Molecular and Kinetic Characterization of Planktonic Nitrospira spp. Selectively Enriched from Activated Sludge

Nitrospira spp. are chemolithoautotrophic nitrite-oxidizing bacteria (NOB), which are ubiquitous in natural and engineered environments. However, there exist few independent biokinetic studies on Nitrospira spp., likely because their isolation and selective enrichment from environmental consortia such as activated sludge can be challenging. Herein, planktonic Nitrospira spp. cultures closely related to Candidatus Nitrospira defluvii (Nitrospira lineage I) were successfully enriched from activated sludge in a sequencing batch reactor by maintaining sustained limiting extant nitrite and dissolved oxygen concentrations. Morphologically, the enrichment consisted largely of planktonic cells with an average characteristic diameter of 1.3 ± 0.6 μm. On the basis of respirometric assays, estimated maximum specific growth rate (μ_max), nitrite half saturation coefficient (K_S), oxygen half saturation coefficient (K_O), and biomass yield coefficient (Y) of the enriched cultures were 0.69 ± 0.10 d^-1, 0.52 ± 0.14 mg-N/L, 0.33 ± 0.14 mg-O₂/L, and 0.14 ± 0.02 mg-COD/mg-N, respectively. These parameters collectively reflect not just higher affinities of this enrichment for nitrite and oxygen, respectively, but also a higher biomass yield and energy transfer efficiency relative to Nitrobacter spp. Used in combination, these kinetic and thermodynamic parameters can help toward the development and application of energy-efficient biological nutrient removal processes through effective Nitrospira out-selection.