The colourless dictyochophyte Ciliophrys sp. Baltic secondarily lacks a plastid but hosts a bacterial endosymbiont

This study characterizes the colourless protist *Ciliophrys* sp. Baltic as a rare example of secondary plastid loss in a photosynthetic lineage, revealing that it compensates for the resulting biosynthetic deficiencies by hosting a metabolically interactive bacterial endosymbiont, *Candidatus* Penulousia baltica.

The Gist

The Story of the "Lost Kitchen" and the "Helpful Squatter"

Imagine a single-celled organism named Ciliophrys sp. Baltic. Think of it as a tiny, free-living marine monster that swims around eating bacteria. It belongs to a family of organisms (the Dictyochophytes) that are usually famous for having chloroplasts—the tiny "solar panels" inside cells that let plants and algae make their own food using sunlight.

But this specific Ciliophrys is different. It's colorless, meaning it has no solar panels. It doesn't photosynthesize. It just eats.

For a long time, scientists were confused: Did this creature lose its solar panels completely, or did it just turn them off and hide them? This paper solves the mystery.

1. The Great Kitchen Demolition

To understand what happened, imagine a house (the cell) that used to have a massive, high-tech kitchen (the plastid/chloroplast) where it cooked its own meals.

• The Evidence: The scientists looked at the creature's genetic blueprint (its DNA and RNA). They found that the blueprints for the kitchen are gone. There are no instructions for the stove, the fridge, or the solar panels.
• The Result: The kitchen hasn't just been turned off; it has been completely demolished. The walls are down, the appliances are gone, and the blueprints are shredded. This is a rare event in nature. Most "non-photosynthetic" plants still keep a tiny, useless kitchen (a leucoplast) just in case. But Ciliophrys sp. Baltic has thrown the whole thing out.

The Twist: Even though the kitchen is gone, the creature still has a few old recipes (enzymes) that used to be cooked in the kitchen. But instead of throwing them away, the creature has moved them to the living room (the cytoplasm) or the basement (the mitochondria) and is trying to use them there. It's like trying to bake a cake in a bathtub because the kitchen is gone.

2. The Uninvited (but Useful) Tenant

Just as the creature was struggling without its kitchen, it was discovered that it has a new roommate living inside it.

• The Tenant: A bacterium named Candidatus Penulousia baltica.
• The Relationship: This bacterium is a "professional squatter." It lives inside the Ciliophrys, eats its food, and generally takes advantage of the host. It's like a tenant who never pays rent but eats all the leftovers.
• The Surprise: Usually, these squatters are just parasites. But in this case, the tenant is actually helping the host survive the kitchen demolition.

3. How the Squatter Saves the Day

Because Ciliophrys lost its kitchen, it lost the ability to make two very important ingredients: Heme (needed for breathing) and a precursor for Lysine (a building block for proteins). Without these, the creature would starve or suffocate.

Here is where the tenant steps in:

• The Heme Handout: The bacterium can still make Heme. It makes extra and shares it with the host.
• The Lysine Loophole: The bacterium makes a chemical called diaminopimelate to build its own cell wall. It doesn't need to turn it into Lysine. However, the host can turn that chemical into Lysine. So, the bacterium leaks some of its chemical, and the host grabs it to finish the job.

The Analogy: Imagine you lost your ability to bake bread (Lysine). Your annoying roommate, who bakes cookies, accidentally leaves a bag of flour (the precursor) on the counter. You grab the flour, bake your bread, and suddenly, you can survive. You didn't ask for help, but the roommate's waste product saved your life.

4. The Bacterium's Secret Weapons

The scientists also looked at the bacterium's genome and found it has a "toolkit" for manipulating its host.

• The Secret Service: It has a special injection system (Type IV secretion) to shoot proteins into the host's cell to control it.
• The Poison: It has toxins that can break down the host's internal structures if the host tries to kick it out.
• The Defense: It has systems to protect itself from viruses.

