Hydrogenases are enzymes that catalyze the reversible reaction H₂ <-> 2H⁺ + 2e^-. They are widespread throughout phylogenetically distinct microorganisms and currently three types of hydrogenases are known: [NiFe]-, [FeFe]- and [Fe]-hydrogenases.

In hydrothermal vent environments, hydrogen is emitted with the hydrothermal fluids and can reach several millimolar. Due to seawater-endmember fluid mixing processes, steep thermal and chemical gradients can persist providing both electron donors and acceptors. Microbially mediated hydrogen oxidation generates energy (in form of ATP), which can drive local biomass synthesis.

In porewater sediments (where hydrogen levels are commonly in the nanomolar range) hydrogen has a direct impact on carbon degradation because it is expected to affect fermentation and hydrogen-based terminal metabolisms.

An activity-based screen was developed within our group to detect hydrogenase enzymes from hydrothermal vent metagenomes. This activity-based screen uses a broad-host range fosmid vector and a hydrogenase mutant (Shewanella oneidensis DhyaB). Several recombinant enzymes were recovered that exhibit H₂-uptake and -evolution ability. On the respective metagenomic fragments the orfs mostly exhibited very low similarity to hydrogenases or any known gene or no similarity at all, suggesting that there may be as yet undescribed hydrogenases out there.