Euglena are single-celled chimeras that defy the simple divide between plants and animals.

Nature · Published 3 October 2026 Swipe more like this →

In short: These microscopic organisms possess both photosynthetic chloroplasts and animal-like feeding behaviors. Because they blur the lines of traditional biological classification, they forced scientists to rethink the structure of life itself. Today, they remain a vital model for understanding how complex cellular traits evolve and shift over time.

Euglena is a genus comprising over 1,000 species of single-celled, flagellated microorganisms. They are defined by their dual nature: most species contain chloroplasts, allowing them to photosynthesize like plants, yet they can also feed heterotrophically like animals. This hybrid identity historically baffled taxonomists who operated under the rigid Linnaean two-kingdom system, eventually prompting the creation of the Kingdom Protista to accommodate such unclassifiable creatures.

The study of these organisms dates back to the 17th century, when Antonie van Leeuwenhoek first documented 'animalcules' under the microscope. Early observers were fascinated by their ability to contract, dilate, and move with agility. By the 19th century, researchers like C.G. Ehrenberg identified their distinctive eyespot, naming the genus from Greek roots meaning 'well' and 'eyeball.' However, the classification of Euglena remained a moving target, with 20th-century scientists debating whether to group them by color or by structural features like flagella.

Modern molecular evidence has revealed a more complex history. It is now understood that photosynthetic euglenoids likely acquired their chloroplasts through secondary endosymbiosis, essentially engulfing an algal cell to co-opt its machinery. Genetic analysis of colorless species, such as those formerly placed in the genus Astasia, shows they retain DNA sequences inherited from photosynthetic ancestors, proving that some lineages lost their ability to photosynthesize over time. This discovery highlights that the genus is not a simple, uniform group but a dynamic lineage shaped by evolutionary loss and gain.

Beyond their evolutionary intrigue, these microbes are ecologically significant. They thrive in quiet inland waters, where they can bloom in such high densities that they visibly color the surface of ponds and ditches. Species like Euglena sanguinea produce a red pigment called astaxanthin to protect their chloroplasts from intense light, a defense mechanism that allows them to shift color as environmental conditions fluctuate. Their resilience and adaptability continue to make them a primary subject for biological research.

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