T W O FIGURES
Barker's investigation on the oxidative metabolism of Prototheca Zopfii ( '36) furnished the first convincing demonstration that during the respiration of simple organic substances by non-proliferating microorganisms assimilatory processes occur to an unexpectedly large extent.
Subsequent studies, by Giesberger ( '36) on various Spirillum species, by Clifton ('37, '39) on Pseudomonas calco-acetica, and on Escherichia coli, by Winzler and Baumberger ( '38) on yeast, by Doudoroff ( '40) on hydrogen bacteria, by Hoover and Allison (mentioned by Burk, '39) on Rhizobium species, all have amply corroborated and extended Barker's results. Thus it seems justified to conclude that the occurrence of such oxidative assimilation is indeed a general phenomenon.
The publication by Winzler and Baumberger ( '38) contains the first indication that assimilation of a considerable part of the substrate is not limited to respiring systems, but that it may occur also during fer1nentation.l This follows from a consideration of their measurements of the heat production during alcoholic fermentation. The addition of 200 mg. dextrose to a suspension of non-proliferating yeast under anaerobic conditions gave rise to the liberation of 15.8 calories as heat instead of the theoretically possible 25 calories. These data led the authors to conclude that only 70.5% of the dextrose had been fermented to ethanol and carbon dioxide, while 29.5% had been assimilated. It must be remarked, however, that this conclusion is based upon computations in which (1) the complete conversion of the added dextrose was taken for granted, and (2) the fermentation was assumed to lead to the production of ethanol and carbon dioxide, and of cell materials as the only conversion products.
Observations made on the metabolism of yeast suspensions in the presence of sugar under anaerobic conditions have furnished evidence of a more direct nature, supporting the occurrence of fermentative assimilation.
"Fermentation" is here used in tlir strictly Pasteurian sense as metabolism in the ahsence of molecular oxygen.