Amoeba Loses Its Sixth Sense and Shrinks When Stripped of Algal Partners

A rare freshwater amoeba called Mayorella viridis completely loses its ability to navigate toward light when stripped of the microscopic green algae living inside its cells. Scientists investigated how this partnership works by creating an experimental model where the amoeba can be switched between symbiotic green states and bleached white states at will.

The study, published in The Journal of eukaryotic microbiology, provides fresh insight into photosymbiosis, a biological strategy where non-photosynthetic organisms host photosynthetic algae. While photosymbiosis is well studied in corals, jellyfish, and paramecia, exploring it in amoebae helps researchers understand how different species form mutually beneficial partnerships and manage nutrient exchanges.

How Researchers Bleached and Re-established Symbiosis in the Amoeba

To examine the exact role of the algal partner, the research team led by scientists in Japan targeted Mayorella viridis, a freshwater species belonging to the phylum Amoebozoa. Within this major lineage, only two species are known to harbor endosymbiotic green algae of the genus Chlorella. The authors combined treatments using the herbicide norflurazon with prolonged cultivation under constant dark conditions for 7 days to eliminate the algae without killing the host.

This process successfully generated an aposymbiotic, or white, strain of the amoeba that lacked detectable Chlorella and chlorophyll autofluorescence. Crucially, the researchers were also able to reconstruct the photosymbiotic system by mixing isolated Chlorella cells with the bleached amoebae. Within 6 days, the algae multiplied inside the host cytoplasm, successfully restoring the green symbiotic state and proving that the partnership can be experimentally reversed.

Why Algae Give Amoebae an Edge During Starvation

Comparative physiological analyses between the green and white strains revealed striking differences in how the organisms handle stress and energy. Under well-fed conditions, both strains measured roughly 4,200 to 4,300 square micrometers in cell size and traveled similar distances of about 1.8 to 1.9 millimeters over 10 minutes. However, under starvation conditions, the bleached white amoebae shrank significantly to an average size of 2,800 square micrometers, whereas the green strains maintained their structural stability at roughly 4,600 square micrometers.

Survival tests under constant light and nutrient-poor conditions showed that approximately 70 percent of the symbiotic green amoebae survived for 28 days, while the white strains experienced a steep population decline, nearly dying out within the same timeframe. The authors attribute this enhanced resilience to photosynthetic products transferred from the Chlorella. Furthermore, under dark and starved conditions where photosynthesis was impossible, the number of internal Chlorella cells dropped significantly over a month, suggesting that the amoebae may digest their algal partners as an emergency nutritional reserve.

How Symbiosis Controls Navigation and Light Responses

The most dramatic behavioral divergence between the two strains involved their response to light. When exposed to moderate light levels of 11.5 micro mols of photons per square meter per second from the side of a dish, the green amoebae migrated away to avoid intense exposure. Under weak light of 0.28 micro mols, they actively gathered toward the source in a display of positive phototaxis.

In stark contrast, the bleached white strain showed zero directed movement, distributing randomly regardless of light intensity. This complete loss of photoresponse indicates that the amoebas navigation relies entirely on the physiological activity of the Chlorella. The researchers suggest that avoiding moderate light helps the organisms prevent cellular damage from excess reactive oxygen species generated by active photosynthesis, balancing the energetic benefits of the partnership with the risks of oxidative stress.

The research was published in The Journal of eukaryotic microbiology on November 1, 2026.