It's a classic "hostage situation," but with a twist: the hostage is actually keeping the captor alive by providing a home, and the captor is keeping the hostage alive by providing food ingredients.

The Big Picture

This paper is important because:

1. It proves a rare event: It shows a free-living organism that has completely lost its "solar panel" organelle, not just turned it off.
2. It reveals a new partnership: It shows how a parasite can accidentally become a life-support system when the host loses a major organ.
3. It's a new model: Scientists can now use this tiny creature and its bacterial roommate as a "test lab" to study how life evolves when it loses major parts and has to rely on others.

In short: A tiny marine monster lost its internal solar-powered factory. Instead of dying, it accidentally adopted a bacterial squatter. The squatter, while mostly a parasite, unknowingly provides the exact ingredients the monster needs to survive, creating a bizarre, new kind of survival team.

Technical Summary

Problem Statement

The study addresses two major gaps in evolutionary biology and protistology:

1. Plastid Loss in Free-Living Ochrophytes: While plastid loss is documented in parasitic lineages (e.g., Syndinea, some Apicomplexa) and rare free-living taxa (e.g., Picozoa, Picophagus flagellatus), the status of the plastid in the genus Ciliophrys (Dictyochophyceae) remained unresolved. Previous studies were conflicting, with some suggesting a cryptic plastid based on an rbcL sequence (later suspected to be contamination) and others finding no morphological evidence.
2. Endosymbiont Diversity in Ochrophytes: Bacterial endosymbionts, particularly "professional" intracellular bacteria like Rickettsiales, are common in protists but have been rarely documented in Ochrophytes. The interaction between a plastid-lacking host and a bacterial endosymbiont, specifically regarding metabolic compensation for organelle loss, was unexplored.

Methodology

The researchers isolated a new strain of Ciliophrys (designated Ciliophrys sp. Baltic) from sand samples in the Baltic Sea (Lithuania). They employed a multi-omics approach:

• Sequencing: Performed Illumina (short-read) and Oxford Nanopore (long-read) DNA sequencing, as well as RNA sequencing, to generate genomic and transcriptomic data.
• Assembly & Binning: Assembled the host genome and transcriptome. Used coverage depth and BLAST-based binning to separate the host sequences from a co-assembled bacterial genome.
• Phylogenetics: Conducted Maximum Likelihood (ML) phylogenetic analyses using 18S rDNA for the host and 16S rDNA/phylogenomics (45 conserved proteins) for the bacterial endosymbiont to determine evolutionary relationships.
• Genomic Annotation & Metabolic Reconstruction: Annotated the host and endosymbiont genomes. Used BUSCO for completeness assessment and KEGG pathway reconstruction to analyze metabolic capabilities.
• Localization Prediction: Used TargetP-2.0 and HMMER searches to predict subcellular localization signals (plastid vs. mitochondrial vs. cytosolic) for key enzymes.
• Microscopy: Utilized Transmission Electron Microscopy (TEM) and Fluorescence In Situ Hybridization (FISH) with a specific probe (CilioEndo16S) to visualize the endosymbiont's location and morphology within the host.

Key Contributions & Results

1. Confirmation of Complete Plastid Loss

• Genomic Evidence: The study found no plastid genome (plastome) in Ciliophrys sp. Baltic, despite deep sequencing. The previously reported rbcL sequence was identified as a contaminant from a photosynthetic relative.
• Loss of Import Machinery: The host lacks genes for the TOC/TIC protein import complexes and SELMA machinery, which are essential for maintaining a plastid.
• Metabolic Deficiencies: The host lacks the entire MEP/DOX pathway (isoprenoids), the shikimate pathway (aromatic amino acids), and most of the FASII pathway (fatty acids), all typically plastid-localized.
• Genetic Footprints: While the organelle is gone, "genetic footprints" remain. Two enzymes from the ancestral plastidial FASII pathway (FabD and FabI) persist but have been retargeted to the mitochondrion (indicated by mitochondrial transit peptides).
• SQDG Anomaly: Surprisingly, the host retains the complete pathway for synthesizing Sulfoquinovosyldiacylglycerol (SQDG), a thylakoid lipid. However, the enzymes (SQD1, CysH, SQD2) lack plastid transit peptides and likely function in the ER or a specialized endomembrane compartment, suggesting a remnant of the plastid's membrane synthesis machinery persists in a modified form.

