AUTHOR=Cua Lynnie S. , Stein Lisa Y. 

TITLE=Characterization of denitrifying activity by the alphaproteobacterium, Sphingomonas wittichii RW1

JOURNAL=Frontiers in Microbiology

VOLUME=5

YEAR=2014

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2014.00404

DOI=10.3389/fmicb.2014.00404

ISSN=1664-302X

ABSTRACT=<p><italic>Sphingomonas wittichii</italic> RW1 has no reported denitrifying activity yet encodes nitrite and nitric oxide reductases. The aims of this study were to determine conditions under which <italic>S. wittichii</italic> RW1 consumes nitrite (NO<sup>−</sup><sub>2</sub>) and produces nitrous oxide (N<sub>2</sub>O), examine expression of putative genes for N-oxide metabolism, and determine the functionality of chromosomal (ch) and plasmid (p) encoded quinol-dependent nitric oxide reductases (NorZ). Batch cultures of wildtype (WT) and a <italic>norZ</italic><sub>ch</sub> mutant of <italic>S. wittichii</italic> RW1 consumed NO<sup>−</sup><sub>2</sub> and produced N<sub>2</sub>O during stationary phase. The <italic>norZ</italic><sub>ch</sub> mutant produced N<sub>2</sub>O, although at significantly lower levels (c.a. 66–87%) relative to the WT. Rates of N<sub>2</sub>O production were 2–3 times higher in cultures initiated at low relative to atmospheric O<sub>2</sub> per unit biomass, although rates of NO<sup>−</sup><sub>2</sub> consumption were elevated in cultures initiated with atmospheric O<sub>2</sub> and 1 mM NaNO<sub>2</sub>. Levels of mRNA encoding nitrite reductase (<italic>nirK</italic>), plasmid-encoded nitric oxide dioxygenase (<italic>hmp</italic><sub>p</sub>) and plasmid-encoded nitric oxide reductase (<italic>norZ</italic><sub>p</sub>) were significantly higher in the <italic>norZ</italic><sub>ch</sub> mutant over a growth curve relative to WT. The presence of NO<sup>−</sup><sub>2</sub> further increased levels of <italic>nirK</italic> and <italic>hmp</italic><sub>p</sub> mRNA in both the WT and <italic>norZ</italic><sub>ch</sub> mutant; levels of <italic>norZ</italic><sub>p</sub> mRNA compensated for the loss of <italic>norZ</italic><sub>ch</sub> expression in the <italic>norZ</italic><sub>ch</sub> mutant. Together, the results suggest that <italic>S. wittichii</italic> RW1 denitrifies NO<sup>−</sup><sub>2</sub> to N<sub>2</sub>O and expresses gene products predicted to detoxify N-oxides. So far, only <italic>S. wittichii</italic> strains within four closely related taxa have been observed to encode both <italic>nirK</italic> and <italic>norZ</italic> genes, indicating a species-specific lateral gene transfer that may be relevant to the niche preference of <italic>S. wittichii</italic>.</p>