To overcome the gaps above and building also on the results of previous studies, projects and analysis, proposals should address the following:

• Taxonomic and functional characterisation of indigenous microbial populations present in the different European porous media geological formations. Relate growth rate and hydrogen-related enzymatic functions to different boundary conditions (e.g. temperature, pressure, rock’s chemical composition, salinity, etc.) should also be considered in the inventory. An interdisciplinary approach is highly recommended. Molecular tools should be applied to profile in terms of microbial taxonomic composition, relative abundances and functional characterisation the indigenous microbial populations present in the different European porous media geological formations (e.g. targeted Polymerase chain reaction (PCR) amplification and high-throughput sequencing of 16S rRNA; analysis of functional genes as molecular biomarker).
• Assess the microbial and geochemical reactions and their interactions between the two to clearly distinguish the reactions of hydrogen (and eventually considering also the impurities due to conversion of depleted natural gas storage to hydrogen storage) within each specific site. The results of the analysis considering multiple geological environments should be reported in order to improve the knowledge regarding the conditions in which reactions take place;
• Development of site-specific bio-chemical modelling. Modelling key microbial activities (i.e., H2S production, methanogenesis, etc) should allow to understand how physical, chemical, and biological processes are coupled in an underground hydrogen storage;
• Development of standardisable and transferable methodologies for sampling autochthonous microbial populations of underground geological formations, including a reliable system to distinguish possible exogenous contamination;
• Development of laboratory methodologies for the processing and analysis of different types of samples collected from the underground storage sites (e.g. liquids, rocks, drilling mud, etc.). The methodologies developed should be described in sufficient detail to allow reliability and reproducibility of the obtained results among different laboratories;
• Application of the methodologies previously indicated for different sites, assuring an exhaustive coverage of the different European sites’ peculiarities. The methodologies should be applied in different laboratories, considering different site samples. Microbial activity should also be observed during different testing periods, possibly up to six months, and in different batch/reactor operation volumes. Different levels of experiment together with modelling will be necessary to reproduce underground storage conditions and enable upscaling to the reservoir scale.
• Definition of guidelines/protocols to support SSOs in the identification and management of risk associated to the storage of H2 in porous media geological formations, as well as future standardisation activities. The guidelines should also propose a fast-track procedure which allow the SSOs to have a preliminary qualitative assessment of the hydrogen storage feasibility, considering the main relevant factors, as well as assist SSOs in the identification of the optimum storage sites.

The knowledge and differences in the national underground storage facilities can be very significant, therefore the EU wide coverage should guarantee full usability of results for EU companies.

Building on the results of the previous activities, the proposals should, as relevant, provide recommendations and dissemination for updating and/or development of new standards at the EU and International levels.