Characterization of the bacterial microbiome associated with centrohelid heliozoans from aquatic environments using full-length 16S rRNA PacBio sequencing

This study utilizes full-length 16S rRNA PacBio sequencing to characterize the bacterial microbiome of freshwater centrohelid heliozoans, revealing a core community dominated by specific Proteobacteria and Bacteroidia while providing the first evidence that these protists harbor opportunistic pathogens and Rickettsiaceae, thereby serving as potential environmental reservoirs for bacteria with pathogenic potential.

The Gist

Imagine the microscopic world of a pond not as a quiet, empty soup, but as a bustling, chaotic city. In this city, there are tiny, floating predators called Centrohelid heliozoans. Think of them as microscopic "sea urchins" or "spiky dandelions" drifting in the water. They have long, sticky arms (axopodia) that they use to grab and eat bacteria, algae, and other tiny critters.

For a long time, scientists knew these creatures ate bacteria, but they didn't know much about the bacteria that lived on or inside them. It was like knowing a bear eats fish, but never having looked inside the bear's stomach to see what kind of fish were actually there, or if the bear was carrying any unexpected passengers.

This paper is the first time scientists took a high-tech "molecular census" of the bacteria living on these spiky predators. Here is what they found, broken down simply:

1. The "Roommates" (The Microbiome)

The researchers took samples from lakes in Russia and a fountain in Turkey. They grew these spiky creatures in the lab and then used a super-advanced DNA scanner (PacBio sequencing) to read the genetic ID cards of every bacterium hanging out on them.

They found that these creatures aren't just eating bacteria; they are hosting a whole community of them.

• The Neighborhood: The bacterial "city" on these creatures is mostly made up of three main groups (Alphaproteobacteria, Bacteroidia, and Gammaproteobacteria).
• The VIPs: Just like in any city, some bacteria are the landlords. The study found that four specific types—Arcicella, Variovorax, Sphingobium, and Pseudomonas—were the "core" residents. They were found on almost every single spiky creature they studied, no matter where it came from.

2. The "Trojan Horses" (The Surprise Discovery)

This is the most exciting part. Scientists often worry that tiny creatures like amoebas act as "Trojan Horses" for dangerous bacteria. They swallow the bad guys, protect them from the environment, and then spit them out, helping the bacteria survive and spread.

The researchers found that these spiky predators are doing exactly that.

• The Bad News: They discovered that the spiky creatures were hosting bacteria known to be opportunistic pathogens. These are bacteria that usually don't hurt healthy people but can cause serious infections in hospitals or in people with weak immune systems.
• The Culprits: The list included famous troublemakers like Pseudomonas (often found in hospital infections), E. coli, Acinetobacter, and even Mycobacterium (related to tuberculosis).
• The Implication: It turns out these tiny, harmless-looking spiky balls might be acting as a "safe house" or a "training ground" for these dangerous bacteria, helping them survive in the wild before they potentially jump to humans or animals.

3. The "New Neighbors" (Rickettsiaceae)

The team also found a family of bacteria called Rickettsiaceae. These are usually strict "squatters" that live inside cells and are often parasites. Finding them on these spiky creatures was a brand-new discovery. It's like finding a specific type of termite that was only known to live in one kind of tree, suddenly showing up in a completely different forest. This suggests these spiky creatures might be a new host for these bacteria.

4. Who Lives Where? (The Neighborhood Effect)

The researchers noticed that the "bacterial city" on the creature depended heavily on two things:

1. The Creature's Family: Different species of spiky creatures had slightly different bacterial communities, just like different dog breeds might have different gut bacteria.
2. The Address: Where the creature was found mattered even more.
   • Creatures from deep, clean lakes had different roommates than those from muddy river bottoms.
   • Creatures from a man-made fountain in Istanbul had a very different bacterial mix compared to those from a wild Russian lake. The "city" in the fountain was full of bacteria that love human-made environments.

The Big Picture

Think of these spiky creatures as microscopic buses.

• They pick up passengers (bacteria) from the water.
• They protect them from the harsh outside world.
• They drive them around the ecosystem.

The big takeaway from this paper is that we used to think these "buses" only carried harmless passengers. Now we know they are also carrying some very dangerous ones. This changes how we understand how diseases move through water. It suggests that to understand where dangerous bacteria come from, we need to look not just at the water, but at the tiny, spiky "buses" swimming in it.

In short: These tiny, spiky water creatures are hosting a diverse mix of bacteria, including some that can make humans sick. They might be the hidden link that helps these germs survive and spread in our lakes and rivers.

Technical Summary

1. Problem Statement

Centrohelid heliozoans are ubiquitous, free-living predatory protists found in freshwater, marine, and soil ecosystems. While their interactions with algae (symbiosis and kleptoplasty) are documented, their associations with bacteria remain largely unexplored. Previous knowledge was limited to only two reports. Given that protists often act as "Trojan horses" or environmental reservoirs for bacterial pathogens, understanding the microbiome of centrohelids is critical for assessing their role in the persistence and dispersal of opportunistic pathogens in aquatic ecosystems.

2. Methodology

The study employed a rigorous molecular and morphological approach to characterize the bacterial communities associated with seven centrohelid isolates.

• Sampling and Culturing:

  • Seven clonal centrohelid strains (G001, G004, G065, G014, G038, GB-2, G007) were isolated from diverse aquatic biotopes in Russia (Lake Teletskoe, Lake Kuchak, Irtysh River, etc.) and Turkey (Istanbul fountain).
  • Strains were maintained in monoculture using Parabodo caudatus flagellates as prey, which were themselves fed Aeromonas sobria.
  • Control Strategy: To distinguish between true host-associated bacteria and environmental/prey contaminants, the researchers established a negative control by sequencing the microbiome of the P. caudatus prey culture (fed A. sobria) and subsequently subtracted these sequences from the centrohelid data.

