Abstract
Extremophiles are remarkable organisms that thrive in the harshest environments on Earth, such as hydrothermal vents, hypersaline lakes and pools, alkaline soda lakes, deserts, cold oceans, and volcanic areas. These organisms have developed several strategies to overcome environmental stress and nutrient limitations. Thus, they are among the best model organisms to study adaptive mechanisms that lead to stress tolerance. Genetic and structural information derived from extremophiles and extremozymes can be used for bioengineering other nontolerant enzymes. Furthermore, extremophiles can be a valuable resource for novel biotechnological and biomedical products due to their biosynthetic properties. However, understanding life under extreme conditions is challenging due to the difficulties of in vitro cultivation and observation since > 99% of organisms cannot be cultivated. Consequently, only a minor percentage of the potential extremophiles on Earth have been discovered and characterized. Herein, we present a review of culture-independent methods, sequence-based metagenomics (SBM), and single amplified genomes (SAGs) for studying enzymes from extremophiles, with a focus on prokaryotic (archaea and bacteria) microorganisms. Additionally, we provide a comprehensive list of extremozymes discovered via metagenomics and SAGs.
Highlights
A mounting paradigm shift toward using sustainable resources has stimulated exploring new efficient approaches in technological processes (Raddadi et al, 2015; Lamers et al, 2016; Krüger et al, 2018)
Enzymes derived from microorganisms thriving under harsh conditions, called extremophiles, can overcome these restrictions, and today, such biocatalysts are in higher demand than ever before (Karan et al, 2012a; Rizk et al, 2012; Johnson, 2014; Raddadi et al, 2015; Sarmiento et al, 2015; Singh et al, 2016; Jorquera et al, 2019)
According to their natural habitats, extremophiles are classified into thermophiles, alkaliphiles, acidophiles, halophiles, and others (Karan et al, 2012a,b; Raddadi et al, 2015)
Summary
A mounting paradigm shift toward using sustainable resources has stimulated exploring new efficient approaches in technological processes (Raddadi et al, 2015; Lamers et al, 2016; Krüger et al, 2018). An alternative approach to achieve biocatalysis in extreme physicochemical conditions is to use enzymes derived from organisms that thrive in extreme conditions (Jorquera et al, 2019) These organisms are called extremophiles and live in harsh environments of elevated temperatures (thermophiles), salt concentration (halophiles), pressure (barophiles), osmotic compound content (osmophiles), heavy metal content (metalophiles), and radiation (radiophiles); acidic or basic pH (acid or alkaliphiles); extreme dryness (xerophiles); extreme cold (psychrophiles); or a combination of different extremes (polyextremophiles) (Figure 1; Kristjánsson and Hreggvidsson, 1995; Gerday and Glansdorff, 2009; Gabani and Singh, 2013; Coker, 2016; Horikoshi, 2016)
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