic carbon acquisition by phosphorus limitation in the green alga<i>Chlorella emersonii</i>

Abstract

Inorganic phosphate (P i ) plays a central role in cellular energy transduction. As a consequence, limitation of growth by phosphate availability can have an important impact on various aspects of metabolism. Since carbon acquisition via CO 2 -concentrating mechanisms (CCMs) in most microalgae is an active process, requiring ATP, it might be expected that phosphate limitation could have an indirect regulatory influence on CCM activity. We grew the green alga Chlorella emersonii Shihira et Krauss in semicontinuous or continuous cultures in nutrient-replete conditions or with orthophosphate as the limiting nutrient. CCM activity was down-regulated by P limitation. K 0.5 (dissolved inorganic carbon) values increased from approximately 4.5 µmol·L –1 in cells growing at close to maximal rates to &gt;12 µmol·L –1 in cells growing at 0.2 d –1 . Maximal rates of photosynthesis decreased by approximately half over the same range of growth rates. Direct measurements of CCM activity showed that internal CO 2 : external CO 2 ratio was markedly decreased under P limitation, and concurrent measurements of stable carbon isotope discrimination were consistent with decreased CCM activity in the P-limited cells.Key words: phosphate, CCM, CO 2 -concentrating mechanism, Chlorella, inorganic carbon acquisition.