“Microbial life in salt caverns and their influence on H2 storage – Current knowledge and open questions.”

Hydrogen will be one of the key components for renewable energy storage in the future energy systems as it can be stored in significant volumes to overcome daily to seasonal energy fluctuations. Subsurface storage in salt caverns will be a first step. These caverns are created by solution mining in underground salt formations. Despite the high salinity in this environment, salt caverns harbor microbial life. These microorganisms can not only survive in these caverns by using unique adaptation mechanisms, but they actually cause several risks to hydrogen storage. Different metabolisms can use hydrogen as electron donor, leading to hydrogen loss and in the worst case also to H2S formation. The knowledge on salt cavern microbiology and subsequent possible effects of hydrogen is still in its infancy and only a limited number of salt caverns have been investigated so far. This review summarizes the current knowledge and key questions about halophilic (salt-loving) microbes, their adaptation strategies, their origin and potential consequences of their metabolisms. It also discusses the major factors influencing microbial activities and potential risks. This review emphasizes that more research and field trials with extensive microbial monitoring are needed before hydrogen storage in a biologically active system can be safely achieved at a global scale.