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Plant and Cell Physiology, 2003, Vol. 44, No. 2 146-155
© 2003 Oxford University Press

Effects of Extracellular pH on the Metabolic Pathways in Sulfur-Deprived, H2-Producing Chlamydomonas reinhardtii Cultures

Sergey Kosourov, Michael Seibert and Maria L. Ghirardi1

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, U.S.A.

Sustained photoproduction of H2 by the green alga, Chlamydomonas reinhardtii, can be obtained by incubating cells in sulfur-deprived medium [Ghirardi et al. (2000b) Trends Biotechnol. 18: 506; Melis et al. (2000) Plant Physiol. 122: 127]. The current work focuses on (a) the effects of different initial extracellular pHs on the inactivation of photosystem II (PSII) and O2-sensitive H2-production activity in sulfur-deprived algal cells and (b) the relationships among H2-production, photosynthetic, aerobic and anaerobic metabolisms under different pH regimens. The maximum rate and yield of H2 production occur when the pH at the start of the sulfur deprivation period is 7.7 and decrease when the initial pH is lowered to 6.5 or increased to 8.2. The pH profile of hydrogen photoproduction correlates with that of the residual PSII activity (optimum pH 7.3–7.9), but not with the pH profiles of photosynthetic electron transport through photosystem I or of starch and protein degradation. In vitro hydrogenase activity over this pH range is much higher than the actual in situ rates of H2 production, indicating that hydrogenase activity per se is not limiting. Starch and protein catabolisms generate formate, acetate and ethanol; contribute some reductant for H2 photoproduction, as indicated by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone inhibition results; and are the primary sources of reductant for respiratory processes that remove photosynthetically generated O2. Carbon balances demonstrate that alternative metabolic pathways predominate at different pHs, and these depend on whether residual photosynthetic activity is present or not.

1 Corresponding author: E-mail, maria_ghirardi@nrel.gov; Fax, +1-303-384-6150.


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