An anaerobic Legionellales symbiont in Anaeramoeba pumila

This study characterizes *Candidatus* Centrionella anaeramoebae, a rare anaerobic Legionellales symbiont of *Anaeramoeba pumila* that has undergone genome reduction, acquired host-derived GTPase genes via horizontal transfer, and lost the symbiosome structure, thereby offering a unique model for the evolutionary transition from aerobic pathogenesis to anaerobic mutualism.

The Gist

Imagine a tiny, single-celled creature living in the dark, oxygen-free mud at the bottom of the ocean. This creature is an amoeba called Anaeramoeba pumila. It's the "little sibling" of its family—much smaller than its cousins—and it has a very unusual living situation.

Usually, amoebas in this family live in a cozy, high-tech apartment complex called a symbiosome. Think of this as a specialized room inside the cell where they keep their bacterial roommates. But this little amoeba, A. pumila, has thrown out the walls. Its bacterial roommates are now free-roaming, hanging out directly in the main living room (the cytoplasm), specifically clustering around the cell's "command center" (the microtubule-organizing center).

Here is the story of what scientists discovered about this unique trio:

1. The New Roommate: A "Legionnaire" Who Loves the Dark

The main bacterial roommate is a new species named Candidatus Centrionella anaeramoebae.

• The Plot Twist: This bacterium belongs to a family called Legionellales. You might know this family from Legionella pneumophila, the germ that causes Legionnaires' disease. Usually, these bacteria are like aerobic vampires—they need oxygen to survive and are famous for being sneaky pathogens that invade human lungs.
• The Transformation: This new Centrionella is a rebel. It has completely lost its need for oxygen. It has evolved into a "dark-dweller," surviving entirely on fermentation (like making yogurt or alcohol) and eating hydrogen gas. It's the first time scientists have found a member of this "Legionnaire" family that is strictly anaerobic (oxygen-hating) and lives in a symbiotic (helpful) relationship rather than a parasitic one.

2. The Metabolic Dance: A Three-Person Tug-of-War

The amoeba and its two bacterial roommates have formed a perfect three-way business partnership to survive in the mud.

• The Host (The Amoeba): It acts like a power plant. It breaks down food and produces waste products: hydrogen gas, acetate, and propionate. In a normal world, this waste would just pile up and poison the system.
• The First Roommate (Centrionella): This bacterium is the recycler. It lives inside the amoeba and eats the hydrogen gas the amoeba produces. It uses this hydrogen to generate its own energy. It also helps manage the cell's internal structure.
• The Second Roommate (Desulfobacter): This is a larger bacterium that lives outside the amoeba but right next to it. It acts as the cleanup crew. It eats the acetate and propionate waste that the amoeba and Centrionella can't handle. In exchange for this cleanup service, it provides the amoeba with Vitamin B12, a crucial nutrient the amoeba can't make on its own.

The Analogy: Imagine a house where the homeowner (the amoeba) cooks a meal and produces smoke (hydrogen) and trash (acetate). Centrionella is a roommate who lives inside the house and eats the smoke to stay warm. The Desulfobacter is a neighbor who comes over to take away the trash in exchange for giving the homeowner a vitamin supplement. Everyone is happy, and the house stays clean.

3. The Spy in the System: Stealing the Host's Blueprints

One of the most fascinating discoveries is how Centrionella controls its host.

• The Secret Weapon: Bacteria usually try to trick a host cell by sending in "fake" proteins that look like the host's own tools. But Centrionella did something much more brazen: it stole the host's actual blueprints.
• The Heist: The bacterium grabbed a gene from the amoeba that controls the cell's skeleton (a protein called Rac1). It didn't just copy it; it duplicated it and mixed it with other parts to create new, custom-made tools.
• The Result: Now, the bacterium has its own version of the amoeba's "remote control." It likely uses this stolen remote to manipulate the amoeba's skeleton, keeping itself anchored right next to the command center (the MTOC) and ensuring it gets passed down when the amoeba divides. It's like a tenant who stole the landlord's master key and rewired the locks to keep the landlord from kicking them out.

