Starch fermentation via a formate producing pathway in Chlamydomonas reinhardii, Chlorogonium elongatum and Chlorella fusca

Starch degradation was investigated during anaerobic dark incubation in the algae Chlamydomonas reinhardii, Chlorogonium elongatum and Chlorella fusca . The pathway of algal formate fermentation was elucidated by determination of the relationship between substrate consumption and product accumulation. The fate of reducing equivalents was also determined. Investigations were done on dependence of pH, fermentation time, cell cycle, and after addition of H₂, hypophosphite and inhibitors of protein synthesis.
A mixed acid fermentation that produced formate, acetate and ethanol (2:1:1) with only small amounts of H₂ and CO₂ was shown for the algal strains used. The failure of inhibition with cycloheximide and chloramphenicol indicated the constitutive presence of all fermenting enzymes. Nevertheless, glycerol, D(–)lactate and stoichiometrical amounts of ethanol and CO₂ were found additionally at extreme pH (pH 4.6 and 7.9), and after addition of H₂ and hypophosphite (7 m M ). During long-term incubation (28 h) fermentation changed from mixed acid to ethanol production. The pathways of algal fermentation did not depend on cell cycle, and fermentation rate corresponded directly to the actual starch content of algal cells. The results gave evidence for synthesis of formate during anaerobic metabolism in algae by a thioclastic cleavage of pyruvate via the enzyme pyruvate formate lyase. This indicated an algal fermentation pathway thought to be present only in procaryotic organisms.