Halophiles are extremophilic salt-loving microorganisms that can survive in an extremely high level of salinity (10-30% NaCl). They belong to all three groups (i.e., bacteria, archaea, and eukaryotes). Halophiles tolerate high salt concentration due to unique cellular adaptations like salt-in strategy, compatible solute strategy, and enzyme adaptations. The chapter describes the classification, physiology, ecology, and mechanisms of adaptations and biotechnological applications of halophiles.
TopIntroduction
Halophiles include diverse group of organisms which include Archaea, Bacteria, and Eukarya and grow in presence of salt (Oren, 2015). The halophiles are isolated mainly from saline soil, water, springs, marshes, brines and lakes. These are classified viz., slight halophiles (0.2-0.5 M salt tolerance), moderate halophiles (0.5-2.5 M salt tolerance), borderline extreme halophiles (2.5-4.0 M salt tolerance) and extreme halophiles (4.0-5.9 M salt tolerance) based on their ability to tolerate salinity (Fig. 1). The ability to tolerate salt depends on parameters such as temperature, pH, nutrients, etc. (Corral et al., 2020). In this way, the halophiles are adapted and limited by specific environmental factors. Halophiles belong to family Halomonadaceae and class Gammaproteobacteria. The halophiles can be aerobes, anaerobes, chemo heterotrophs, photo heterotrophs, or photoautotrophs (Edbeib et al., 2016). The halophiles have been reported to be isolated from Dead Sea of Israel (Wei et al., 2015), lake Urmia near Iran (Mehrshad et al., 2015a), Tuzkoy salt mines of Turkey (Mutlu & Guven, 2015), Great salt lake, Utah in United States of America (Tazi et al., 2014), Rambla Salada, Murcia, Spain (Luque et al., 2014), etc. The sources for isolation of halophiles are represented in Fig. 2.
Figure 1.
Classification of group of halophiles
Figure 2.
Sources for isolation of halophiles
TopHistory Of Halophiles
The first halophilic microorganism came into account in 2700 BC (Bass-Becking, 1931) and found in hyper saline area. In 1920s and 1940s halophilic bacteria were isolated from different sources, e.g., fish, animal hides, and anchovies. Elazari isolated various extreme halophiles, Halobacterium trapanicum and Micrococcus morrhuae and moderate halophiles Chromohalobacterium marismortui, Pseudomonas halestrogus, and Flavobacterium halmephium from the Dead Sea (Edbeib et al., 2016). The ecology, physiology and biochemistry of halophiles have been reported (Oren, 2015). The genome sequencing of halophiles has immense interest. The first genome sequence has been studied of Halobacterium NRC-1 (Ng et al., 2000). During the last three years, the genome sequence of eight halophiles has been reported. From these, four halophiles belong to the Halomonas genus. The in-silico studies help to know exact biology of halophiles (Oren, 2014). The in-silico post-genomic and genetic engineering studies have developed new pathways of growth optimization for halophilic microorganisms (Yue et al., 2014).
Cellular Adaptation
Microorganisms which cannot tolerate high salt concentration or saline environment tend to lose water because of which cells shrink resulting in complete loss of cell structure and function (Fig. 3a). Halophiles produce osmoprotectants which enhances the osmotic activity of their cytoplasm (Fig. 3b). Some of the halophiles are able to produce excess salt concentration in the cell in order to have equal salt concentrations with that of the environment (Fig. 3c) (Edbeib et al., 2016).
Figure 3.
Adaptations of halophiles to extreme saline environments. a) The non-halophile macromolecules are compromised, the water flows outside cell leading to turgor effect. b) Moderate halophiles maintain synthesize compatible organic solutes. c) Extreme halophiles adapt to saline conditions through balance of cellular and outside salt concentrations