The Role of the C4 Pathway in Carbon Accumulation and Fixation in a Marine Diatom

Abstract

The role of a C 4 pathway in photosynthetic carbon fixation by marine diatoms is presently debated. Previous labeling studies have shown the transfer of photosynthetically fixed carbon through a C 4 pathway and recent genomic data provide evidence for the existence of key enzymes involved in C 4 metabolism. Nonetheless, the importance of the C 4 pathway in photosynthesis has been questioned and this pathway is seen as redundant to the known CO 2 concentrating mechanism of diatoms. Here we show that the inhibition of phosphoenolpyruvate carboxylase (PEPCase) by 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate resulted in a more than 90% decrease in whole cell photosynthesis in Thalassiosira weissflogii cells acclimated to low CO 2 (10 mM), but had little effect on photosynthesis in the C 3 marine Chlorophyte, Chlamydomonas sp. In 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate-treated T. weissflogii cells, elevated CO 2 (150 mM) or low O 2 (80-180 mM) restored photosynthesis to the control rate linking PEPCase inhibition with CO 2 supply in this diatom. In C 4 organic carbon-inorganic carbon competition experiments, the 12 C-labeled C 4 products of PEPCase, oxaloacetic acid and its reduced form malic acid suppressed the fixation of 14 C-labeled inorganic carbon by 40% to 50%, but had no effect on O 2 evolution in photosynthesizing diatoms. Oxaloacetic acid-dependent O 2 evolution in T. weissflogii was twice as high in cells acclimated to 10 mM rather than 22 mM CO 2 , indicating that the use of C 4 compounds for photosynthesis is regulated over the range of CO 2 concentrations observed in marine surface waters. Short-term 14 C uptake (silicone oil centrifugation) and CO 2 release (membrane inlet mass spectrometry) experiments that employed a protein denaturing cell extraction solution containing the PEPCKase inhibitor mercaptopicolinic acid revealed that much of the carbon taken up by diatoms during photosynthesis is stored as organic carbon before being fixed in the Calvin cycle, as expected if the C 4 pathway functions as a CO 2 concentrating mechanism. Together these results demonstrate that the C 4 pathway is important in carbon accumulation and photosynthetic carbon fixation in diatoms at low (atmospheric) CO 2 .Diatoms are important marine photoautotrophic protists that account for up to 25% of the primary production on Earth (Falkowski and Raven, 1997). They are also important fractionators of stable carbon isotopes that are used to evaluate trophic relationships in marine food webs (Checkley and Entzeroth, 1985;Fry and Wainright, 1991) and marine carbon cycling in modern and ancient oceans (Francois et al., 1993;Hayes, 1993). As a result, the mechanisms of uptake and fixation of inorganic carbon (C i ) in these organisms have been the topics of extensive research (Korb et al., 1997;Tortell et al., 2000;Rost et al., 2003;Tchernov et al., 2003). In particular, the existence and role of a C 4 pathway for photosynthetic carbon fixation in marine diatoms has been the subject of research for over 2...

Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features

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A Metabolomic Approach to Study Major Metabolite Changes during Acclimation to Limiting CO2 in Chlamydomonas reinhardtii

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Carbon acquisition of marine phytoplankton: Effect of photoperiod length

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