A tiny ocean organism's relationship with an alga is shedding light on an unexplained process that occurred more than a billion years ago and drove the evolution of plants and algae.

The colorless organism, named Hatena (meaning "enigma" in Japanese) by the Japanese scientists that described it the October 14 issue of Science, alternates between two phases: one allows it to host, and another allows it to devour a green alga. The stages demonstrate remarkably quick changes in both the host organism and the symbiont cell that could have played a key role in the evolution of photosynthesis, the process by which modern plants and algae make their own food by converting light energy into chemical energy.

Surprisingly, photosynthesis did not originate in plants and algae, but arose first in bacteria. Algae figured out a way to engulf bacteria, which eventually evolved into a chloroplast, a specialized cellular component responsible, in this case, for photosynthesis. How it happened, though, has been a mystery. Now biologists Noriko Okamoto and Isao Inouye of the University of Tsukuba in Japan have observed a similar process taking place in the wild algae Hatena.

Hatena's life cycle alternates between a host phase and a predator phase. As a host, the otherwise colorless Hatena harbors a green alga cell known as Nephroselmis, which makes Hatena appear green. The green cell splits in two, always producing one colorless daughter cell and one green daughter cell. The colorless cell develops a feeding apparatus that it uses to engulf a new Nephroselmis. Once devoured, Nephroselmis becomes a functioning part of the host and the host's feeding apparatus--now no longer needed--degenerates.

The green daughter goes off on its own, presumably dividing into more green daughters, however the scientists were unable to culture the wild Hatena and thoroughly follow its life cycle. But this stage could hint at what needs to happen to make the chloroplast a permanent part of the cell. "Whatever you need to make that a permanent part is not occurring here. Maybe in a hundred millions years it will figure it out," says biologist Debashish Bhattacharya, director of the Interdisciplinary Program in Genetics at the University of Iowa.

In other experiments, Okamoto and Inouye tried to feed Hatena other strains of Nephroselmis to see if the organism would behave the same way. But although it engulfed the alga, it did not undergo the modifications, suggesting a specialized relationship between the two. Determining whether that relationship is genetic is the next puzzle the scientists will have to solve.