Endosymbiotic Interactions in Aquatic Eukaryotes

Endosymbiosis in aquatic eukaryotes represents a fundamental evolutionary process by which free-living microbes became integrated into host cells, giving rise to organelles such as chloroplasts and mitochondria. In modern ecosystems, a wide variety of protists, invertebrates and even vertebrates maintain intimate associations with photosynthetic or chemoautotrophic symbionts. These partnerships range from facultative interactions, in which partners can survive independently, to obligate associations that are essential for host development and survival. Research on model systems such as Paramecium bursaria, Tetrahymena–Synechocystis synthetic assemblages and the Hydra–Chlorella partnership has revealed diverse strategies of host control, nutrient exchange and structural accommodation. Endosymbiosis underpins primary productivity in plankton, resilience of reef ecosystems and biotechnological innovations in bioenergy and pollution remediation. By elucidating the cellular, molecular and evolutionary dynamics of these interactions, scientists are uncovering the mechanisms that stabilise partnerships, regulate symbiont loads and drive long-term co-evolution.

Research from Nature Portfolio

Researchers have developed a defined synthetic model system that co-cultures a cyanobacterial symbiont with a ciliate host under controlled conditions. This assembly remains stable for over 100 generations, and selected lineages demonstrate progressively enhanced coexistence without extinction, offering a real-time window into the earliest stages of primary endosymbiosis. In parallel, advanced cryofixation and three-dimensional electron microscopy of a ciliate–algal system have uncovered direct contacts between host mitochondria, the perialgal vacuole membrane and the algal cell wall. These observations reveal active remodelling of host organelles to facilitate metabolic exchange and establish intimate structural interfaces that sustain long-term symbiosis.

Endosymbiotic Interactions in Aquatic Eukaryotes publication trend

The graph below shows the total number of articles in endosymbiotic interactions in aquatic eukaryotes across all publications each year (not limited to Nature Index journals).

Technical terms

Endosymbiosis: A close association in which one organism lives within the cells of another.

Photosymbiosis: A symbiotic partnership involving a photosynthetic organism supplying fixed carbon to its host.

Primary endosymbiosis: The initial uptake of a prokaryotic phototroph by a eukaryotic host, leading to plastid formation.

Secondary endosymbiosis: The acquisition of a photosynthetic eukaryote by another eukaryotic host, resulting in complex plastids.

Perialgal vacuole membrane (PVM): The host-derived membrane compartment enclosing endosymbiotic algae.

Host control: Mechanisms by which the host regulates symbiont proliferation, distribution and metabolic exchange.

References

1. Synthetic symbiosis between a cyanobacterium and a ciliate toward novel chloroplast-like endosymbiosis. Scientific Reports (2023).
2. Intracellular symbiosis of algae with possible involvement of mitochondrial dynamics. Scientific Reports (2017).
3. Tracking the early events of photosymbiosis evolution. Trends in Plant Science (2023).
4. Shining a Light on Exploitative Host Control in a Photosynthetic Endosymbiosis. Current Biology (2015).
5. Metabolic co-dependence drives the evolutionarily ancient Hydra–Chlorella symbiosis. eLife (2018).