A Unique Feature of Hydrogen Recovery in Endogenous Starch-to-Alcohol Fermentation of the Marine Microalga, <i>Chlamydomonas perigranulata</i>

Abstract

A unicellular marine green alga, Chlamydomonas perigranulata, was demonstrated to synthesize starch through photosynthesis, store it in a cell, and ferment it under anaerobic conditions in the dark to produce ethanol, 2,3-butanediol (butanediol), acetic acid, and carbon dioxide (CO2). Previous fermentation data of an algal biomass cultivated outdoors in a 50-L tubular photo-bioreactor showed good carbon (C) recovery in the fermentation balance, with a higher ratio to alcohols and, therefore, lower ratio to CO2 in the C distribution of products than what would be expected from the embden-Myerhof-Parnas pathway. These findings led to a proposed concept for a CO2-ethanol conversion system (CDECS). The above data were evaluated in terms of hydrogen (H) recovery with the following results: C recovery at 105% was well balanced, although H recovery was as high as 139%, meaning an additional gain of H through fermentation. This finding was reproduced wholly in a set of experiments carried out in the same month of the following year, October, whereas another set of experiments was carried out in the following June provided ordinary fermentation results in terms of C and H recoveries with poor growth. Further analyses of these data revealed that butanediol is equal to ethanol as a product from a putative conversion system from CO2 to the detected fermentation products, leading to the revision of the CDECS concept to a CO2-alcohol conversion system (CDACS). The relevance of the CDACS will be discussed in relation to the cultivation conditions employed by chance.