Nitrospirae

Nitrospirae is a bacterial phylum comprising primarily chemolithoautotrophic organisms that play a key role in the global nitrogen cycle by catalyzing the oxidation of nitrite to nitrate. Members of the phylum include the genera Nitrospira, Nitrobacter, and other related lineages that are found in a wide range of habitats such as freshwater, marine, soil, and engineered systems like wastewater treatment plants.

Morphologically, Nitrospirae are Gram‑negative rods or filaments, usually 0.5–1.5 µm in width and several micrometres long, and they lack motility structures. Their cell membranes contain characteristic lipopolysaccharides, and many species possess periplasmic nitrite oxidoreductase (NXR), the enzyme responsible for nitrite oxidation. Some Nitrospira species also exhibit complete ammonia oxidation (comammox), converting ammonia directly to nitrate in a single organism.

Physiologically, these bacteria are obligate chemolithoautotrophs that derive energy from the oxidation of nitrite (or ammonia in the case of comammox) and fix carbon dioxide via the reductive tricarboxylic acid (rTCA) cycle or the Calvin–Benson–Bassham pathway. Growth is typically slow, with doubling times ranging from several hours to days, reflecting the low energy yield of nitrite oxidation.

Ecologically, Nitrospirae contribute to nitrogen removal in natural and engineered ecosystems, influencing water quality and soil fertility. In wastewater treatment, Nitrospirae are integral to nitrification stages, and their resilience to environmental stressors such as low oxygen and inhibitory compounds is of practical interest.

Genomic analyses have revealed diverse metabolic capabilities, including genes for alternative electron donors, resistance to heavy metals, and stress response mechanisms. Recent phylogenomic studies suggest that the phylum encompasses several distinct families and orders, expanding the known diversity beyond the classical nitrite‑oxidizing lineages.

Overall, Nitrospirae represent a crucial functional group in nitrogen transformation processes, with ongoing research uncovering new ecological roles and biotechnological applications.