Denitrifying haloarchaea within the genus Haloferax display divergent respiratory phenotypes, with implications for their release of nitrogenous gases.

Abstract

Haloarchaea are extremophiles, generally thriving at high temperatures and salt concentrations, thus, with limited access to oxygen. As a strategy to maintain a respiratory metabolism, many halophilic archaea are capable of denitrification. Among them are members of the genus Haloferax, which are abundant in saline/hypersaline environments. Three reported haloarchaeal denitrifiers, Haloferax mediterranei, Haloferax denitrificans and Haloferax volcanii, were characterized with respect to their denitrification phenotype. A semi-automatic incubation system was used to monitor the depletion of electron acceptors and accumulation of gaseous intermediates in batch cultures under a range of conditions. Out of the species tested, only H. mediterranei was able to consistently reduce all available N-oxyanions to N2 , while the other two released significant amounts of NO and N2 O, which affect tropospheric and stratospheric chemistries respectively. The prevalence and magnitude of hypersaline ecosystems are on the rise due to climate change and anthropogenic activity. Thus, the biology of halophilic denitrifiers is inherently interesting, due to their contribution to the global nitrogen cycle, and potential application in bioremediation. This work is the first detailed physiological study of denitrification in haloarchaea, and as such a seed for our understanding of the drivers of nitrogen turnover in hypersaline systems.