The pattern of carbon fixation in the marine unicellular alga Phaeodactylum tricornutum

Abstract

The short- and long-term fixation of 14CO2 by Phaeodactylum tricornutum was studied using methods of fractionation that allowed examination of all the products labelled with 14C. There was no doubt that the major pathway of CO2 fixation was into 3-phosphoglycerate, but there was also significant incorporation by β-carboxylation by means of phosphoenol-pyruvate carboxylase and transamination into aspartate. At short time intervals (10 sec), 90% of the radioactive products found were accounted for by 3-phosphoglycerate and aspartate. The lipids associated with the photosynthetic apparatus contained a high proportion of the 14C fixed, which at 60 sec was located mainly in the carbohydrate portion of the lipids. At 30 and 300 min, the chlorophylls, carotenoids and the long-chain fatty acids were heavily labelled with 14C. The monogalacto-diglycerides, the digalacto-diglycerides and the sulpholipids each had a characteristic long-chain fatty acid composition. The cell proteins and a reserve polysaccharide were also labelled with 14C at short time intervals and increased their radioactivity in a linear fashion up to the longest period studied, 300 min. The activity of the enzymes ribulose diphosphate carboxylase and phosphoenolpyruvate carboxylase was sufficient to account for the pattern of fixation found.