• Morphological Identification:

  • Host cells were identified using Differential Interference Contrast (DIC), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) to analyze cell coverings (scales and spicules).
  • Strains were classified into genera: Acanthocystis, Raineriophrys, and Heterophrys-like organisms (HLO).

• Sequencing and Bioinformatics:

  • Technique: Full-length 16S rRNA gene sequencing using the PacBio Sequel II platform (SMRTbell library).
  • Processing: Reads were processed using DADA2 to generate Amplicon Sequence Variants (ASVs). Taxonomy was assigned using the GTDB r220 database.
  • Analysis: Statistical analysis included alpha/beta diversity metrics (Chao1, Shannon, Bray-Curtis), PERMANOVA for community differences, and LEfSe (Linear Discriminant Analysis Effect Size) to identify biomarkers.
  • Phylogenetics: Bayesian Inference (MrBayes) and Maximum Likelihood (IQ-TREE) were used to reconstruct phylogenetic trees for specific bacterial families (Burkholderiaceae, Pseudomonadales, Rickettsiaceae).

3. Key Contributions

• First Molecular Characterization: This is the first study to provide a comprehensive molecular profile of the bacterial microbiome associated with centrohelid heliozoans.
• Methodological Rigor: The study utilized full-length 16S rRNA sequencing (PacBio) for higher taxonomic resolution compared to short-read methods, coupled with a strict prey-control subtraction method to ensure data accuracy.
• Discovery of Pathogenic Reservoirs: It provides the first evidence that centrohelids harbor bacteria with known pathogenic potential, expanding the known repertoire of protist hosts for opportunistic pathogens.
• Novel Host Range: The study reports the first detection of Rickettsiaceae (specifically Candidatus Megaira) in centrohelids.

4. Key Results

A. Microbiome Composition

• Diversity: The associated bacterial communities comprised 5 phyla, 6 classes, and 58 genera.
• Dominant Taxa: The community was dominated by Pseudomonadota (54.8%) and Bacteroidota (41.7%).
  • Classes: Alphaproteobacteria (41%), Bacteroidia (41%), and Gammaproteobacteria (13.5%).
  • Genera: Arcicella (34.7%), Sphingobium (21.4%), Pseudomonas (7.9%), Sphingomonas (6.5%), Azospirillum (5.8%), Shinella (5.5%), Flavobacterium (5.0%), Variovorax (3.7%), and Rhodococcus (3.5%).
• Core Microbiome: Arcicella, Variovorax, Sphingobium, and Pseudomonas were identified as the core microbiome, present across multiple strains.

B. Host Specificity and Habitat Influence

• Taxonomic Influence: Host taxonomy significantly influenced community composition (PERMANOVA, p=0.003). Acanthocystis strains clustered separately from Raineriophrys and Heterophrys-like strains.
• Habitat Influence: Habitat origin had an even stronger effect (PERMANOVA, p=0.0001). Microbiomes clustered distinctly by:
  1. Planktonic isolates (dominated by Variovorax).
  2. Littoral/Sediment isolates (dominated by specific Alphaproteobacteria like Mesorhizobium and Megaera).
  3. Anthropogenic/Artificial niche isolates (rich in diverse taxa including Cypionkella, Duganella, etc.).
• Nitrogen Fixation: Strains from oligotrophic Lake Teletskoe (G001, G004) were dominated by nitrogen-fixing bacteria (Azospirillum, Shinella, Rhodococcus), suggesting a symbiotic role in nutrient acquisition.

C. Pathogenic and Symbiotic Associations

• Opportunistic Pathogens: The study detected 12 genera of opportunistic pathogens, including Acidovorax, Acinetobacter, Escherichia, Mycobacterium, Pseudomonas, Ralstonia, and Sphingomonas.
  • Specific species identified included Pseudomonas aeruginosa, Sphingomonas koreensis (linked to meningitis), Acinetobacter lwoffii, and Escherichia coli.
  • These pathogens were found even in low abundance, suggesting centrohelids may act as reservoirs.
• Rickettsiaceae: The study identified Rickettsiaceae in two strains:
  • ASV110 clustered with Candidatus Megaira polyxenophila (a known symbiont of algae and ciliates).
  • ASV27 clustered with Rickettsiaceae incertae sedis.
  • This expands the known host range of Rickettsiaceae to include centrohelids.

D. Phylogenetic Insights

• Unassigned Burkholderiales sequences formed a novel clade closely related to Limnobacter, suggesting the discovery of new taxa.
• Pseudomonas ASVs formed well-supported clades with known pathogens like P. aeruginosa.

5. Significance

• Ecological Impact: The findings suggest that centrohelids are not just passive predators but active participants in microbial ecology, potentially facilitating the persistence of bacteria in nutrient-poor environments through nitrogen-fixing symbionts.
• Public Health Relevance: By identifying centrohelids as potential reservoirs for opportunistic pathogens (including P. aeruginosa and E. coli), the study highlights a previously overlooked vector for pathogen dispersal in aquatic and anthropogenic environments.
• Evolutionary Biology: The discovery of Rickettsiaceae associations suggests a broader evolutionary history of intracellular interactions between these bacteria and diverse protist hosts beyond the well-studied ciliates and amoebae.
• Future Directions: The paper calls for further research into the functional nature of these associations (mutualism vs. parasitism) and the potential role of centrohelids as vectors for human and animal pathogens.