Among the biological systems that produce hydrogen, the most efficient ones achieve it through reactions catalyzed by enzymes with iron-sulfur clusters which require hypoxic microenvironments to work. The bacterium Rhizobium etli, during its symbiotic relationship with the common bean Phaseolus vulgaris, can transform nitrogen gas into ammonia in a process called nitrogen fixation. In exchange the plant provides the bacteria with carbon sources and a protected niche inside its root, where Rhizobium etli reaches a hypoxic state. We will exploit this microenvironment to produce hydrogen in Rhizobium etli introducing a pathway assembled with elements from Clostridium acetobutylicum, Desulfovibrio africanus and Chlamydomonas reinhardtii, while maintaining nitrogen fixation. The two goals of our project are to make Rhizobium etli a powerful agent in environmental protection by nitrifying soils and producing hydrogen from solar energy, and to standardize the work in Rhizobials.