4. Why This Matters

This discovery is a huge deal for evolutionary biology.

• From Villain to Hero: It shows how a lineage of bacteria known for being dangerous pathogens (Legionellales) can evolve into a helpful, oxygen-free partner. It's a snapshot of evolution in action, showing how life adapts to extreme environments.
• The "Lost Apartment": Most of the amoeba's cousins live in that special "symbiosome" room. A. pumila lost this room. This suggests that when the bacteria get really good at manipulating the host (like stealing the Rac1 remote), they might not need a separate room anymore. They can just take over the whole house.
• Convergent Evolution: Interestingly, this bacterium evolved to live without oxygen in a way that is very similar to a completely different group of bacteria (Anoxychlamydiales). Nature found the same solution to the "no oxygen" problem twice, independently.

In a Nutshell

Scientists found a tiny amoeba in the deep sea that lives without oxygen. It hosts a bacterial roommate that used to be a dangerous, oxygen-loving pathogen but has transformed into a hydrogen-eating partner. This bacterium is so integrated into the host's life that it stole the host's own genetic "remote control" to stay in place. Together with a second bacterial neighbor, they form a perfect three-way team that recycles waste and shares nutrients, proving that even in the darkest, most oxygen-free corners of the ocean, life finds a way to cooperate.

Technical Summary

1. Problem Statement

The study addresses several gaps in microbial ecology and evolutionary biology:

• Evolutionary Transition: While the order Legionellales is well-known for containing aerobic intracellular pathogens (e.g., Legionella pneumophila), the evolutionary trajectory toward anaerobic mutualism in oxygen-depleted environments remains poorly understood.
• Host-Symbiont Complexity: The phylum Anaeramoebae typically hosts sulfate-reducing bacteria within specialized membrane-bound organelles called symbiosomes. However, the newly discovered miniature amoeba Anaeramoeba pumila lacks these symbiosomes, and the identity and metabolic integration of its symbionts were unknown.
• Metabolic Gaps: The metabolic capabilities of A. pumila's mitochondrion-related organelles (MROs) and how they interact with intracellular bacteria in the absence of a symbiosome needed elucidation.

2. Methodology

The researchers employed a multi-omics and imaging approach to characterize the host (A. pumila isolate LANTAAN) and its associated microbiota:

• Cultivation & Enrichment: A. pumila was cultivated in anoxic marine sediment conditions. Enrichment of host cells versus free-living bacteria was achieved via cold-shock and Histopaque density gradient centrifugation.
• Advanced Imaging:
  • FIB-SEM (Focused Ion Beam Scanning Electron Microscopy): Used for 3D tomography to visualize the spatial relationship between the host nucleus, microtubule-organizing centers (MTOCs), hydrogenosomes, and symbiont clusters.
  • Fluorescence In Situ Hybridization (FISH): Used to confirm the presence and localization of specific bacterial symbionts within the host.
  • Immunofluorescence: Used to localize MROs and cytoskeletal elements.
• Genomics & Metagenomics:
  • Sequencing: Combined Illumina short-reads, Oxford Nanopore long-reads, and RNA-seq data.
  • Assembly & Binning: Metagenomic assembly was performed (using Flye/Canu/Unicycler), followed by manual binning to separate the host nuclear genome from prokaryotic symbiont genomes.
  • Phylogenomics: Multi-gene phylogenetic trees were constructed to place the symbiont within the Legionellales and the host within the Anaeramoebae.
• Metabolic Reconstruction: Comparative genomics and pathway analysis were used to reconstruct the metabolic networks of the host and symbionts, focusing on energy metabolism, secretion systems, and horizontal gene transfer (HGT).

