4.1. Characteristics of High-Temperature Environments and Habitats with High Mineral Content
In addition to the dark and low-nutrient underground environments, hypersaline habitats like salterns and cold habitats like glaciers, there are other extreme environments in Slovenia and central Europe: mineral and thermal springs. Thermal springs represent extreme habitats due to their high temperature that is often accompanied by specific mineral content. Although the mineral springs range in temperature, they have high mineral content, e.g., magnesium and sulphides. They are often used as a source of mineral water, and hot springs are sometimes used for spas, depending on other characteristics.
4.2. Thermophiles in Central-European Thermal Springs
A condensed overview of the thermophiles that inhabit American hot springs was presented by Ashcroft (2002) 
, albeit European examples were not included. Nevertheless, thermal springs (similar to extremely cold environments) worldwide appear to have unexpectedly similar chemical compositions and temperatures at their point of emergence 
. Southern Europe is one of the areas with the highest density of hot springs 
. However, not much is known about the microorganisms that inhabit these diverse habitats.
Slovenia has a lot of thermal springs and some mineral springs (13 large spas, with further smaller ones), which are mainly in the eastern and northeastern regions: Moravske toplice, Lendava, Radenci, Rogaška, Ptuj, Zreče, Topolščica, Dobrna, Laško, Olimia, Čatež, Dolenjske, and Šmarješke toplice, Banovci, Mala Nedelja, Rimske terme, Atomske toplice, Snovik, and Cerkno. Invanjševci ob Ščavnici and Strunjan are the only spas in the Primorska region. There are up to 60 natural thermal springs, such as those of Kostanjevica na Krki, Klunove Toplice, and Klevevž, and the thermal springs Ljubljanica and Tolminka. The Slovenian thermal springs range in both temperature and geochemical conditions, and thus it is expected that different microbial communities will inhabit these different habitats.
Several types of extremophile microorganisms inhabit different environments in the central European moderate climate: oligotrophic microorganisms inhabit caves and other subsurface environments, while psychrophiles are found in snow and ice, halophiles in salterns and other hypersaline environments along the Mediterranean coast, and thermophiles in thermal springs. Their unique properties provide us with opportunities to study their different metabolic pathways and adaptive mechanisms. Their production of antimicrobial substances can be very useful in medicine and biotechnology, while the enzymes that they produce are thermostable and stable to high and low pH levels. Alternatively, extremophiles can also produce harmful compounds, which they use as an energy source. Lithoautotrophic extremophiles could be used to sequester atmospheric carbon dioxide and use it as an energy source.
The use of metagenomics has also provided enhanced information on microbial diversity, particularly for extremophilic environments, compared to previous cultivation-dependent and classical molecular techniques. However, gene sequencing cannot provide information on their metabolic pathways and ecology, as closely related organisms can have very different physiologies. Thus, we need to apply other “-omics” approaches to better understand their diversity, interactions, metabolic pathways, biogeochemical cycles, and dynamics, such as transcriptomics, proteomics, and metabolomics. This knowledge will provide greater insight into the roles of extremophiles in extreme environments and their potential biotechnological and medical applications.