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The microbial communities in sulfate-rich, saline formation fluids of a natural gas reservoir in Lower Saxony, Germany were investigated to enhance the knowledge about microbial communities in potential carbon dioxide sequestration sites. This investigation of the initial state of the deep subsurface microbiota is necessary to predict their influence on the long-term stability and storage capacity of such sites. While the bacterial 16S rDNA gene library was comprised of sequences affiliating with the Firmicutes, the Alphaproteobacteria, the Gammaproteobacteria and the Thermotogales, the archaeal 16S rDNA libraries were simply dominated by two phylotypes related to the genera Methanolobus and Methanoculleus. The monitoring of the archaeal communities in different formation fluid samples by T-RFLP and Real-Time-PCR indicated that these two methanogenic genera dominated at all, whereas the proportion of the two groups varied. Thus, methylotrophic and autotrophic methanogenesis seems to be of importance in the reservoir fluids, dependent on the provided reduction equivalents and substrates and it also may influence the fate of CO 2 in the subsurface.

This chapter gives the reader an introduction into the microbiology of deep geological systems with a special focus on potential geobiotechnological applications and respective risk assessments. It has been known for decades that microbial activity is responsible for the degradation or conversion of hydrocarbons in oil, gas, and coal reservoirs. These processes occur in the absence of oxygen, a typical characteristic of such deep ecosystems. The understanding of the responsible microbial processes and their environmental regulation is not only of great scientific interest. It also has substantial economic and social relevance, inasmuch as these processes directly or indirectly affect the quantity and quality of the stored oil or gas. As outlined in the following chapter, in addition to the conventional hydrocarbons, new interest in such deep subsurface systems is rising for different technological developments. These are introduced together with related geomicrobiological topics. The capture and long-termed storage of large amounts of carbon dioxide, carbon capture and storage (CCS), for example, in depleted oil and gas reservoirs, is considered to be an important options to mitigate greenhouse gas emissions and global warming. On the other hand, the increasing contribution of energy from natural and renewable sources, such as wind, solar, geothermal energy, or biogas production leads to an increasing interest in underground storage of renewable energies. Energy carriers, that is, biogas, methane, or hydrogen, are often produced in a nonconstant manner and renewable energy may be produced at some distance from the place where it is needed. Therefore, storing the energy after its conversion to methane or hydrogen in porous reservoirs or salt caverns is extensively discussed. All these developments create new research fields and challenges for microbiologists and geobiotechnologists. As a basis for respective future work, we introduce the three major topics, that is, CCS, underground storage of gases from renewable energy production, and the production of geothermal energy, and summarize the current stat of knowledge about related geomicrobiological and geobiotechnological aspects in this chapter. Finally, recommendations are made for future research.

In this study, the hydrochemical characteristic changes of open-pit groundwater (OPGW) near a closed metalliferous mine proposed for drinking and irrigation usage were investigated. Water samples from an OPGW in O'Kiep were collected in April (dry season) and August (wet season), 2017. Data analysis methods, such as the coefficient of variation, correlation coefficient, piper trilinear plot analysis and sodium adsorption ratio (SAR) among others were used to assess of the seasonal variation in the OPGW quality. The hydrochemical characteristics of the OPGW were also compared with drinking water guidelines. Based on the results, it was evident that the OPGW quality varied seasonally. The most abundant cation and anions were Ca 2+ and SO 4 2− , with recorded concentrations of 631 and 6020 mg/L, (dry season); 541 and 4450 mg/L (wet season), respectively. The OPGW was relatively rich in SO 4 2− > Cl − > Mg 2+ > Ca 2+ > Na + > K + during the dry season; however, these cations and anions were reduced during the wet season with Cl − and Na + subsequently increasing. PHREEQC indicated that cation exchanges played a significant role in the OPGW chemistry with Sb and Ca appearing to be the minerals with the highest precipitation potential followed by Cu 7 S 4 and Blaubleil, respectively. Covellite, CdSe 2 and NiSe were likely to be in equilibrium while Ca 3 Sb 2 will remain in a dissolution state. Furthermore, the seasonal fluctuation in the OPGW quality characteristics was attributed to the water table level fluctuations and the water quality did not satisfy the guidelines stipulated by the South African National Standard, SANS241-1 (2015) and World Health Organisation, WHO (2011). The trilinear piper plot classified the OPGW into three types: Ca 2+ -Mg 2+ -Cl − -SO 4 2 ; Ca 2+ -Mg 2+ -F − ; and Ca 2+ -Mg 2+ -SO 4 2− . Overall, the SAR values were within the permissible levels for irrigation purposes; however, long-term use of the OPGW might be detrimental for plant growth.

Water insecurity is a growing concern globally and the role of groundwater and aquifers in buffering the effects of climate variability and change is increasingly acknowledged due to extended periods of drought in arid regions. Water security can only be fully realised with a more robust understanding of groundwater as a water resource, especially in the O'Kiep area which is in Namaqualand, South Africa. Results of analyses of groundwater samples (n ¼ 8) indicated that pH values met acceptable standards for drinking, while the total dissolved solids (TDS) and electrical conductivity (EC) were however not within the recommended limits. The chemical composition of the groundwater indicated Eh values À 34.1 to À 87.2 mV, indicative of reducing geochemical conditions with the most abundant ions being Cl -, SO 4 2-, NO 3 -, F -, Na þ , Ca 2þ , Mg 2þ and K þ . Furthermore, some of the ions were not within the guidelines, i.e. Cl À , SO 4 2þ , Na þ , Ca 2þ and Mg 2þ , but all the potential toxic elements,