Abstract
Microorganisms from the Halobacteria class, also known as haloarchaea, inhabit a wide range of ecosystems of which the main characteristic is the presence of high salt concentration. These environments together with their microbial communities are not well characterized, but some of the common features that they share are high sun radiation and low availability of oxygen. To overcome these stressful conditions, and more particularly to deal with oxygen limitation, some microorganisms drive alternative respiratory pathways such as denitrification. In this paper, denitrification in haloarchaea has been studied from a phylogenetic point of view. It has been demonstrated that the presence of denitrification enzymes is a quite common characteristic in Halobacteria class, being nitrite reductase and nitric oxide reductase the enzymes with higher co-occurrence, maybe due to their possible role not only in denitrification, but also in detoxification. Moreover, copper-nitrite reductase (NirK) is the only class of respiratory nitrite reductase detected in these microorganisms up to date. The distribution of this alternative respiratory pathway and their enzymes among the families of haloarchaea has also been discussed and related with the environment in which they constitute the major populations. Complete denitrification phenotype is more common in some families like Haloarculaceae and Haloferacaceae, whilst less common in families such as Natrialbaceae and Halorubraceae.
Highlights
These species have been taxonomically classified in 44 genera and 10 families to look for denitrification patterns at these taxonomic levels (Table 2)
We propose that the common ancestor of some of the current complete denitrifier haloarchaea carried only NirK and nitric oxide reductase (Nor)
Acquisition of genomes, taxonomic classification and subsequent enzyme annotation have shown that details on denitrification at genus level in haloarchaea are still scarce, due to the lack of genomes from microorganisms belonging to the same taxonomic level
Summary
Saline and hypersaline environments are found worldwide. Hypersaline environments can be divided into two groups: thalassohalines, derived from seawater and containing Na+ and Cl− as predominant ions, and athalassohalines, which display variable ion composition depending on the area where these environments develop [1,2]. The biodiversity of these ecosystems depends on several factors such as total salinity and ionic composition, pH, temperature, radiation, water accessibility and oxygen availability. Due to lower oxygen solubility in these environments, some microorganisms have developed different strategies. Some examples are the production of gas vesicles ( called gas vacuoles) that enable cells to float to the surface of the brines where oxygen concentration is higher or the use of N-compounds as final electron acceptor, such as nitrate or nitrite using denitrification pathway [3,4]
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