A very few fungi and yeast produce succinate. The fermentation of filter paper cellulose by several anaerobic fungi has been studied in the absence and presence of methanogenic bacteria. In the absence of methanogens, Neocallimastix sp. strain L2,

N. frontalis RE1, N. patriciarum CX, Piromonas communis P and Sphaeromonas communis FG10 have been found, respectively, to produce 0.48, 0.59, 0.39, 0.81 and

O. 26 mol succinate/10 mol hexose. In the presence of methanogenic bacteria, the production of succinate is reduced significantly for all the fungi, a result attributed to the process of hydrogen transfer between the fungi and bacteria (137).

Muratsubaki elucidated the pathway by which succinate is produced by the anaerobic growth of Saccharomyces cerevisiae on glucose (138). The activity of fumarate reductase, which catalyzes the conversion of fumarate to succinate, is three times greater under anaerobic conditions than under aerobic conditions. However, succinate dehydrogenase activity is completely lost after ten hours of fermentation.

These observations indicate that for this organism the citric acid cycle has been modified to become a reductive pathway leading to succinate production during the anaerobic growth of S. cerevisiae on glucose (138).