Deciphering the impact of light-dark cycles on the interaction between cyanophage YongM and Nostoc sp. FACHB-596

Biological control of cyanobacterial blooms via cyanophages represents an eco-friendly strategy that has received increasing attention. Nevertheless, in-depth understanding of their infection efficiency, especially for freshwater cyanophage, still remains elusive. To address the issues, the broad-host-range cyanophage YongM was selected to explore its interaction with the representative cyanobacterium Nostoc sp. FACHB-596 (FACHB -596). This study presents the first de novo genome assembly of FACHB-596, in which incomplete microcystin biosynthetic gene clusters were identified. Subsequent tracking of YongM infection showed that host growth was significantly inhibited by 81.6% at 48 h post-infection, while the maximum quantum efficiency of photosystem II (Fv/Fm) decreased by 29.2% within 24 h. Interestingly, the infection efficiency of YongM was demonstrated to be diurnal rhythm-dependent, reaching a maximum under a 16 h:8 h light-dark cycle. To reveal the underlying mechanisms of the diurnal rhythm-dependent infection, comparative transcriptomics under different light conditions were conducted. Our results demonstrate that the enhanced infection efficiency under the 16 h:8 h cycle is driven by the host's diurnal metabolic reprogramming during the light-to-dark transition; specifically, it stimulates host glycogenolysis and the pentose phosphate pathway (with zwf and gnd upregulated by 2.1- and 3.4-fold), which optimizes energy allocation for phage replication, while simultaneously upregulating type IV pili and DNA-binding proteins (e.g., a 10.10-fold increase in comEA) to accelerate phage DNA adsorption and uptake. The study can provide new insights for regulating infection efficiency of cyanophages, with important implication for eco-friendly strategies in the control of cyanobacterial blooms in the future practices.