Illuminating the dark majority: photobiology of nonphotosynthetic bacteria.

Nonphotosynthetic bacteria represent the majority of prokaryotic diversity across virtually all terrestrial and host-associated environments, yet their interactions with light have long been overlooked. Recent discoveries have revealed that, even in the absence of canonical photosystems, these organisms possess an array of photoreceptors that couple light sensing to rapid membrane-potential changes, stress-response pathways, gene regulation, and motility. Here, we review three principal classes of light-induced phenomena in nonphotosynthetic bacteria: (1) electrophysiological responses, where light stimulation elicits membrane-potential modulation; (2) stress-response modulation, including light-activated sigma factors, DNA-repair enzymes, antioxidant defences and circadian-like and anticipatory transcriptional programs that align physiology with environmental light-dark cycles; (3) phototaxis, in which light gradients rewire chemoreceptor signaling to bias flagellar rotation. By illuminating how nonphotosynthetic bacteria exploit light as an environmental cue, we aim to inspire new strategies in antimicrobial therapy, synthetic biology, and environmental biotechnology.