Growth Kinetics and Yield Coefficients of the Extreme Thermophile Thermothrix thiopara in Continuous Culture

Abstract

Thermothrix thiopara did not appear to be stressed at high temperature (72°C). Both the actual and theoretical yields were higher than those of analogous mesophilic sulfur bacteria, and the specific growth rate (μ max ) was more rapid than that of most autotrophs. The specific growth rate (0.58 h −1 ), specific maintenance rate (0.11 h −1 ), actual molar growth yield at μ max ( Y max = 16 g mol −1 ), and theoretical molar growth yield ( Y G = 24 g mol −1 ) were all higher for T. thiopara (72°C) than for mesophilic (25 to 30°C) Thiobacillus spp. The growth efficiencies for T. thiopara at 70 and 75°C (0.84 and 0.78) were significantly higher than at 65°C (0.47). Corresponding specific maintenance rates were highest at 65°C (0.41 h −1 ) and lowest at 70 and 75°C (0.11 and 0.15 h −1 , respectively). Growth efficiencies of metabolically similar mesophiles were generally higher than for T. thiopara. However, the actual yields at μ max were higher for T. thiopara because its theoretical yield was higher. Thus, at 70°C, T. thiopara was capable of deriving more metabolically useful energy from thiosulfate than were mesophilic sulfur bacteria at 25 and 30°C. The low growth efficiency of T. thiopara reflected higher maintenance expenditures. T. thiopara had higher maintenance rates than Thiobacillus ferroxidans or Thiobacillus denitrificans , but also attained higher molar growth yields. It is concluded that sulfur metabolism may be more efficient overall at extremely high temperatures due to increased theoretical yields despite increased maintenance requirements.