Chemolithoorganotrophic growth of Nitrosomonas europaea on fructose

The nitrifying bacterium Nitrosomonas europaea can obtain all its carbon for growth from CO2 and all its energy and reductant for growth from the oxidation of NH3 and is considered an obligate chemolithoautotroph. Previous studies have shown that N. eurapaea can utilize limited amounts of certain organic compounds, including amino acids, pyruvate, and acetate, although no organic compound has been reported to support the growth of N. europaea. The recently completed genomic sequence of N. europaea revealed a potential permease for fructose. With this in mind, we tested if N. europaea could utilize fructose and other compounds as carbon sources to support growth. Cultures were incubated in the presence of fructose or other organic compounds in sealed bottles purged of CO2. In these cultures, addition of either fructose or pyruvate as the sole carbon source resulted in a two- to threefold increase in optical density and protein content in 3 to 4 days. Studies with [C-14] fructose showed that >90% of the carbon incorporated by the cells during growth was derived from fructose. Cultures containing mannose, glucose, glycerol, mannitol, citrate, or acetate showed little or no growth. N. europaea was not able to grow with fructose as an energy source, although the presence of fructose did provide an energy benefit to the cells. These results show that N. europaea can be grown in CO2-free medium by using fructose and pyruvate as carbon sources and may now be considered a facultative chemolithoorganotroph.