C sub(3) and C sub(4) Pathways of Photosynthetic Carbon Assimilation in Marine Diatoms Are under Genetic, Not Environmental, Control

Marine diatoms are responsible for up to 20% of global CO sub(2) fixation. Their photosynthetic efficiency is enhanced by concentrating CO sub(2) around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. Diatoms have been regarded as C sub(3) photosynthesizers, but recent metabolic labeling and genome sequencing data suggest that they perform C sub(4) photosynthesis. We studied the pathways of photosynthetic carbon assimilation in two diatoms by short-term metabolic super(14)C labeling. In Thalassiosira weissflogii, both C3 (glycerate-P and triose-P) and C4 (mainly malate) compounds were major initial (2-5 s) products, whereas Thalassiosira pseudonana produced mainly C3 and C6 (hexose-P) compounds. The data provide evidence of C sub(3)-C sub(4) intermediate photosynthesis in T. weissflogii, but exclusively C sub(3) photosynthesis in T. pseudonana. The labeling patterns were the same for cells grown at near-ambient (380 mu L L super(-1)) and low (100 mu L L super(-1)) CO sub(2) concentrations. The lack of environmental modulation of carbon assimilatory pathways was supported in T. pseudonana by measurements of gene transcript and protein abundances of C sub(4)-metabolic enzymes (phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase) and Rubisco. This study suggests that the photosynthetic pathways of diatoms are diverse, and may involve combined CO sub(2)-concentrating mechanisms. Furthermore, it emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C sub(4)-metabolic enzymes as evidence for C sub(4) photosynthesis.