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Extreme environments on Earth host a large diversity of microbial life. Bacteria, archaea, and fungi are able to survive under one or several extreme conditions including extreme ranges of temperature, pressure, pH or salinity. Despite extensive research on extremophilic microorganisms, a relatively unexplored frontier within the study of the deep biosphere is the survey of the diversity of microorganisms inhabiting deep geothermal reservoirs used for energy production. These sites offer unique access to investigate life in the deep biosphere. The conditions in these reservoirs are often within the range of the known limits of life, which makes them a suitable habitat for various extremophilic microorganisms. Moreover, microbial-driven processes such as microbially induced scaling or corrosion can decrease the efficacy of geothermal power plant systems. The present review summarizes the current knowledge and uncertainties surrounding microbial life in deep geothermal reservoirs. As the knowledge in deep geothermal fluids is still scarce, the microbial diversity in analogous environments, such as surface geothermal springs, deep-sea hydrothermal vents or deep subsurface environments, is also summarized here. The high diversity of microorganisms inhabiting these analogous environments suggests that deep geothermal fluids may host an unsuspected microbial diversity. Moreover, the challenges associated to the study of microorganisms in geothermal fluids are reviewed. These include notably challenges linked to sampling, DNA extraction from low biomass samples, DNA amplification and sequencing of unknown communities, and biases induced by comparison of the sequences obtained to reference databases. Such biases are even stronger concerning fungi and archaea, as specific databases are less extensive than those for bacteria. A broader knowledge on microorganisms in deep geothermal fluids may not only allow to reduce the negative impact of microbial activity in geothermal power plants, but could also provide new insights into the evolution of microorganisms and their survival in extreme environments. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) can be found in a variety of deep subsurface environments such as sedimentary basins, oil fields and mines. However, the origin, composition and fate of DOM within deep geothermal reservoirs used for energy production is relatively unknown. With well depths reaching a few kilometers, these sites give access to investigate deep subsurface environments. Natural DOM as well as artificial DOM (e.g., from chemical scaling inhibitors) might serve as nutrients for microorganisms or affect chemical properties of the fluids by complexation. Its composition might reveal hydraulic connections to organic-rich strata, giving insights to the fluid flow within the reservoir. This review presents an overview of a total of 143 fluid samples from 22 geothermal sites (mainly central Europe), from the literature and compiling data to address the importance of DOM in geothermal fluids and how it might affect geothermal operation. The environmental conditions of the sites included varied greatly. Temperatures range from 34 to 200 ∘ C , depths from 850 to 5000 m, chloride content from 0.1 to 160 g L - 1 , and dissolved organic carbon (DOC) concentrations from 0.1 to 30.1 g L - 1 . The DOC concentrations were found to be generally lower in the fluids with temperatures below 80 ∘ C . DOC concentrations were higher in fluids with temperatures above 80 ∘ C and showed a decrease towards 200 ∘ C . Microbial degradation might be the main driver for low DOC concentrations in the lower temperature range (below 80 ∘ C ), while thermal degradation likely accounts for the decline in DOC in the temperature region between 80 ∘ C and 200 ∘ C . This review shows that DOM can be found in a variety of geothermal reservoirs and that it could be an additional essential tool to better understand fluid chemistry and reservoir conditions, and to optimize geothermal operation. [ABSTRACT FROM AUTHOR]

Knowledge of associations between fungal hosts and their bacterial associates has steadily grown in recent years as the number and diversity of examinations have increased, but current knowledge is predominantly limited to a small number of fungal taxa and bacterial partners. Here, we screened for potential bacterial associates in over 700 phylogenetically diverse fungal isolates, representing 366 genera, or a tenfold increase compared with previously examined fungal genera, including isolates from several previously unexplored phyla. Both a 16 S rDNA-based exploration of fungal isolates from four distinct culture collections spanning North America, South America and Europe, and a bioinformatic screen for bacterial-specific sequences within fungal genome sequencing projects, revealed that a surprisingly diverse array of bacterial associates are frequently found in otherwise axenic fungal cultures. We demonstrate that bacterial associations with diverse fungal hosts appear to be the rule, rather than the exception, and deserve increased consideration in microbiome studies and in examinations of microbial interactions. Robinson et al. explore bacterial associations in over 700 phylogenetically diverse fungal isolates with 16 S rRNA sequencing of fungal collections from across the globe and bioinformatic screening of publicly available fungal genome sequencing projects. This study reveals that bacterial associations with fungal hosts are widespread, and represents a vast increase in the representation of explored fungal-bacterial relationships. [ABSTRACT FROM AUTHOR]