Haloferax mediterranei, an Archaeal Model for Denitrification in Saline Systems, Characterized Through Integrated Physiological and Transcriptional Analyses

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.

Exploring the Molecular Machinery of Denitrification in Haloferax mediterranei Through Proteomics

“…The predominance of these two proteins in the micelles could indicate that they are more highly synthesized than the other catalytic subunits. This is consistent with a recent study showing that the highest peaks of expression during denitrification were reached by the genes that encode them (nirK and nosZ) (Torregrosa-Crespo et al, 2020 ). Despite these observations, this hypothesis should be addressed using quantitative proteomics.…”

“…In the last years, one of their members has been used as model organism for the study of denitrification: Haloferax mediterranei . It is an haloarchaeon able to carry out the complete reduction of nitrate to dinitrogen through the four key enzymes of denitrification: nitrate, nitrite, nitric oxide, and nitrous oxide reductases (Torregrosa-Crespo et al, 2018 , 2019 , 2020 ).…”

“…Probably, it is the most unknown denitrification enzyme in haloarchaea. So far, only in vivo experiments have been performed to test its functionality in the presence of oxygen or low pH (Torregrosa-Crespo et al, 2020 ), but its structure remains unknown. A priori , based on the gene sequence, it must be a typical nitrous oxide reductase containing two multicopper sites: a binuclear CuA electron- transferring center and a tetranuclear copper sulfide catalytic center, named the “CuZ center” (Pauleta et al, 2017 ).…”

“…To date, much of the experimental work on denitrification in Haloarchaea has been focused on physiological studies or, more recently, transcriptional analysis (Torregrosa-Crespo et al, 2019 , 2020 ). These approaches cannot get a global perspective on protein associated with denitrification.…”

Microorganisms and Their Metabolic Capabilities in the Context of the Biogeochemical Nitrogen Cycle at Extreme Environments

“…In halophilic ecosystems, such as salt marshes or salted ponds, the low oxygen solubility and high ionic strength promotes the development of denitrification in those geographical regions in which nitrate or nitrite are present in soil or water (from natural sources or as part of the pesticides and fertilizers used for agricultural purposes) [39,44,105,161]. In those environments, partial denitrification results in the emission of NO and N 2 O gases to the atmosphere, thus contributing to global warming [39,51,105].…”