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BIOENERGETICS AND PHOTOSYNTHESIS

Expression and Inhibition of the Carboxylating and Decarboxylating Enzymes in the Photosynthetic C₄ Pathway of Marine Diatoms¹

Department of Geosciences, Princeton University, Princeton, New Jersey 08540

There is evidence that the CO₂-concentrating mechanism in the marine diatom Thalassiosira weissflogii operates as a type of single-cell C₄ photosynthesis. In quantitative-PCR assays, we observed 2- to 4-fold up-regulation of two phosphoenolpyruvate carboxylase (PEPC) gene transcripts in Thalassiosira pseudonana cells adapted to low pCO₂, but did not detect such regulation in Phaeodactylum tricornutum grown under similar conditions. Transcripts encoding phosphoenolpyruvate carboxykinase did not appear to be regulated by pCO₂ in either diatom. In T. pseudonana and T. weissflogii, net CO₂ fixation was blocked by 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (a specific inhibitor of PEPC), but was restored by about 50% and 80%, respectively, by addition of millimolar concentrations of KHCO₃. In T. pseudonana, T. weissflogii, and P. tricornutum, rates of net O₂ evolution were reduced by an average of 67%, 55%, and 62%, respectively, in the presence of 20 µM quercetin, also an inhibitor of PEPC. Quercetin promoted net CO₂ leakage from inhibited cells to levels in excess of the equilibrium CO₂ concentration, suggesting that a fraction of the HCO₃^– taken up is fated to leak back into the medium as CO₂ when PEPC activity is blocked. In parallel to these experiments, in vitro assays on crude extracts of T. pseudonana demonstrated mean inhibition of 65% of PEPC activity by quercetin. In the presence of 5 mM 3-mercaptopicolinic acid (3-MPA), a classic inhibitor of phosphoenolpyruvate carboxykinase, photosynthetic O₂ evolution was reduced by 90% in T. pseudonana. In T. weissflogii and P. tricornutum, 5 mM 3-MPA totally inhibited net CO₂ fixation and O₂ evolution. Neither quercetin nor 3-MPA had a significant inhibitory effect on photosynthetic O₂ evolution or CO₂ uptake in the marine chlorophyte isolates Chlamydomonas sp. or Dunaliella tertiolecta. Our evidence supports the idea that C₄-based CO₂-concentrating mechanisms are generally distributed in diatoms. This conclusion is discussed within the context of the evolutionary success of diatoms.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Patrick J. McGinn (pmcginn{at}princeton.edu).

www.plantphysiol.org/cgi/doi/10.1104/pp.107.110569

^* Corresponding author; e-mail pmcginn{at}princeton.edu.

Received October 9, 2007; accepted November 6, 2007; published November 9, 2007.

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On the Inside

Peter V. Minorsky
Plant Physiol. 2008 146: 1-2. [Full Text]