Prophage induction shifts community composition and functional capacity in a Sargassum-derived multispecies biofilm

This study demonstrates that chemical induction of prophages in a *Sargassum*-derived biofilm triggers a shift in microbial community composition—specifically reducing *Vibrio* abundance while promoting other genera—and alters functional capacity by depleting key metabolic modules, highlighting prophage dynamics as a critical mechanism for structuring and adapting the *Sargassum* microbiome.

The Gist

The Big Picture: A Floating City Under Siege

Imagine the Sargassum (a type of floating seaweed) not just as a plant, but as a massive, floating city in the middle of the ocean. This city is teeming with life, but not just fish and crabs; it's covered in a thick, invisible layer of bacteria called a biofilm. Think of this biofilm as the city's "skin" or "microscopic neighborhood."

For a long time, scientists noticed this seaweed was growing like crazy, forming huge "blooms" that clogged beaches and hurt tourism. They wondered: What makes this seaweed so successful?

This paper investigates the relationship between the seaweed's bacterial neighborhood and its invisible tenants: viruses (specifically bacteriophages, or "phages").

The Cast of Characters

1. The Host (Sargassum): The floating seaweed.
2. The Tenants (Bacteria): The microscopic community living on the seaweed. The most famous (and abundant) tenant is a genus called Vibrio. Think of Vibrio as the "landlords" of this neighborhood—they run the show, fix the plumbing (nutrients), and keep the place running.
3. The Dormant Agents (Prophages): These are viruses that have already moved into the bacteria's DNA. They are sleeping agents. They aren't killing the bacteria right now; they are just hiding inside, waiting for a signal to wake up.
4. The Alarm (Mitomycin C): In the lab, the scientists used a chemical called Mitomycin C. Think of this as a "wake-up call" or a "fire alarm" that forces the sleeping viruses to wake up, burst out of the bacteria, and go on a rampage.

The Experiment: Pulling the Fire Alarm

The scientists took a sample of this seaweed's bacterial neighborhood and set up two groups in a lab:

• Group A (Control): Just let the bacteria chill.
• Group B (Induced): They pulled the "fire alarm" (added Mitomycin C) to wake up the sleeping viruses.

What happened?
When the alarm went off, the sleeping viruses woke up, exploded out of their bacterial hosts, and killed a huge chunk of the Vibrio population. It was like a sudden eviction notice for the landlords.

The Aftermath: A Neighborhood Shake-Up

Here is where it gets interesting. When the dominant "landlords" (Vibrio) were wiped out by the viruses, the neighborhood didn't just go quiet. It changed completely.

• The Power Vacuum: With the Vibrio gone, other bacteria that were usually quiet and less common (like Pseudoalteromonas, Alteromonas, and Cobetia) suddenly had room to grow. They moved into the empty houses.
• The Result: The entire makeup of the community shifted. The "Vibrio-dominated" city became a "mixed-species" city.

The Secret Weapons: What the Viruses Were Hiding

The scientists looked closely at the viruses that woke up and found they were carrying some very special "tools" (genes) that they had stolen from their bacterial hosts over time.

1. The "Boss" Switch (Quorum Sensing): One virus had a gene called AphA. In bacteria, this is like a "volume knob" for communication. It tells the bacteria when to build biofilms (stick together) or when to act tough. The virus had stolen this switch. It's like a spy who stole the mayor's walkie-talkie; the virus can now eavesdrop on the bacteria or even trick them into changing their behavior.
2. The "Toxin" Toolkit: Some viruses carried genes for toxins (like Zot or chitinase). These are weapons that help the bacteria infect other creatures or eat specific foods (like chitin from shrimp shells). By carrying these, the virus makes its host a better survivor.
3. The "Nutrient" Factory: The bacteria were also responsible for recycling nutrients like nitrogen (fertilizer for the seaweed). When the viruses killed the Vibrio, the city lost its ability to process nitrogen efficiently. The "fertilizer factory" went offline.

