Abstract
Nitrification plays a crucial role in global nitrogen cycling and treatment processes. However, the relationships between the nitrifier guilds of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) are still poorly understood, especially in freshwater habitats. This study examined the physiological interactions between the AOB and NOB present in a freshwater aquarium biofilter by culturing them, either together or separately, in a synthetic medium. Metagenomic and 16S rRNA gene sequencing revealed the presence and the draft genomes of Nitrosomonas-like AOB as well as Nitrobacter-like NOB in the cultures, including the first draft genome of Nitrobacter vulgaris. The nitrifiers exhibited different growth rates with different ammonium (NH4+) or nitrite concentrations (50–1,500 μM) and the growth rates were elevated under a high bicarbonate (HCO3-) concentration. The half-saturation constant (Ks for NH4+), the maximum growth rate (μmax), and the lag duration indicated a strong dependence on the synergistic relationships between the two guilds. Overall, the ecophysiological and metagenomic results in this study provided insights into the phylogeny of the key nitrifying players in a freshwater biofilter and showed that interactions between the two nitrifying guilds in a microbial community enhanced nitrification.
Highlights
Nitrification, the biological oxidation of ammonia (NH3) to nitrate (NO3−) via nitrite (NO2−), is a vital oxidative process that links reduced and oxidized inorganic nitrogen to sustain the global nitrogen cycle in soils (Wang et al, 2015) and aquatic systems (Hou et al, 2013). This process is mainly carried out by two different but interdependent guilds of lithoautotrophic microorganisms, namely the ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas and Nitrosospira) and the ammonia-oxidizing archaea (AOA) as one guild that convert NH3 to NO2− using the key enzymes ammonia monooxygenase and hydroxylamine dehydrogenase (Sedlacek et al, 2016), and the nitrite-oxidizing bacteria (NOB) (e.g., Nitrospira and Nitrobacter) as another that convert NO2− to NO3− with the enzyme nitrite
We explored the relationships between ammonia-oxidizing and nitrite-oxidizing populations found in freshwater systems by instead culturing one or both of these nitrifying guilds from a microbial community inhabiting a freshwater biofilter in a synthetic medium
The concentrations of NH4+ in the influent to the biofilter were in the range of 500 μM; a synthetic medium containing 500 μM NH4+ was used to culture the nitrifiers to simulate the in situ process
Summary
Nitrification, the biological oxidation of ammonia (NH3) to nitrate (NO3−) via nitrite (NO2−), is a vital oxidative process that links reduced and oxidized inorganic nitrogen to sustain the global nitrogen cycle in soils (Wang et al, 2015) and aquatic systems (Hou et al, 2013). This process is mainly carried out by two different but interdependent guilds of lithoautotrophic microorganisms, namely the ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas and Nitrosospira) and the ammonia-oxidizing archaea (AOA) as one guild that convert NH3 to NO2− using the key enzymes ammonia monooxygenase (encoded by the amoCAB genes) and hydroxylamine dehydrogenase (encoded by the haoAB genes) (Sedlacek et al, 2016), and the nitrite-oxidizing bacteria (NOB) (e.g., Nitrospira and Nitrobacter) as another that convert NO2− to NO3− with the enzyme nitrite. The tight coupling between AOB and NOB means that even minor perturbations in the abundance of ammonia oxidizers can lead to large changes in the abundance of nitrite oxidizers (Knapp and Graham, 2007), and subsequently to erratic nitrification activities (Graham et al, 2007)
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