## 3 Results
### Microbial Activities  
The importance of microbial growth is recognized in hydrocarbon reservoirs (e.g., biodegradation of petroleum by the action of microorganisms) [93] and in the feasibility of UGS (e.g., microbial transformation of stored H2) [90; 94]. According to [PERSON] et al. [94], possible outcomes of storing hydrogen might involve the microbial alteration of hydrogen, which could lead to the creation of unwanted byproducts like hydrogen sulphide, methane, and acids. Additionally, these processes can cause corrosion, blockages, and the dissolution of reservoir materials. Various microorganisms, especially sulphate reducers, methanogens, and acetogens, mediate these changes. The primary cause of hydrogen loss in subsurface reservoirs is microbial metabolism (e.g., increasing percentage of H2 was transferred to that bacterium, which was able to utilize the preferred electron acceptor), making it a critical concern for underground gas storage projects [62]. [PERSON] et al. [62] performed geochemical modeling to examine the hydrogen loss associated with hydrogen dissolution and fluid-rock interactions using PHREEQC as a function of temperature and pressure. The introduction of hydrogen into reservoirs accelerates electron release for bacteria and archaea, leading to significant hydrogen consumption by existing electron acceptors and environmental risks associated with the products of microbial metabolism [68]. All these interactions are greatly affected by some physicochemical processes that occur in reservoirs during hydrogen storage, including changes in wettability, interfacial tension, diffusion, adsorption, and solubility [58]. Observations made on-site indicate that the underground storage of hydrogen functions similarly to a natural chemical reactor, producing methane. This production process is driven by the metabolic activities of methanogenic bacteria, which use hydrogen and carbon dioxide to produce methane and water [69]. According to [PERSON] [69], biotic reactions are recognized as the primary cause of hydrogen loss during storage, occurring under standard reservoir conditions (35 MPa and 130 C) and giving rise to additional concerns. For instance, the sulphate-reducing reaction generates an acidic environment and leads to sulphide precipitation near the wellbore, resulting in corrosion of materials and decreased permeability [69]. In the study developed by [18], \"if the diffusivity of bacteria is neglected with respect to the effective diffusivity of the injected gas, then only large-scale spatial waves arise\". A minimal, yet existing, diffusivity of bacteria results in the emergence of extra small-scale linear oscillations, with their duration serving as the inherent parameter of the phenomenon, correlating with the ratio of bacteria-to-gas diffusivity. Based on the review by [PERSON] et al. [90], _Homoacetogenic_ bacteria also use H\({}_{2}\) and CO\({}_{2}\) in a metabolic reaction known as acidogenesis, producing acetate and methane. Acetate can easily convert into acetic acid through protonation [95]. In the presence of sulphate in a reservoir, H\({}_{2}\) can be converted into H\({}_{2}\)S by sulphate-reducing bacteria like the _Desulfovibrio genus_, which use it as an electron donor [96]. Finally, iron (III)-reducing, bacteria use available iron oxide present in the mineral phase of the rock [97]. Biologically produced H\({}_{2}\)S has the same impact as abiotically produced H\({}_{2}\)S. According to [PERSON] et al. [98], microbial activities can also lead to issues such as pore blockage due to mineral precipitation and microbial films due to increased microbial population density, which can affect injectivity or production. [PERSON] et al. [99] studied the impact of biochemical reactions on H\({}_{2}\) storage in sandstones (depleted gas fields) by considering injection, production cycle lengths, cushion gas, volumes, and microbial model parameters.