Bloom-forming bacteria heavily invest in anti-phage defense

Bloom-forming bacterial blooms, defined by high cell density and low diversity, significantly impact ecosystem function. Bacteriophages, viruses that infect bacteria, are typically seen as a controlling force in these blooms. This study examined the viral ecology of an extended estuarine bloom of green sulfur bacteria. By combining direct cell and viral counts, as well as metagenomic and metaproteomic data, researchers analyzed host and phage activity throughout the bloom's duration.

The results indicated a decrease in virus-like particles (VLPs) as cell densities increased, suggesting a reduction in lytic infection rates. The primary organism responsible, GSB-TRL01 (from the Prosthecochloris genus), was found to possess a large conjugative plasmid containing five anti-phage defense systems. Additionally, GSB-TRL01's genome encoded 13 more defense systems, suggesting a strong anti-phage defense capability.

When compared to the average of five defense systems per microbial genome, this abundance of anti-phage defenses highlights GSB-TRL01's robust protective mechanisms. The proteome analysis revealed proteins from ten different defense systems on GSB-TRL01's genome and four systems from the conjugative plasmid, underscoring the organism's heavy investment in anti-phage defense. This defense investment likely led to reduced lysis at high cell densities, enabling the bloom to persist for weeks to months.

To assess whether this anti-phage defense strategy is widespread among bloom-forming organisms, the researchers compared genomes of potential bloomers to those of non-blooming organisms. They discovered that bloomers' genomes were significantly enriched with anti-phage defense systems. This finding challenges traditional notions of phage ecology in bloom-forming systems and suggests that bacteria adapted to high-density growth have evolved mechanisms to minimize their susceptibility to phage attacks.