On the verge of a chimera

Many would like to move and operate more efficiently. Hydras go far with this, by depositing tiny algae thalli (among others Chlorella) to cover almost all available space within its own cells. They have been doing it for tens of millions of years. The strategy was original to hydras, albeit the majority of hydra species are not green – they have lost it secondarily. Nevertheless, brown hydras have retained some of the ability to interact with algae again. 

A key aspect of the symbiosis for the host relies on a supply of sugar assimilates, whereas the alga relies on a sourcing of nitrogen compounds. Chlorella strains involved in a symbiotic regime are not capable of obtaining nitrogen in non-host environments, making them existentially dependent on hydra cells. For hydras, cooperation provides a significant competitive advantage over aposymbiotic individuals in times of starvation, allowing them to grow their populations more rapidly.

It wouldn't be all rosy if partners only benefit from such a relationship. Like any other green autotroph, chlorella chloroplasts will run like crazy when exposed to too much light. Once they can't absorb any more photons, their entire photosynthetic machinery collapses and many cellular components are at risk of damage. In this case, both the algae cells and the hydra are harmed. 

When a symbiont becomes (fully) integrated and lives in tighter coexistence with its host, it is likely to tend to simplify itself evolutionarily. Chlorella lost genes linked to the uptake of inorganic nitrogen and the synthesis of glutamine, the main mobile nitrogen mediator, as a result of adapting to a steady influx of nutrients from its host.

Hamada, M., Schröder, K., Bathia, J., Kürn, U., Fraune, S., Khalturina, M., Khalturin, K., Shinzato, Ch., Satoh, N., Bosch, T. (2018): Metabolic co-dependence drives the evolutionarily ancient Hydra–Chlorella symbiosis. eLife 7: 1–37. Dostupné z: https://doi.org/10.7554/eLife.35122.001

Miyokawa, R., Kanaya, H., Itoh, T., Kobayakawa, Y., Kusumi, J. (2021): Immature symbiotic system between horizontally transmitted green algae and brown hydra. Scientific Reports: 1–11. Dostupné z: doi: 11. 10.1038/s41598-021-82489-6