Fermentative Metabolism of Chlamydomonas reinhardii

Abstract

The anaerobic photodissimilation of acetate by Chlamydomonas reinhardii F-60 adapted to a hydrogen metabolism was studied utilizing manometric and isotopic techniques. The rate of photoanaerobic (N2) acetate uptake was approximately 20 moles per milligram chlorophyll per hour or one-half that of the photoaerobic (air) rate. Under N2, cells produced 1.7 moles H2 and 0.8 mole CO2 per mole of acetate consumed. Gas production and acetate uptake were inhibited by monofluoroacetic acid (MFA), 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) and by H2. Acetate uptake was inhibited about 50% by 5% H2 (95% N2 (3) suggested that acetate increased gas production by consuming ATP which regulated the unspecified sequence of reactions giving rise to CO2 and H2. Healey (18) modifying a mechanism put forward by Jones and Myers (20) to explain the Kok effect in blue-green algae, proposed a flow of electrons from acetate via the citric acid cycle into PSI resulting in the photoevolution of H2 from reduced Fd. The operation of an anaerobic and light-dependent citric acid cycle which affects the stoichiometric conversion of acetate to CO2 and H2 had been documented in the photosynthetic purple bacteria ( 13).The present communication summarizes the results of a detailed investigation of the anaerobic photometabolism of acetate by C. reinhardii F-60, with reference to stoichiometry of gas (CO2 and H2) production, incorporation into cellular components, and sensitivity of the process to a variety of inhibitors. The stoichiometric relationships observed, together with the isotopic distribution following assimilation of ["4C]acetate, constitute strong evidence for the conclusion that anaerobic carbon oxidation occurs in part through the reactions of the glyoxylate and citric acid cycles. MATERIALS AND METHODSAlgal Growth Conditions. Chlamydomonas reinhardii (Dangeard) F-60, a mutant strain with an incomplete photosynthetic carbon reduction cycle but with an intact photosynthetic electron transport chain, was obtained from R. K. Togasaki, Indiana University. Cells were grown in batch cultures on an acetatesupplemented medium (14)

Interaction between Starch Breakdown, Acetate Assimilation, and Photosynthetic Cyclic Electron Flow in Chlamydomonas reinhardtii

“…Whereas P700 ϩ reduction is accelerated in the WT, it is markedly slower in sta6, showing that, when respiration is blocked by myxothiazol and ATP is depleted in the cells, sta6 cells are no longer capable of assimilating acetate, confirming that acetate assimilation is ATP-dependent (2, 3). Likewise, acetate cannot be assimilated under dark anaerobic conditions (28), suggesting that the ATP produced by glycolysis and fermentation is not enough to assimilate acetate. It explains why the burst of reducing power we observe after myxothiazol addition in the WT in acetate is only transient and correlates with the rate measured in stt7-9 dum11 in the same conditions.…”

Metabolic repression of transcription in higher plants.

“…In alga1 cells fed with acetate, the glyoxysome, a single membrane-bound organelle, is involved in using acetyl-COA for gluconeogenesis through the glyoxylate cycle (Beevers, 1969;Monroy and Schwartzbach, 1984;Gibbs et al, 1986;Steinbiss and Zetsche, 1986;Rikin and Schwartzbach, 1989). The presence of acetate inhibits the expression of photosynthetic genes under light but activates the expression of glyoxysome enzymes for the metabolism of acetate (Monroy and Schwartzbach, 1984;Gibbs et al, 1986;Steinbiss and Zetsche, 1986;Kindle, 1987;Rikin and Schwartzbach, 1989). These observations suggest that in algae acetate is a more favorable exogenous carbon source than CO, , probably because the utilization of CO, involves the expression of more complex photosynthetic pathways.…”

Hydrogen Photoevolution Indicates an Increase in the Antenna Size of Photosystem I in Chlamydobotrys stellata during Transition from Autotrophic to Photoheterotrophic Nutrition

“…In the first step of acetate dissimilation, the acetate is converted to acetyl-CoA, a reaction that involves a pyrophosphoryl split of ATP, which is coupled to the acetylation of CoA by free acetate (13 It has to be emphasized that the increased ATP/NAD(P)H demand in the photoheterotrophic cells brings about not only changes in the antenna size and energy distribution (state transition), but it governs the photosystem stoichiometry as well. This is reflected in an increased number of PSI reaction centers ( Fig.…”

“…Another possible interpretation of acetate effect on H2 photoevolution has to be considered also. During acetate metabolism, both C reinhardtii and C stellata consume ATP and produce NAD(P)H (13,31). The increased level of NAD(P)H may serve as the necessary electron donor system to PSI and in turn, via Fd, may accelerate the activation of hydrogenase in a reductive process (13,24).…”

“…During acetate metabolism, both C reinhardtii and C stellata consume ATP and produce NAD(P)H (13,31). The increased level of NAD(P)H may serve as the necessary electron donor system to PSI and in turn, via Fd, may accelerate the activation of hydrogenase in a reductive process (13,24). Other factors that may also result in differences in the activation of hydrogenase enzyme in the photoheterotrophic and autotrophic algae may be associated with the different 02-evolving activities of PSII or with differences in the reductive deoxygenation of the latent hydrogenase enzyme as well as in differences in protein synthesis of hydrogenase and its activators (12).…”