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Background  The production of biohydrogen (H 2 ) as a promising future fuel in anaerobic hyperthermophiles has attracted great attention because H 2  formation is more thermodynamically feasible at elevated temperatures and fewer undesired side products are produced. However, these microbes require anoxic culture conditions for growth and H 2  production, thereby necessitating costly and time-consuming physical or chemical methods to remove molecular oxygen (O 2 ). Therefore, the development of an O 2 -tolerant strain would be useful for industrial applications.    Results  In this study, we found that the overexpression of  frhAGB -encoding hydrogenase genes in  Thermococcus onnurineus  NA1, an obligate anaerobic archaeon and robust H 2  producer, enhanced O 2  tolerance. When the recombinant FO strain was exposed to levels of O 2  up to 20% in the headspace of a sealed bottle, it showed significant growth. Whole transcriptome analysis of the FO strain revealed that several genes involved in the stress response such as chaperonin β subunit, universal stress protein, peroxiredoxin, and alkyl hydroperoxide reductase subunit C, were significantly up-regulated. The O 2  tolerance of the FO strain enabled it to grow on formate and produce H 2  under oxic conditions, where prior O 2 -removing steps were omitted, such as the addition of reducing agent Na 2 S, autoclaving, and inert gas purging.    Conclusions  Via the overexpression of  frhAGB  genes, the obligate anaerobic archaeon  T. onnurineus  NA1 gained the ability to overcome the inhibitory effect of O 2 . This O 2 -tolerant property of the strain may provide another advantage to this hyperthermophilic archaeon as a platform for biofuel H 2  production.    Electronic supplementary material  The online version of this article (10.1186/s13068-019-1365-3) contains supplementary material, which is available to authorized users.

biotechnology for biofuels

Biohydrogen production of obligate anaerobic archaeon Thermococcus onnurineus NA1 under oxic conditions via overexpression of frhAGB-encoding hydrogenase genes