A cell of two halves
On Isonoura Beach, in the sands of Wakayama Prefecture, lives a microscopic flagellate with a double life. First observed in 2000 by biologists Noriko Okamoto and Isao Inouye, Hatena arenicola is a single-celled predator that can transform into a photosynthesizing, plant-like organism. This transformation is not its own doing; it occurs when the protist engulfs a specific type of green alga from the genus Nephroselmis.
Once the alga is captured, a strange remodeling begins. The predator, a colorless cell about 30-40 micrometers long, develops a complex feeding tube to hunt. After consuming a Nephroselmis cell, the protist loses this feeding apparatus. The captured alga also changes. It loses its own flagella and cytoskeleton, while its plastid—the photosynthetic part—swells to ten times its normal size, eventually occupying most of the host cell's interior. The alga's other internal components, like its mitochondria and Golgi body, are degraded. The host, now green and photosynthetic, uses the alga's red eyespot to guide its movement toward light, a behavior called phototaxis.
The division of labor
The relationship between Hatena and Nephroselmis is a work-in-progress in evolutionary terms, a process known as secondary endosymbiosis. The alga is not a fully integrated organelle like a chloroplast. This is most evident during cell division. When the green, photosynthesizing Hatena cell divides, the entire algal symbiont is passed to only one of the two daughter cells.
The result is two distinct offspring. One daughter cell is green, retaining the algal partner and the photosynthetic lifestyle. The other daughter cell is born colorless and must fend for itself. It develops a feeding apparatus de novo at the exact spot where its sibling has an eyespot and takes on the role of a predator. This colorless cell must then hunt and successfully capture a new Nephroselmis alga from the environment to begin the cycle again. This "half-plant, half-predator" life cycle is a example of how complex life can evolve by incorporating other organisms.