Systematics of haloarchaea and biotechnological potential of their hydrolytic enzymes

Abstract

Halophilic archaea, also referred to as haloarchaea, dominate hypersaline environments. To survive under such extreme conditions, haloarchaea and their enzymes have evolved to function optimally in environments with high salt concentrations and, sometimes, with extreme pH and temperatures. These features make haloarchaea attractive sources of a wide variety of biotechnological products, such as hydrolytic enzymes, with numerous potential applications in biotechnology. The unique trait of haloarchaeal enzymes, haloenzymes, to sustain activity under hypersaline conditions has extended the range of already-available biocatalysts and industrial processes in which high salt concentrations inhibit the activity of regular enzymes. In addition to their halostable properties, haloenzymes can also withstand other conditions such as extreme pH and temperature. In spite of these benefits, the industrial potential of these natural catalysts remains largely unexplored, with only a few characterized extracellular hydrolases. Because of the applied impact of haloarchaea and their specific ability to live in the presence of high salt concentrations, studies on their systematics have intensified in recent years, identifying many new genera and species. This review summarizes the current status of the haloarchaeal genera and species, and discusses the properties of haloenzymes and their potential industrial applications. SYSTEMATICS OF HALOPHILIC ARCHAEAHalophilic micro-organisms are found in the three domains of life, and are extremophiles living in hypersaline environments in the presence of salt (generally NaCl) as a specific requisite for their growth [1]. Hypersaline environments contain higher salt concentrations than seawater (3.5 % total dissolved salts) [2]. According to the optimum NaCl concentration for growth, micro-organisms fall into one of the following categories: extreme halophiles with optimum growth at 15-30 % (2.5-5.2 M) NaCl; moderate halophiles with optimum growth at 3-15 % (0.5-2.5 M) NaCl; slight halophiles with optimum growth at 1-3 % (0.2-0.5 M) NaCl; non-halophiles with optimum growth at less than 1 % (0.2 M) NaCl; and halotolerant micro-organisms which are non-halophiles showing the ability to tolerate high NaCl concentrations [3]. Halophiles can live in hypersaline environments because they are able to sustain osmotic balance. They use two main strategies to withstand the adverse consequences of water loss when exposed to high salt concentrations. The first strategy used by extremely halophilic archaea (haloarchaea) and halophilic anaerobic bacteria such as members of Halanaerobiales is known as the 'salt-in-cytoplasm' strategy, during which salts such as NaCl or KCl accumulate in the cytoplasm at concentrations similar to the extracellular ones [4,5]. Another mechanism to cope with high salt concentrations is the accumulation of organic molecules such as amino acids, sugars and polyols, known as compatible solutes or osmolytes. Compatible solutes are small and highly soluble molecules that can...

Metagenomic- and cultivation-based exploration of anaerobic chloroform biotransformation in hypersaline sediments as natural source of chloromethanes

“…Due to dominant presence of halophilic microbes in hypersaline environments (60), and in an attempt to find halophilic microbes capable of CF metabolism, two media were used for halophilic bacteria and archaea enrichment: modified growth medium (MGM) and DBCM2 medium (DBC) (61). The media were boiled and flushed with nitrogen to remove oxygen.…”

Interaction of two strongly divergent archaellins stabilizes the structure of theHalorubrumarchaellum

“…The CC and CG spacers were found 3 times, AA, AC, AT and TT once Interestingly, for the other haloarchaeal genera (Halopiger, Natrialba, Halobiforma, Natronolimnobius and Natrarchaeobius) the situation with archaellins is very similar to that of Halorubrum. Despite the rather high similarity of their archaellins with that of the Halorubrum species, all these haloarchaea belong to a clade (the order Natrialbales, the family Natrialbaceae) evolutionarily distinct from Halorubrum (order Haloferacales, family Halorubraceae) (Amoozegar et al, 2017). It is likely that operons from two highly diverged paralogs, having a common origin with Halorubrum flaB1 and flaB2 genes were exchanged between these taxa by horizontal gene transfer.…”

Haloferax volcanii for biotechnology applications: challenges, current state and perspectives

“…They are found in hypersaline environments around the world where the salt concentrations is higher than sea water (around 3.5% dissolved salt), in various geographical niches such as salt lakes, deep sea, salt pans, saline soils, salt mines or salt marshes (Oren 2015). Based on their salt preference, halophiles can be split into three different categories: mild halophiles growing optimally at 1-3% (0.2-0.5 M) NaCl, moderate halophiles with optimum growth at 3-15% (0.5-2.5 M) and extreme halophiles with optimal growth at 15-20% (2.5-5.2 M) NaCl (Amoozegar et al 2017;De Lourdes et al 2013). Below 0.2 M NaCl (< 1%), microbes are classified as nonhalophiles and a key difference is that the growth of mesophilic organisms is impaired at higher salinities, where archaeal species dominate.…”