Cyanophage Infections in a Sponge Intracellular Cyanobacterial Symbiont.

Sponges are sessile animals that play crucial roles in marine ecosystems by facilitating nutrient cycling, enhancing biodiversity, and structuring benthic habitats. Microbial symbionts, including cyanobacteria, are vital to sponges, aiding in nutrient cycling, metabolism, and defence. However, due to the sponge's ability to concentrate phages from seawater, extracellular sponge symbionts are particularly vulnerable to phage infection. By contrast, little is known about the susceptibility of intracellular sponge symbionts to phage predation. Here, we present evidence that Candidatus Synechococcus feldmannii, a facultative, horizontally transmitted cyanobacterial endosymbiont of the sponge Petrosia ficiformis, is susceptible to cyanophages. We analysed four Ca. S. feldmannii genomes and found evidence for phage interactions in two, including CRISPR spacers matching sipho- and T4-like cyanophages. One genome harboured a prophage region resembling freshwater cyanobacterial prophages, featuring conserved regions associated with Type VI secretion systems, similar to Wolbachia endosymbionts prophages. Additionally, we developed a method for isolating cyanophages directly from purified sponge bacteriocytes (specialised sponge cells harbouring symbionts) and identified nine T4-like cyanophages with less than 60% similarity to known relatives. Collectively, our findings indicate that Ca. S. feldmannii is susceptible to cyanophages and suggest potential functional parallels between phages infecting endosymbionts across different animal hosts.