Why Does This Matter?

This study tells us that the relationship between seaweed, bacteria, and viruses is a delicate dance.

• The "Lysogenic" Cycle: Most of the time, these viruses are sleeping (lysogeny), helping the bacteria survive and maybe even helping the seaweed grow by providing nutrients.
• The "Lytic" Cycle: But when stress hits (like too much sun or pollution), the viruses wake up (induction). They kill the dominant bacteria, which changes the whole community.

The Takeaway:
The massive growth of Sargassum isn't just about the seaweed itself; it's about the invisible war and peace treaty happening in its microscopic skin. The viruses act as a traffic cop for the bacterial community. When they stay asleep, the "Vibrio" landlords run the show. When they wake up, they clear the board, allowing new players to take over.

This helps explain why Sargassum blooms can change so drastically from one place to another. The "mood" of the viruses (sleeping vs. waking) might be the secret switch that decides which bacteria rule the seaweed, and ultimately, how well the seaweed grows and survives.

In a Nutshell

Imagine a city where the police (viruses) are usually asleep inside the houses of the mayor (Vibrio). One day, the police wake up, arrest the mayor, and the city government collapses. Immediately, a different group of citizens takes over the government. The city still functions, but it runs very differently. This paper shows that this "police wake-up" is a major reason why the Sargassum seaweed's bacterial world is so dynamic and successful.

Technical Summary

1. Problem Statement

Pelagic Sargassum (specifically S. natans and S. fluitans) has experienced unprecedented range expansion and massive blooms in the Atlantic Ocean over the last 15 years, causing significant ecological and socioeconomic disruption. While the algal microbiome is known to be crucial for the host's nutrient acquisition and ecological success, the mechanisms regulating the structure and function of these microbial communities remain poorly understood.

Previous research indicated that Sargassum-associated bacteria harbor a high density of integrated prophages (temperate bacteriophages) compared to surrounding seawater. However, the ecological impact of prophage induction (the switch from lysogeny to the lytic cycle) on the Sargassum microbiome's community composition, metabolic potential, and biofilm dynamics was unknown. The authors hypothesized that inducing prophages would lyse dominant bacterial hosts, thereby opening niche space for other community members and altering the functional capacity of the biofilm.

2. Methodology

The study utilized an in vitro multispecies biofilm system derived from natural Sargassum samples collected off the coast of Florida.

• Biofilm Cultivation: Biofilms were established from scraped Sargassum tissue in 10% Sargassum-supplemented seawater.
• Induction Treatment: The biofilms were divided into control and treatment groups. The treatment group was exposed to Mitomycin C (MC) at 1 µg/mL, a known DNA-damaging agent that triggers the SOS response and prophage induction. Samples were harvested pre-induction and after a 12-hour recovery period post-induction.
• Phenotypic Assay: Biofilm biomass was quantified using Crystal Violet staining and optical density (OD540) measurements.
• Metagenomics: Shotgun metagenomic sequencing (Illumina HiSeq 4000) was performed on all samples.
• Bioinformatic Analysis:
  • Viral Identification: Tools like geNomad, COBRA (for contig extension), and Virathon were used to identify and dereplicate viral genomes.
  • Prophage Detection: PropagAtE was used to calculate coverage-based Virus-to-Host Ratios (VHR) to identify induced prophages.
  • Binning: MetaWRAP (CONCOCT, MetaBAT2, MaxBin2) was used to reconstruct bacterial Metagenome-Assembled Genomes (bMAGs).
  • Host Prediction: Tools like iPHoP and BLASTn were used to link viruses to specific bacterial hosts.
  • Functional Analysis: Anvi'o and KEGG pathways were used to assess metabolic modules.
  • Structural Biology: AlphaFold 3 and ChimeraX were used to compare the structural similarity of prophage-encoded vs. host-encoded proteins.

