Maximum yield and standard enthalpy of growth of iron-oxidising bacteria

The standard enthalpy of combustion of acidophilic, iron-oxidising lithotrophic cells was determined to be − 23.98 kJ g(cells)− 1. This value is identical to that reported for the enthalpy of combustion for the bacteria E. coli and M. methylophilus. Using an average value for the standard entropy of formation of cell mass gave a Gibbs energy for the growth reaction of + 57 kJ equiv− 1. This figure is significantly different from that previously reported. The ability of iron-oxidising cells to produce biomass is evaluated in terms of a thermodynamic model. The theoretical yield of biomass was found to be most sensitive cytoplasmic pH and to the value of the Fe3+/Fe2+ couple (E°2). E°2 is in turn dependent on the speciation of iron in solution and process water quality. The observed yields of cell mass are ca. 30% of the maximum theoretical yield calculated from thermodynamic principles.