| Summary: | The heat output of steady-state cells of Klebsiella aerogenes growing in Carbon limited and Carbon-sufficient media was measured with an LKB flow-microcalorimeter. Problems associated with oxygen shortage were eliminated by using low substrate concentrations, higher aeration rates,and high pump rates through the microcalorimeter. The recorded heat output was extrapolated to the heat production by cells in the chemostat, and this was converted to specific heat output, DeltaH<sub> p</sub>/kJ (g cell)<sup>-1</sup>. i.e. the heat output of formation of 1 g cell during one generation time. This enabled direct comparison of the heat production for different dilution rates and growth under different conditions. In C-limited cultures both DeltaH<sub>p</sub> and molar growth yield increased with increasing dilution rate and attained constant values at D > 0.5 h<sup> -1</sup> . In C-limited media DeltaH<sub>p</sub> was the same irrespective of substrate concentration and carbon source. Although the specific heat output was much greater in C-sufficient than in C-limited chemostats, it was independent of the concentration of limiting nutrient and glucose, provided glucose was supplied in excess. The increased heat output produced by adding small amounts of different substrates to glucose-limited chemostats depended on the added C-source; four types of substrate were recognised. The additional heat output increased linearly with the amount of added acetate, but not with glucose and pyruvate. Small amounts of uncouplers disturbed the steady-state heat output and the increased heat output was related to the stimulation of the ATPase system. The enhanced heat output when the pH was increased to 8, was associated with proton translocating ATPase activity. The maintenance energy obtained from the variation of the growth yield and specific heat output with dilution rate for glucose- and glycerol-limited chemostats, was partly growth rate-dependent, m<sub>g</sub>, and partly growth rate-independent, m<sup>'</sup><sub>s</sub> These values were related to the maintenance coefficients reported in the literature.
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