Reports on the effect of temperature on the yield of thermophiles seem somewhat contradictory. During batch cultivation of B. stearothermophilus, Coultate and Sundaram (1975) found that yield on glucose decreases with increase in temperature. They proposed that this was due to inefficient co-ordination of oxidative and non-oxidative phases of metabolism, leading to accumulation of incompletely utilised glucose carbon as organic acids. Acetate accumulation has also been reported in mesophiles (Holms and Bennett, 1971), where yields seem to remain constant over a wide temperature range, but drop sharply above the optimum temperature (Sundaram, 1986). However, incomplete utilisation of glucose does not seem to be significant during continuous culture of B. acidocaldarius, particularly if it is glucose limited (Farrand et al., 1983). During culture of thermophilic Bacillus spp. isolated from TAD process in glucose mineral medium, decrease in biomass yield with increase or decrease in temperature away from the optimum, was associated with increased accumulation of overflow metabolites as volatile fatty acids, with acetate being the predominant metabolite (Ugwuanyi, 2008).
A Bacillus sp. studied by Matsche and Andrew (1973) gave constant yield on glucose between temperature of 45° and 60°C. However, for B. caldotenax, yield increased with temperature between 50° and 70°C (Kuhn et al., 1980). In spite of contradictions relating to yield, maintenance coefficient and death rates of thermophiles increase consistently with increase in temperature (Brock, 1986). It is envisaged that high growth rates and maintenance requirements, and low yield will lead to enhanced breakdown of organic materials, thus combining the low sludge production advantage of anaerobic digestion with the efficiency of aerobic processes. This is in addition to the accumulation of useful metabolic products at rates higher than would be expected of their mesophilic counterparts (Sonnleitner and Fiechter, 1983c), and could be the foundation for the exploitation of thermophiles in waste treatment, biotransformation and production of biochemicals.
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