Thermophiles are remarkable for their ability to grow rapidly at their optimum temperature (Couillard et al., 1989). Several thermophiles with temperature optima between 55° and 70°C have generation times of the order of 11 to 16min as compared to 26min for mesophilic B. subtilis (Brock, 1967). However, based on theoretical expectations (of growth rate) from their high growth temperatures, it is believed that thermophiles do not grow efficiently when compared to mesophiles (Sonnleitner and Fiechter, 1983a). Thus biomass accumulation, particularly in batch cultures of thermophiles is low. This may be due to an inability to sustain their growth rate or due to low catalytic efficiency of some key thermophile enzymes, or high substrate affinity (Ks) constant of the thermophiles for their principal carbon sources (Brock, 1967). They exhibit very short exponential growth phases, due perhaps to inadequate oxygen supply at high temperature, and cell density, particularly in batch culture (Kuhn et al., 1980,1979). It is also possible that thermophiles exhibit enhanced susceptibility to toxic metabolites at high temperatures. During growth of thermophilic Bacillus spp isolated from TAD of potato process waste in glucose mineral medium Ugwuanyi (2008) reported peak specific growth rate of B. stearothermophilus as 2.63^h-1 at 1.0vvm aeration rate and 60°C, declining with decrease in aeration rate and with increase or decrease in growth temperature. B. coagulans had maximum specific growth rate of 1.98^h-1 at 55°C and 1.0vvm while B. licheniformis had its peak of 2.56^h-1 at 50°C and 1.0vvm. For both organisms also, the specific growth rates declined as the aeration rates decreased.
Growth, yield and maintenance requirements of a variety of thermophiles have been studied in batch and continuous culture. A yield of 65g cells mole-1 glucose was reported for B. stearothermophilus nondiastaticus growing optimally at 55° to 58°C in batch conditions. This value is lower, than the range of 70 to 95g mole-1 glucose reported for a variety of mesophilic aerobes (Payne, 1970). The lower yield is believed to be due to the high maintenance requirement of thermophiles (Sundaram, 1986). Low yield of thermophiles has also been reported during TAD of slaughterhouse effluent (Couillard et al., 1989). In continuous cultures however, thermophiles exhibit yields similar to mesophiles. Thus, B. caldotenax yielded 89g mole-1 glucose at 65° and 70°C (Kuhn et al., 1980), while B. acidocaldarius gave 83.5g mole-1 glucose at 51°C pH 4.3 (Farrand et al., 1983). Thermophilic populations isolated from TAD of potato process waste namely B. licheniformis, B.coagulans and B. stearothermophilus have been studied for their yield under conditions similar to waste digestion (Ugwuanyi, 2008). Peak biomass yield of 72.72gmol-1 was obtained at 50°C for batch culture of B. stearothermophilus growing in glucose mineral medium.
There is also variability in yield of thermophiles based on oxygen uptake. For instance, B. caldotenax has a yield of 50g mole-1 at 65° and 70°C compared to 56g mole-1 at 30°C, and 28g mole-1 at 35°C for E. coli, or 52g mole-1 at 35°C for Klebsiella aerogenes, suggesting that thermophiles may be more efficient than mesophiles within their permissive temperature in continuous culture (with regards to yield on oxygen). Similar variability based on oxygen supply was reported in respect of TAD associated Bacillus spp growing in glucose mineral medium (Ugwuanyi, 2008). Some thermophiles, particularly Bacillus spp have high maintenance requirement, which has been postulated as the reason, in addition to high decay rate, for their low yield in batch culture (Couillard et al., 1989). For instance, B. caldotenax has a maintenance requirement of 4.1 and 20 mmoles g -1 h-1 of glucose and oxygen respectively at 70°C and 3.8 and 20 mmoles respectively at 65°C. These figures are up to ten times the maintenance need of mesophiles. However, wide variations in maintenance requirements exist among thermophiles and this may be influenced by the environment.
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