Oxygen-limited metabolism in the methanotroph 5GB1C.

The bacteria that grow on methane aerobically (methanotrophs) support populations of non-methanotrophs in the natural environment by excreting methane-derived carbon. One group of excreted compounds are short-chain organic acids, generated in highest abundance when cultures are grown under O-starvation. We examined this O-starvation condition in the methanotroph 5GB1. The 5GB1 genome contains homologs for all enzymes necessary for a fermentative metabolism, and we hypothesize that a metabolic switch to fermentation can be induced by low-O conditions. Under prolonged O-starvation in a closed vial, this methanotroph increases the amount of acetate excreted about 10-fold, but the formate, lactate, and succinate excreted do not respond to this culture condition. In bioreactor cultures, the amount of each excreted product is similar across a range of growth rates and limiting substrates, including O-limitation. A set of mutants were generated in genes predicted to be involved in generating or regulating excretion of these compounds and tested for growth defects, and changes in excretion products. The phenotypes and associated metabolic flux modeling suggested that in 5GB1, formate and acetate are excreted in response to redox imbalance. Our results indicate that even under O-starvation conditions, 5GB1 maintains a metabolic state representing a combination of fermentation and respiration metabolism.