3. Key Contributions & Results

A. Discovery of Candidatus Centrionella anaeramoebae

• Taxonomy: The primary symbiont was identified as a novel gammaproteobacterium belonging to the order Legionellales. It is described as Candidatus Centrionella anaeramoebae gen. nov., sp. nov.
• Phylogeny: It belongs to the provisional order UBA12402 (now proposed as Ca. Centrionellales), a deep-branching lineage distinct from the well-studied Legionella and Coxiella.
• Genome Characteristics: The genome is highly reduced (1.52 Mbp, 1,249 genes) with a low GC content (36.4%). It retains a complete Dot/Icm Type IVB secretion system, a hallmark of Legionellales used for host manipulation.

B. Metabolic Adaptation to Anaerobiosis

• Anaerobic Metabolism: Unlike its aerobic relatives, Ca. C. anaeramoebae has lost the aerobic respiratory chain. It relies entirely on substrate-level phosphorylation, arginine fermentation (via the arginine deiminase pathway), and hydrogen oxidation.
• Hydrogenase: It encodes a bidirectional [NiFe]-hydrogenase (likely of archaeal origin via HGT) capable of oxidizing hydrogen, suggesting a syntrophic role in consuming host-derived $H_2$.
• Host Manipulation via HGT: Strikingly, the symbiont acquired Rac1-like GTPase genes directly from the host (A. pumila) via Horizontal Gene Transfer. These genes underwent domain shuffling and duplication, creating effectors with eukaryotic-like domains (Ankyrin repeats, Kinase domains). This suggests a unique strategy where the symbiont directly co-opts host cytoskeletal regulators rather than just mimicking them.

C. Host Characteristics: Secondary Loss of Symbiosome

• Phylogeny: A. pumila is phylogenetically nested within Anaeramoebae but exhibits secondary losses of ancestral traits:
  • No Symbiosome: Unlike other Anaeramoeba species, A. pumila lacks the membrane-bound symbiosome. Symbionts are free in the cytoplasm, specifically clustered around the MTOC (Microtubule-Organizing Center).
  • Genomic Reduction: The host genome shows massive intron loss (only 22 introns vs. >30,000 in relatives) and a shift from AT-rich to GC-rich DNA.
• MROs: The host possesses functional hydrogenosomes that produce $H_2$, acetate, and propionate, providing the metabolic basis for the symbiosis.

D. Tripartite Syntrophy

• Third Partner: A free-living (or loosely associated) Desulfobacter sp. was identified.
• Metabolic Integration: This bacterium consumes the fermentation end-products ($H_2$, acetate, propionate) via dissimilatory sulfate reduction and synthesizes Vitamin B12 (cobalamin), which the host requires.
• Consortium: The system forms a tripartite syntrophic consortium:
  1. A. pumila (Host): Produces fermentation products via hydrogenosomes.
  2. Ca. C. anaeramoebae (Intracellular): Consumes $H_2$ and acetate; manipulates host cytoskeleton.
  3. Desulfobacter sp. (Extracellular/Loosely associated): Consumes remaining products and provides B12.

4. Significance

• Evolutionary Model: This study provides a rare example of the transition from aerobic pathogenesis to anaerobic mutualism within the Legionellales. It demonstrates that the core machinery for host manipulation (Dot/Icm system) is retained even when the lifestyle shifts from pathogenic to symbiotic.
• Convergent Evolution: The metabolic strategies of Ca. C. anaeramoebae (arginine fermentation, Na+/H+ antiporters) parallel those of the unrelated Anoxychlamydiales, suggesting convergent evolution for survival in anaerobic intracellular niches.
• Mechanism of Symbiosome Loss: The findings suggest that the symbiosome in A. pumila was secondarily lost, possibly due to the small cell size of the host or active suppression by symbiont effectors (like the Rac1-like proteins) that anchor the bacteria to the MTOC instead of a vacuole.
• HGT in Symbiosis: The acquisition of host GTPase genes by the symbiont represents a sophisticated mechanism of host control, blurring the line between pathogen effectors and symbiont genomic integration.

In summary, this paper describes a novel, highly reduced anaerobic Legionellales symbiont that has adapted to an intracellular lifestyle in a miniature amoeba by repurposing its ancestral virulence machinery, acquiring host genes, and engaging in a complex tripartite metabolic exchange.