Abstract
Haloferax mediterranei (R4) belongs to the group of halophilic archaea, one of the predominant microbial populations in hypersaline environments. In these ecosystems, the low availability of oxygen pushes the microbial inhabitants toward anaerobic pathways and the presence of N-oxyanions favor denitrification. In a recent study comparing three Haloferax species carrying dissimilatory N-oxide reductases, H. mediterranei showed promise as a future model for archaeal denitrification. This work further explores the respiratory physiology of this haloarchaeon when challenged with ranges of nitrite and nitrate concentrations and at neutral or sub-neutral pH during the transition to anoxia. Moreover, to begin to understand the transcriptional regulation of N-oxide reductases, detailed gas kinetics was combined with gene expression analyses at high resolution. The results show that H. mediterranei has an expression pattern similar to that observed in the bacterial Domain, well-coordinated at low concentrations of N-oxyanions. However, it could only sustain a few generations of exponential anaerobic growth, apparently requiring micro-oxic conditions for de novo synthesis of denitrification enzymes. This is the first integrated study within this field of knowledge in haloarchaea and Archaea in general, and it sheds lights on denitrification in salty environments.
Highlights
The ability to maintain a respiratory metabolism in lieu of oxygen is widespread
The media contained different KNO3 or KNO2 concentrations depending on the assay: 2, 5, 10, 20, 200, and 2000 mM KNO3; 1, 2, 5, 10, and 40 mM KNO2
Haloferax mediterranei was exposed to different initial concentrations of nitrate with 1 vol% initial O2 in the headspace, and aerobic respiration and denitrification was monitored by frequent gas measurements (Molstad et al, 2007)
Summary
The ability to maintain a respiratory metabolism in lieu of oxygen is widespread. Among the many types of anaerobic dissimilatory pathways, denitrification is the most energetically profitable. Less is known about saline and hypersaline ecosystems, H. mediterranei, the Saline Model for Denitrification where anaerobic respiration is prominent because the high salt concentrations result in low oxygen availability (RodríguezValera et al, 1985; Sherwood et al, 1991, 1992; Oren, 2013). Such systems are becoming increasingly interesting in terms of denitrification and N-oxide emissions because anthropogenic activities currently lead to contamination by nitrogenous compounds like nitrates and nitrites (MartínezEspinosa et al, 2007; Ochoa-Hueso et al, 2014; TorregrosaCrespo et al, 2018). Their extent on a global scale is on the rise due to desertification, resulting from climate change (Torregrosa-Crespo et al, 2018)
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