Ectoine bio-milking in methanotrophs: A step further towards methane-based bio-refineries into high added-value products

Abstract

This communication showed for the first time that the methanotrophic strain Methylomicrobium alcaliphilum 20Z (M. alcaliphilum 20Z) can efficiently synthesize and excrete (through a tailored bio-milking process) ectoine under continuous mode using methane (CH4) as the sole energy and carbon source. First, three consecutive 50 h fed batch fermentations consisting of alternating high salinity (6% NaCl for 24 h) and low salinity (0% NaCl for 24 h) cultivation stages were carried out in triplicate to determine the influence of sudden modifications in media salinity on ectoine synthesis and excretion. The results demonstrated that M. alcaliphilum 20Z exhibited a rapid response to osmotic shocks, which resulted in the release of the accumulated ectoine under hyposmotic shocks and the immediate uptake of the previously excreted ectoine during hyperosmotic shocks. A second experiment was carried out under continuous cultivation mode in two sequential stirred tank reactors operated at NaCl concentrations of 0 and 6%. Cells exhibited a constant intra-cellular ectoine concentration of 70.4 ± 14.3 mg g biomass−1 along the entire operation period when cultivated at a NaCl concentration of 6%. The centrifugation of the cultivation broth followed by a hyposmotic shock resulted in the excretion of ∼70% of the total intra-cellular ectoine. In brief, this research shows the feasibility of the continuous bioconversion of diluted CH4 emissions into high added-value products such as ectoine, which can turn greenhouse gas (GHG) abatement into a sustainable and profitable process.