Mixotrophy drives niche expansion of verrucomicrobial methanotrophs.

Aerobic methanotrophic bacteria have evolved a specialist lifestyle dependent on consumption of methane and other short-chain carbon compounds. However, their apparent substrate specialism runs contrary to the high relative abundance of these microorganisms in dynamic environments, where the availability of methane and oxygen fluctuates. In this work, we provide in situ and ex situ evidence that verrucomicrobial methanotrophs are mixotrophs. Verrucomicrobia-dominated soil communities from an acidic geothermal field in Rotokawa, New Zealand rapidly oxidised methane and hydrogen simultaneously. We isolated and characterised a verrucomicrobial strain from these soils, Methylacidiphilum sp. RTK17.1, and showed that it constitutively oxidises molecular hydrogen. Genomic analysis confirmed that this strain encoded two [NiFe]-hydrogenases (group 1d and 3b), and biochemical assays revealed that it used hydrogen as an electron donor for aerobic respiration and carbon fixation. While the strain could grow heterotrophically on methane or autotrophically on hydrogen, it grew optimally by combining these metabolic strategies. Hydrogen oxidation was particularly important for adaptation to methane and oxygen limitation. Complementary to recent findings of hydrogenotrophic growth by Methylacidiphilum fumariolicum SolV, our findings illustrate that verrucomicrobial methanotrophs have evolved to simultaneously utilise hydrogen and methane from geothermal sources to meet energy and carbon demands where nutrient flux is dynamic. This mixotrophic lifestyle is likely to have facilitated expansion of the niche space occupied by these microorganisms, allowing them to become dominant in geothermally influenced surface soils. Genes encoding putative oxygen-tolerant uptake [NiFe]-hydrogenases were identified in all publicly available methanotroph genomes, suggesting hydrogen oxidation is a general metabolic strategy in this guild.

Growth and neutral lipid synthesis by Yarrowia lipolytica on various carbon substrates under nutrient-sufficient and nutrient-limited conditions.

Growth and TAG production by Yarrowia lipolytica were compared for cells cultured in nitrogen-complete medium containing waste glycerol derived from biodiesel production, as well as pure glycerol, dextrose, or canola oil as the carbon sources. Growth and TAG production were also analyzed for Y. lipolytica cells cultured in nitrogen-limited media containing either pure glycerol or glycerol plus dextrose. Significantly greater amounts of TAGs were synthesized by Y. lipolytica cultured in minimal media compared to rich media (approximately 3-fold on dry weight basis when grown on glycerol). Cultures in minimal medium containing glycerol yielded 31% TAGs on a dry cell weight (dcw) basis, while cultures in minimal medium containing glycerol plus dextrose produced 38% TAGs (dcw), with glycerol consumption favored over dextrose consumption. Our results suggest that Y. lipolytica could serve as a source of TAGs for biodiesel production using crude waste glycerol generated by biodiesel synthesis.