3. Key Contributions

• Ecological Relevance of In Vitro Models: Demonstrated that the in vitro biofilm community composition closely mirrors natural Sargassum microbiomes, with 64% of identified viruses and 17 of 28 bMAGs present in environmental samples.
• Mechanism of Community Structuring: Provided direct evidence that prophage induction is a primary driver of bacterial community turnover in Sargassum biofilms, specifically targeting dominant Vibrio populations.
• Discovery of Phage-Encoded Regulatory Genes: Identified a prophage-encoded AphA transcriptional regulator, a key master regulator of Vibrio quorum sensing (QS) and biofilm formation, suggesting phages can directly manipulate host social behaviors.
• Metabolic Consequences: Quantified the loss of specific metabolic functions (e.g., nitrogen cycling, vitamin biosynthesis) resulting from the lysis of specific bacterial taxa.

4. Key Results

A. Impact on Biofilm Growth and Community Composition

• Biofilm Reduction: MC treatment caused a significant decrease in biofilm biomass compared to controls, indicating that induction leads to cell lysis and reduced growth.
• Shift in Dominance:
  • Pre-Induction: The community was dominated by Vibrio species.
  • Post-Induction: There was a significant reduction in Vibrio abundance (13 out of 17 Vibrio bMAGs decreased).
  • Niche Opening: The reduction in Vibrio allowed for the proliferation of other genera, specifically Pseudoalteromonas, Cobetia, and Alteromonas.
• Viral Dynamics: The treatment led to a significant increase in the abundance of Vibrio-infecting temperate phages. Four specific prophages (all predicted to infect Vibrio) drove the differences in viral community composition between pre- and post-induction samples.

B. Prophage-Encoded Virulence and Regulation

• AphA Discovery: One induced prophage (prophage.8) encoded a gene homologous to AphA, a transcriptional regulator that controls quorum sensing and biofilm formation in Vibrio at low cell densities.
  • The prophage-encoded AphA shared only ~34% amino acid identity with the host but showed high structural similarity (RMSD 1.216 Å).
  • This suggests the phage acquired the gene to potentially "eavesdrop" on or manipulate the host's QS network to favor lysogeny or lysis decisions.
• Other Virulence Factors: Induced phages also encoded genes for zonula occludens toxin (Zot) (affecting tight junctions in animals) and chitinase (potentially aiding in nutrient acquisition from chitinous holobiont members like bryozoans).

C. Metabolic Shifts

• Loss of Metabolic Modules: Prophage induction resulted in a relative loss of 17 metabolic modules in the community, primarily because the dominant Vibrio hosts were lysed.
• Key Depleted Functions:
  • Nitrogen Metabolism: Loss of dissimilatory nitrate reduction and nitrate assimilation pathways. Given that Vibrio are known nitrogen fixers in Sargassum, this suggests induction could temporarily disrupt the nitrogen supply to the algal host.
  • Vitamin Biosynthesis: Reduced capacity for cobalamin (Vitamin B12) and thiamine biosynthesis.
  • Energy Metabolism: Decreases in cytochrome-c oxidase and acetogenesis pathways.

5. Significance

This study establishes prophage induction as a critical biotic mechanism regulating the Sargassum microbiome. The findings suggest that:

1. Community Flexibility: The microbiome is highly dynamic; environmental stressors (like UV radiation or chemical stressors) that induce prophages can rapidly shift the community from a Vibrio-dominated state to one dominated by Pseudoalteromonas or Cobetia.
2. Host-Phage Co-evolution: The presence of phage-encoded regulators like AphA indicates a sophisticated level of interaction where phages may actively manipulate host physiology (biofilm formation, virulence) to optimize their own transmission.
3. Ecological Implications: Since Vibrio are key nitrogen fixers and vitamin producers for Sargassum, frequent prophage induction could alter the nutrient cycling and physiological health of the algae, potentially influencing the success of Sargassum blooms in a changing ocean environment.

In conclusion, the paper demonstrates that viral infection strategies are not merely destructive but are integral to the structural and functional plasticity of the Sargassum holobiont.