Isolation and Some Properties of a Mesophilic and Mixotrophic Iron-oxidizing Bacterium, OKM-9

An iron-oxidizing bacterium strain, OKM-9, isolated from mud obtained from the bottom of a pond, Minamikata Ohike, in Okayama prefecture, Japan, grew well in an FeSO₄•7H₂O (3%)-medium (pH 2.5) with 0.03% yeast extract. However, the strain could not grow either in an FeSO₄•7H₂O (3%)-medium without yeast extract or in a yeast extract (0.03%)-medium (pH 2.5) without Fe²⁺. The strain did not use elemental sulfur as an energy source and did not have the activity to fix carbon dioxide. Strain OKM-9 could grow in an FeSO₄•7H₂O (3%)-medium with twenty different L-amino acids instead of yeast extract. Incorporation of [U-¹⁴C] glutamic acid into the cells was dependent on the energy produced by the oxidation of Fe²⁺. Strain OKM-9 did not grow heterotrophically using amino acids and hexoses as a sole energy and carbon source. The results that strain OKM-9 absolutely required ferrous iron (Fe²⁺) as a sole energy source and yeast extract or L-amino acids as a carbon source for growth strongly suggest that the strain is a mixotrophic iron-oxidizing bacterium. Strain OKM-9 was a Gram-negative and rod-shaped bacterium (0.4-0.6×1.6-2.2 μm) and the mean G+C content of the DNA of the bacterium was 59.6 mol%. The optimum temperature and pH for growth were 30°C and 2.1, respectively. However, the strain could not grow at temperatures above 45°C. Iron-oxidizing activities of strain OKM-9 measured with intact cells and the plasma membrane were 14.3 and 5.7 μl O₂ uptake/mg protein/min, respectively. The pyridine ferrohemochromes prepared from the plasma membrane of this strain showed absorption peaks characteristic of α-bands of heme a and b, but not heme c, at 587 and 557 nm, respectively. The results suggest that the cytochromes composing an iron-oxidation system of strain OKM-9 are different from those of the well-known mesophilic iron-oxidizing bacteria Thiobacillus ferrooxidans and Leptospirillum ferrooxidans.