Very little is known about the electron transfer reactions that occur during aerobic respiration on soluble iron by members of the Actinobacteria, a phylum of Eukaryotes that contains Metallosphaera sedula. We tested the hypothesis that iron oxidation is expressed constitutively in this moderately thermophilic, Gram-positive organism. M. sedula was cultured organotrophically on yeast extract at pH 1.9 and mixotrophically on yeast extract and soluble ferrous iron at pH 1.5, both at 65o C. Initial velocities of iron oxidation were determined from the linear increases in absorbance at 350 nm due to the generation of oxidized iron. The time courses of ferrous iron oxidation as catalyzed by intact cells of M. sedula were monitored using an integrating cavity absorption meter (ICAM) that permitted the acquisition of accurate absorbance data in suspensions of intact cells that scatter light. Regardless of the immediate growth history of the organism, M. sedula was observed to readily oxidize soluble iron, and the kinetics of aerobic respiration on soluble iron by the intact organism conformed to the Michaelis–Menten formalism. It was thus evident that the ability to respire aerobically on iron was expressed in this organism under both growth conditions, which was consistent with the hypothesis. Electron transfer reactions among colored cytochromes in the intact eukaryotic cells were subsequently monitored in the ICAM in the presence of much higher concentrations of the intact cells. When whole cells were mixed with soluble iron at pH 1.5, the aerobic iron respiratory chain of M. sedula was dominated by the redox status of two abundant cellular chromophores that had maximum absorbance peaks at 442, 485, and 578 nm in the reduced state. The intracellular chromophores were reduced within the time that it took to mix a suspension of the intact bacterium with soluble ferrous iron. The reduced chromophores then returned to their original oxidized states as the organisms catalyzed the complete oxidation of the limiting concentration of ferrous iron by the excess concentration of molecular oxygen. The kinetic behaviors of these reduced chromophores were consistent with the hypothesis that their oxidation represented the rate-limiting step in the overall aerobic respiratory process.

Aerobic respiration on soluble iron is expressed constitutively by Metallosphaera sedula

255th National Meeting (2018)

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