Abstract
Marine diatoms are responsible for up to 20% of global CO(2) fixation. Their photosynthetic efficiency is enhanced by concentrating CO(2) around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. Diatoms have been regarded as C(3) photosynthesizers, but recent metabolic labeling and genome sequencing data suggest that they perform C(4) photosynthesis. We studied the pathways of photosynthetic carbon assimilation in two diatoms by short-term metabolic (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(3)-C(4) intermediate photosynthesis in T. weissflogii, but exclusively C(3) photosynthesis in T. pseudonana. The labeling patterns were the same for cells grown at near-ambient (380 microL L(-1)) and low (100 microL L(-1)) CO(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(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(2)-concentrating mechanisms. Furthermore, it emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C(4)-metabolic enzymes as evidence for C(4) photosynthesis.
Highlights
Marine diatoms are responsible for up to 20% of global CO2 fixation
Short-term photosynthetic 14C labeling was studied in T. weissflogii and T. pseudonana grown at near-ambient (380 mL L21) or low (100 mL L21) CO2 concentration
(formed by Rubisco) and malate as primary products of photosynthesis and sugar-P as secondary products (Fig. 1). This labeling pattern resembles that of a C3-C4 intermediate flowering plant such as Flaveria linearis (Monson et al, 1986), rather than a pure C4 plant, in which malate is the dominant initial product and C3 compounds appear as intermediates (Hatch and Slack, 1966)
Summary
Marine diatoms are responsible for up to 20% of global CO2 fixation Their photosynthetic efficiency is enhanced by concentrating CO2 around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. This study suggests that the photosynthetic pathways of diatoms are diverse, and may involve combined CO2concentrating mechanisms. It emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C4-metabolic enzymes as evidence for C4 photosynthesis. By increasing the ratio of CO2 to O2, this diminishes the wasteful process of photorespiration Despite their great ecological impact, photosynthetic carbon acquisition by diatoms is still poorly understood.
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