Fermentative Metabolism of Chlamydomonas reinhardtii

Abstract

ABSTRACIEvidence is presented to substantiate a chloroplastic respiratory pathway in the green alga, Chlamydomonas reinhardtii, whereby reducing equivalents generated during the degradation of starch enter the thylakoidal chain at the plastoquinone site catalyzed by NADH-plastoquinone reductase. In this formulation, the reduced plastoquinone is oxidized either by the photoevolution (photosystem I) of H2 under anaerobic conditions or by 02 during dark respiration.In the preceding paper (7), we reported on the effect of light on the fermentative mode of starch breakdown in a strain of Chlamydomonas reinhardtii adapted to a hydrogen metabolism.When compared to cells kept in the dark, starch breakdown in the illuminated cells was decreased and the fermentative pattern was modified. Inasmuch as the rate of starch breakdown was restored on addition of the uncoupler, FCCP3, the light-induced inhibition was attributed to the role of the energy charge in regulating the glycolytic flux, in a way similar to that proposed to account for the Pasteur effect in this alga (15). The major modification of the fermentative pattern which was not reversed by FCCP or DCMU was a drop in ethanol yield relative to starch consumed. While, in the dark, ethanol formation accounted for the oxidation of the bulk of the reduced pyridine nucleotides generated in the glycolytic pathway, ethanol production in the light was a minor sink for reducing equivalents. To account for this difference, we (7) proposed that, in illuminated cells, most of the reducing equivalents formed during the oxidation of glyceraldehyde 3-P to glycerate 3-P were diverted to the thylakoidal chain at the plastoquinone site (1) with this transfer being catalyzed by NADH-plastoquinone oxido-reductase (9), thus halting the reduction of acetyl-CoA to acetaldehyde and subsequently to ethanol. PSI would catalyze the reoxidation of the reduced plastoquinone and the electrons would bleed off as H2.Making use of DBMIB, a plastoquinone antagonist (3) which is known to prevent not only reduced plastoquinone reoxidation but also noncyclic and cyclic photosynthetic electron transport, we report in this brief communication data substantiating the 'Supported by Department of Energy DE-AC02-76-ER03231 and National Science Foundation PCM 83-04147.

Photochemical and Nonphotochemical Fluorescence Quenching Processes in the Diatom Phaeodactylum tricornutum

“…This process involves oxygen-dependent electron flow via the PQ pool and may utilize the same or a similar antimycin A-sensitive component as cyclic electron transport around PSI (Ravenel and Peltier, 1991). Chlororespiration was first studied in green algae (Bennoun, 1982;Gfeller and Gibbs, 1985) and has since been reported in brown algae and higher plants (Owens, 1986b;Caron et al, 1987;Garab et al, 1989). Although past studies have shown that chlororespiration results in the dark reduction of the PQ pool (Bennoun, 1982), these studies did not consider the effects of transthylakoid proton gradient formation that would accompany this electron flow.…”

“…In addition, chlororespiratory electron flow involves the donation of electrons from a membrane-bound NADP(H)-oxidoreductase to the PQ pool and the formation of a transthylakoid proton gradient (Bennoun, 1982;Gfeller and Gibbs, 1985;Garab et al, 1989;Wilhelm and Duval, 1990). Chlororespiration has been reported in higher plants as well as green and brown algae (Gfeller and Gibbs, 1985;Caron et al, 1987;Garab et al, 1989;Wilhelm and Duval, 1990). In cyanobacteria, both photosynthetic and respiratory electron transport pathways intersect at the site of the PQ pool (Dominy and Williams, 1987).…”

Anaerobic Carbon Metabolism by the Tricarboxylic Acid Cycle

“…Although we have provided evidence of a reductive pathway from OAA to succinate in this alga during anaerobiosis, we do not know the relative flux of carbon between the oxidative and reductive paths of the TCAC. We therefore do not know the significance of the reductive pathway in the maintenance of oxidative TCAC carbon flow relative to other potential processes such as H2 evolution (9,10 (16) and in Euglena (27). It has been shown that fumarate reduction may be linked to NADH oxidation and ATP synthesis (7 and references therein) thereby providing an excellent strategy for anaerobic fermentation.…”

Fermentative Metabolism of Chlamydomonas reinhardii

“…The yields of acetate and ethanol, two major compounds of starch breakdown in the dark, were strikingly diminished during exposure to illumination, whereas CO2 evolution was enhanced by illumination. Evidence was presented in a more recent paper (15) to substantiate a chloroplastic respiratory pathway in C. reinhardii whereby reducing equivalents generated during starch breakdown were eliminated as H2 rather than coupled to the reduction of acetyl * CoA to ethanol. To account for the low yield of acetate and ethanol, we (14,15) postulated that acetate or the intermediate acetyl * CoA formed during the anaerobic photodissimilation of starch was converted to undetermined cellular material.…”