The genus Nitrospira represents a group of bacteria fundamental to life-sustaining processes on Earth. These microorganisms are ubiquitous, inhabiting nearly every oxic environment globally. As a widespread member of the phylum Nitrospirota, Nitrospira is a primary driver of the global nitrogen cycle, a biogeochemical process that transforms nitrogen into forms usable by plants and other organisms. Much of the knowledge about them comes from genomic data rather than traditional isolation methods.
The Critical Role in the Nitrogen Cycle
Nitrification, the process that converts ammonia into nitrate, is traditionally understood as a two-step biological sequence carried out by two distinct groups of microbes. The first step involves ammonia-oxidizing bacteria or archaea, which convert ammonia (\(text{NH}_3\)) into nitrite (\(text{NO}_2^-\)). This intermediate compound, nitrite, is highly toxic to most life forms, even at relatively low concentrations.
Nitrospira performs the second, necessary step of this detoxification process, acting as a canonical nitrite-oxidizing bacterium (NOB). They oxidize the toxic nitrite into nitrate (\(text{NO}_3^-\)), a far less toxic compound that plants can readily assimilate as a nutrient for growth. This conversion is executed by the enzyme Nitrite Oxidoreductase (NXR), which is positioned in the cell’s periplasmic space, allowing for efficient substrate capture.
Diverse Habitats and Ecological Importance
Nitrospira is the most phylogenetically diverse and widespread group of nitrite-oxidizing bacteria, found in a vast array of natural and engineered environments. In natural settings, they are abundant in soils, marine sediments, freshwater systems, and even geothermal springs. Their ability to thrive under low-substrate conditions allows them to outcompete other nitrifiers in environments with low nutrient concentrations, such as unfertilized soils or oligotrophic waters.
In human-designed systems, Nitrospira plays a prominent role in maintaining water quality. They are the predominant NOB found in most wastewater treatment plants (WWTPs), where they remove nitrogenous pollutants before water is discharged. Furthermore, they are active in recirculating aquaculture systems and home aquariums, colonizing biological filters to prevent the buildup of nitrite that would otherwise be lethal to aquatic life.
The Discovery of Complete Nitrifiers
For over a century, the oxidation of ammonia to nitrate was viewed as an obligate two-step process requiring two separate organisms, a concept challenged in 2015. Researchers made the unexpected discovery of a single organism within the Nitrospira genus, specifically Nitrospira inopinata, that could perform the entire nitrification sequence alone. This organism was termed a “Comammox” bacterium, short for COMplete AMMonia OXidizer.
The genome of Comammox Nitrospira contains the genetic machinery for both ammonia and nitrite oxidation. This includes the gene for Nitrite Oxidoreductase (NXR) for the second step, but also the genes for Ammonia Monooxygenase (AMO) and Hydroxylamine Dehydrogenase (HAO), which are required for the first step of converting ammonia to nitrite.
Comammox Nitrospira are now recognized as abundant ammonia oxidizers in various environments, including groundwater biofilters and specific engineered systems. The presence of these organisms, which were previously overlooked, provides an explanation for why some environments showed high rates of nitrification even when traditional ammonia-oxidizing bacteria were scarce. Phylogenetic analysis shows that all discovered Comammox strains belong to sublineage II of the Nitrospira genus.
Managing Nitrospira in Human Technology
The management of Nitrospira populations is a central component in the design and operation of biological water treatment systems. Engineers intentionally construct biofilters with high surface areas, such as plastic media or sand, to provide a stable colonization site for these slow-growing microbes. Precise control over environmental factors is used to optimize the growth and activity of Nitrospira and other nitrifying organisms.
For example, maintaining a \(text{pH}\) range between 7.6 and 8.0 and a temperature around \(39^circtext{C}\) can favor the activity of certain Nitrospira species, such as Nitrospira moscoviensis$. In many wastewater processes, dissolved oxygen (DO) levels are carefully managed. Nitrospira has a high affinity for oxygen, allowing them to remain active even when DO is kept low to suppress other microbial processes.