2. Discovery of a Novel Endosymbiont: Candidatus Penulousia baltica

• Identification: The culture harbors a dense population of an intracellular bacterium belonging to the Rickettsiaceae family, specifically the "Megaira group" (Clade B).
• Taxonomy: The bacterium is described as a new species, Candidatus Penulousia baltica. It is distinct from the genus Megaera and represents a new genus-level lineage within the Rickettsiaceae.
• Localization: TEM and FISH confirmed the bacteria reside freely in the host cytoplasm (not in vacuoles), densely populating the cell.
• Genome Characteristics: The genome is a 1.51 Mb circular chromosome (35.38% GC). It shows signs of reductive evolution (pseudogenes, fragmented rRNA operon) but retains a complex arsenal of secretion systems.

3. Metabolic Integration and Host-Symbiont Cooperation

The study reveals a unique metabolic interplay where the endosymbiont compensates for the host's plastid loss:

• Haem Provision: The host lacks the haem biosynthesis pathway (essential for respiration). Ca. P. baltica retains a complete haem pathway, suggesting it supplies haem to the host.
• Lysine Precursor Exchange: The host lost the de novo lysine pathway but retains the final two enzymes (converting diaminopimelate to lysine). The endosymbiont produces diaminopimelate (for peptidoglycan) but lacks the decarboxylase to convert it to lysine. This suggests a metabolic cross-feeding where the host scavenges the bacterial precursor to synthesize lysine.
• Unique Metabolic Features: Unlike many Rickettsiales, Ca. P. baltica possesses a Histidine Utilization (Hut) pathway (likely acquired via Horizontal Gene Transfer from Gammaproteobacteria), potentially allowing it to utilize host histidine for energy.

4. Virulence and Host Manipulation Arsenal

Ca. P. baltica encodes a sophisticated toolkit for host interaction, typical of "professional" endosymbionts:

• Secretion Systems: Type IV (T4SS) and Type I (T1SS) secretion systems.
• Effector Proteins: A large repertoire of proteins with Ankyrin (ANK), Leucine-Rich (LRR), and Tetratricopeptide (TPR) repeats, used to manipulate host cell processes.
• Toxins: Includes a tripartite toxin (MakABE) that disrupts actin and the Golgi, and a palatin-like phospholipase to escape autophagy.
• Defense: Encodes Type II and Type IV Toxin-Antitoxin (TA) systems and Gene Transfer Agents (GTAs), likely for phage defense and population stability.

Significance

1. Evolutionary Model for Plastid Loss: Ciliophrys sp. Baltic represents the third confirmed case of complete plastid loss in free-living Ochrophytes (after Picophagus and Actinophrys). It provides a model to study the "genetic footprints" of organelle loss, showing how metabolic pathways are dismantled or retargeted (e.g., to mitochondria) and how specific lipids (SQDG) might persist in modified compartments.
2. Redefining Endosymbiosis: The study expands the known host range of the Rickettsiaceae "Megaira group" to include Dictyochophytes. It challenges the view of these bacteria as purely parasitic by demonstrating a potential mutualistic metabolic dependency (haem and lysine provision) that may have evolved to compensate for the host's organelle loss.
3. New Model System: The establishment of a culturable Ciliophrys strain with a defined endosymbiont creates a powerful single-cell model system for investigating the evolutionary transition from parasitism to mutualism and the metabolic consequences of organelle elimination in eukaryotes.