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Plant and Cell Physiology Advance Access published online on December 24, 2008
Plant and Cell Physiology, doi:10.1093/pcp/pcn200
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Photosynthetic Carbon Assimilation in the Coccolithophorid Emiliania huxleyi (Haptophyta): Evidences for the Predominant Operation of the C3 Cycle and the Contribution of β-Carboxylases to the Active Anaplerotic Reaction

Yoshinori Tsuji, Iwane Suzuki and Yoshihiro Shiraiwa*

Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8572, Japan

Corresponding author: Yoshihiro Shiraiwa, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan Tel:+81-29-853-4668 Fax:+81-29-853-6614 E-mail: emilhux{at}biol.tsukuba.ac.jp


  Abstract

The coccolithophorid Emiliania huxleyi (Haptophyta) is a representative and unique marine phytoplankton species that fixes inorganic carbon by photosynthesis and calcification. We examined the initial process of photosynthetic carbon assimilation by analyses of metabolites, enzymes, and genes. When the cells were incubated with a radioactive substrate (2.3 mM NaH14CO3) for 10 s under illumination, 70% of 14C was incorporated into the 80% methanol-soluble fraction. Eighty-five and 15% of 14C in the soluble fraction were incorporated into phosphate esters (P-esters), including the C3 cycle intermediates and a C4 compound, aspartate, respectively. A pulse-chase experiment showed that 14C in P-esters was mainly transferred into lipids, while 14C-aspartate, -alanine, and -glutamate levels remained almost constant. These results indicate that the C3 cycle functions as the initial pathway of carbon assimilation and that β-carboxylation contributes to the production of amino acids in subsequent metabolism. Transcriptional analysis of β-carboxylases such as pyruvate carboxylase (PYC), phosphoenolpyruvate carboxylase (PEPC), and PEP carboxykinase (PEPCK) revealed that PYC and PEPC transcripts were highly increased under illumination, whereas the PEPCK transcript decreased remarkably. PEPC activity was higher in light-grown cells than in dark-adapted cells. PYC activity was detected in isolated chloroplasts of light-grown cells. According to deduced amino-terminal sequence analysis, PYC and PEPC are predicted to locate in the chloroplasts and mitochondria, respectively. These results suggest that E. huxleyi possesses unique carbon assimilation mechanisms in which β-carboxylation by both PYC and PEPC play important roles in different organelles.

Keywords: β-carboxylation - coccolithophorid - Emiliania huxleyi (Haptophyta) - phosphoenolpyruvate carboxylase - photosynthetic carbon fixation - pyruvate carboxylase

(Received October 31, 2008; Accepted December 16, 2008)
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