## 4 Mycobacteria Ecological Niches
### Extreme Environments  
The physiological characteristics of mycobacteria referred to in the above topics show their extraordinary ability to live in extreme habitats. There are reports of NTM being found in caves, namely in sulfur caves in Romania [223]. These caves possess a unique environment where life can be found in a gas chemocline, with the atmospheric lower part of the cave being mainly composed of carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), and hydrogen sulfide (H\({}_{2}\)S), while the atmospheric upper part shows a similar composition to the atmospheric air found in Earth's surface [223]. Additionally, both elemental sulfur (S\({}^{0}\) and sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) cover the cave walls resulting in a pH lower than 1. Therefore, all these extreme conditions (e.g., local anaerbiosis, high CO\({}_{2}\) levels, extremely low pH, and absence of sunlight) make life in the cave almost impossible. However, strikingly, microbial biofilms can be detected in cave walls with NTM dominating among those communities [223], showing that NTM are capable to survive and multiplicate in niches with pH lower than 1, with limited availability of carbon and free energy sources, and with low humidity. The most likely source of energy in this environment is sulfur oxidation with H\({}_{2}\)S and S\({}^{0}\) as reductants and atmospheric O\({}_{2}\) as oxidant, however, ferric iron can also be a potential oxidant [223]. This was not the first timethat NTM were reported as having chemolithoautotrophic growth. In the analysis of an Angkor monument sandstones, the ability of five NTM isolates to grow both chemolithoautotrophically using S\({}^{0}\) and chemoorganoheterotrophically using organic substances was described [224]. Those isolates were phylogenetically related to _M. cosmeticum_ and _M. pallens_. This autotrophic growth capacity with hydrogen [225] and the chemolithotrophic growth on carbon monoxide as the sole carbon and energy source under aerobic conditions have been reported previously in _M. flavescens_, _M. gastri_, _M. neaurum_, _M. parafortuitum_, _M. peregriunun_, _M. phlei_, _M. smegmatis_, _M. tuberculosis_, and _M. vaccae_[226]. Additionally, the role of the assimilatory sulfur metabolism in virulence, antibiotic resistance, and antioxidant defense of _M. tuberculosis_ has been examined [227]. NTM are also able to survive in extremely alkaline environments, with reports from Rio Grande, USA, where _M. fortutium_ complex, _M. scrofulaceum_, _M. semiae_, _M. gordonae_, _M. smegmatis_, _M. interjecium_, _M. lentiflavour_, _M. nonchromo_, _M. celatum_, _M. chitae_, _M. pheli_, and _M. kansasii_ have been found [228]. The Yellowstone National Park possesses a variety of extreme environments that have been the target of research for several decades. A study focusing on the geobiology of a microbial endolithic community in the Yellowstone geothermal environment showed that 37% of the total bacterial rRNA detected belonged to _Mycobacterium_ species [229]. Later, _M. parascrofulaceum_ was isolated in Norris Geyser Basin, Yellowstone National Park, a system that possesses pH 3.0 and maximum temperatures of 56 \({}^{\circ}\)C [230]. _M. parascrofulaceum_ was also found in hot springs from Zambia and Kenya with a pH range between 5 and 10 [231], suggesting that this species may be ubiquitous in hot springs, independently of the pH value. Another recent study by [PERSON] et al. (2018) investigated the occurrence of NTM in the extreme environment of the water zone of the Hranice Abyss, Czech Republic, the deepest flooded pit cave in the world [232]. This cave is characterized by its acidic water, high concentration of CO\({}_{2}\), and the biofilm communities of slime bacteria. Interestingly, _M. arupense_, _M. avium_, _M. forentinum_, _M. gordonae_, _M. intracellular_, _M. mucogenicum_, and _M. sediminis_ were detected in this study. Furthermore, _M. elephantis_ was isolated from low impact rock surfaces [233], _M.