Bacterial blooms are characterized by unusually high cell densities and exceptionally low diversity and can profoundly alter ecosystem function and services. Bacteriophages have long been considered an important cause of mortality in blooms, acting as a mechanism for control. Here, we characterize the viral ecology of a long-lasting estuarine bloom of green sulfur bacteria (Chlorobiota). We combined direct cell and viral counts with metagenomic and metaproteomic data to characterize host and phage activity at different time points. The abundance of virus-like particles (VLPs) decreased at high cell densities, suggesting reduced lytic infection rates. The dominant organism, GSB-TRL01 (genus Prosthecochloris), apparently contained a large conjugative plasmid encoding five different anti-phage defense systems. The organism's genome encoded 13 additional defense systems. Compared to the average of five defense systems per microbial genome, this enrichment suggests robust anti-phage defense capabilities. Proteins from ten different defense systems on GSB-TRL01's genome and four systems from the conjugative plasmid were expressed in the proteome. This suggests that GSB-TRL01 invests heavily in anti-phage defense, leading to reduced lysis at high cell densities and allowing blooms to persist for weeks to months. To determine whether this ability is widespread among bloom forming organisms, we compared genomes of putative bloomers to those of non-blooming organisms. We found that bloomer genomes were significantly enriched with anti-phage defense systems. This challenges traditional paradigms of phage ecology in bloom-forming systems and suggests that microbes adapted to high-density growth may have evolved mechanisms to reduce their susceptibility to phage attack. ### Competing Interest Statement The authors have declared no competing interest. Simons Foundation, https://ror.org/01cmst727, 824763 U.S. National Science Foundation, 2426305, 2205993 Brien O’Brien and Mary Hasten Scholarship Fund

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Tiny Living Beings · Episode

Nature Microbiology - Grounding microbial conservation in ecological principles

YouTube video by Charles Francis Delwiche

CRISPR–Cas systems transcriptionally tune diverse innate defences using CRISPR RNA-like guides to balance antiviral protection with fitness, and hyperactivate these defences when compromised, establis...

Microbial communities often contain diverse bacteria and their specific viruses (bacteriophages). Lytic phages frequently select for host bacteria to evolve resistance, in-turn threatening phage extinction. However, in nature phage-bacteria interactions do not occur in isolation, but within complex communities containing other species. To test how community context can alter resistance evolution and phage persistence, we experimentally evolved Pseudomonas aeruginosa bacteria and a Pseudomonas-targeting phage with and without a community representing a cystic fibrosis disease pulmonary infection. Phages selected for rapid evolution of receptor-mediated resistance, regardless of community context. However, susceptible bacteria often persisted due to resistance-growth trade-offs. A diverse community favored increased frequencies of susceptibility, fueling higher phage densities and fewer phage extinctions. Our findings show that community complexity can constrain the evolution of phage resistance in bacteria, promoting host-parasite coexistence and fostering microbial diversity. ### Competing Interest Statement The authors have declared no competing interest. Yale STARS program Howard Hughes Medical Institute

Bacteria produce natural products to adapt to their environments, with phage interactions as major ecological and evolutionary drivers. While some nat…

Published in Microbiology spectrum

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Podcast Episode · Night Science · July 13 · 38m

Most naturally occurring bacteria are lysogens, encoding one or more temperate phages (prophages) integrated into their genome. As prophages are induced by the bacterial SOS response, DNA-damaging ant...

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Locked in a constant arms race, bacteria and their phage predators have evolved various defenses and counter-defenses. Compared to the numerous identified defenses, phage-encoded counter-defenses are ...

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