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Appl Environ Microbiol. 1986 January; 51(1): 180-187
Copyright © 1986, American Society for Microbiology. All Rights Reserved.

Microbial Ecophysiology of Whey Biomethanation: Intermediary Metabolism of Lactose Degradation in Continuous Culture

² Department of Bacteriology, University of Wisconsin, Madison, Wisconsin 53706, ¹ and Michigan Biotechnology Institute and the Departments of Biochemistry and Microbiology, Michigan State University, East Lansing, Michigan 48824²

ABSTRACT

The intermediary carbon and electron flow routes for lactose degradation during whey biomethanation were studied in continuous culture. The chemostat was operated under lactose-limited conditions with a 100-h retention time. The carbon balance observed for lactose degradation was 4.65 mmol of CH₄, 4.36 mmol of CO₂ and 1.15 mmol of cellular carbon per mmol of lactose consumed, with other intermediary metabolites (i.e., acetate, lactate, etc.) accounting for less than 2% of the lactose consumed. The carbon and electron recoveries for this biomethanation were 87 and 90%, respectively. ¹⁴C tracer studies demonstrated that lactose biomethanation occurred in three distinct but simultaneous phases. Lactose was metabolized primarily into lactate, ethanol, acetate, formate, and carbon dioxide. During this hydrolytic phase, 82% of the lactose was transformed into lactate. These metabolites were transformed into acetate and H₂-CO₂ in a second, acetogenic, phase. Finally, the direct methane precursors were transformed during the methanogenic phase, with acetate accounting for 81% of the methane formed. A general scheme is proposed for the exact carbon and electron flow route during lactose biomethanation, which predicts the prevalent microbial populations in this ecosystem.

^* Corresponding author.