AbstractMethylomicrobium alcaliphilum is an alkaliphilic and halotolerant methanotroph. The physiological responses of M. alcaliphilum to high NaCl concentrations, were studied using RNA sequencing and metabolic modeling. This study revealed that M. alcaliphilum possesses an unusual respiratory chain, in which complex I is replaced by a Na+ extruding NQR complex (highly upregulated under high salinity conditions) and a Na+ driven adenosine triphosphate (ATP) synthase coexists with a conventional H+ driven ATP synthase. A thermodynamic and metabolic model showing the interplay between these different components is presented. Ectoine is the main osmoprotector used by the cells. Ectoine synthesis is activated by the transcription of an ect operon that contains five genes, including the ectoine hydroxylase coding ectD gene. Enzymatic tests revealed that the product of ectD does not have catalytic activity. A new Genome Scale Metabolic Model for M. alcaliphilum revealed a higher flux in the oxidative branch of the pentose phosphate pathway leading to NADPH production and contributing to resistance to oxidative stress.