Enrichment of Nitrogen-Fixing Bacteria in a Nitrogen-Deficient Wastewater Treatment System

Anthropogenic nitrogen fixation is
essential to sustain a global
population of 7.7 billion. However, there has been a long-standing
desire to find cheaper and more environmentally friendly alternatives
to the Haber–Bosch process. In this study, we developed a new
strategy of nitrogen fixation by enriching free-living N2-fixing bacteria (NFB) in reactors fed with low nitrogen wastewater,
analogous to those usually found in certain industrial effluents such
as paper mills. Our reactors fixed appreciable quantities of nitrogen
with a rate of 11.8 mg N L–1 day–1. This rate is comparable to recent “breakthrough”
nitrogen-fixing technologies and far higher than observed in low C/N
reactors (fed with organic matter and nitrogen). NFB were quantified
using quantitative polymerase chain reaction (qPCR) of the nifH (marker gene used to identify biological nitrogen fixation)
and 16S rRNA genes. The nifH gene
was enriched by a factor of 10 in the nitrogen-fixing reactors (compared
to controls) attaining 13% of the bacterial population (1:4.2 copies
of nifH to 16S rRNA). The Illumina
MiSeq 16S rRNA gene amplicon sequencing of reactors
showed that the microbial community was dominated (19%) by Clostridium pasteurianum. We envisage that nitrogen-enriched
biomass could potentially be used as a biofertilizer and that the
treated wastewater could be released to the environment with very
little post-treatment.