Selective regulation of partial nitrification/denitrification via light irradiation: Microbial niche differentiation...

Short-cut nitrogen removal via partial nitrification/denitrification (PN/PDN) offers a promising, energy-efficient pathway for wastewater treatment. Recently, light irradiation has emerged as a precise, non-invasive modulator to stabilize nitrite accumulation by selectively regulating microbial community structure and metabolic functions. This review systematically synthesizes the wavelength-dependent impacts of light, from ultraviolet (UV) through visible spectra, on key nitrogen‑transforming microorganisms by examining photobiological responses at molecular and physiological levels. Short-wavelength irradiation (UV/blue light) exerts strong selective pressure in nitrifiers, suppressing nitrite-oxidizing bacteria activity by 30-75% while leaving ammonia-oxidizing bacteria and archaea largely unaffected. This selective suppression is attributed to a triple-stress cascade in NOB, comprising enzymatic sensitivity, accumulated DNA damage, and a bioenergetic crisis triggered by blue-light-using FAD (BLUF)-mediated inhibition of the tricarboxylic acid cycle. In denitrification, light acts as a metabolic switch, with yellow light accelerating complete nitrogen-removal kinetics and blue light arresting the process at the nitrite stage. This light-driven PDN is fundamentally governed by electron competition, wherein light-stimulated cytochrome c oxidase (Complex IV) monopolizes electron flow, starving downstream nitrite reductase and thereby stabilizing nitrite accumulation. Thus, light irradiation is repositioned from an environmental stressor to a precise metabolic modulator capable of reshaping microbial community dynamics. Crucially, the selection of the optimal light intensity required factoring in that the actual photobiological load was dynamically controlled by the sludge concentration, light exposure duration, and the physical shielding effects of the microalgal -bacterial consortium. Understanding these wavelength-specific responses provides a practical alternative to chemical inhibitors for controlling the nitrification process. Ultimately, addressing engineering constraints such as biological instability and energy tradeoffs will pave the way for integrating light-driven technologies into next-generation, low-carbon wastewater treatment paradigms.