Oct. 26, 2005— In an act straight out of "Invasion of the Bodysnatchers," a marine microbe has been caught red-handed merging with green algae on a Japanese beach.

By engulfing a single cell of algae, the single-celled Hatena microbe is able to remake itself from a sleek, colorless predator into a fat, verdant sunbather.

The strange switcheroo resembles a pivotal evolutionary step in which early, single-celled organisms took in and eventually formed permanent, long-term relationships with what are today the green, solar-energy-capturing chloroplasts in modern plant cells.

“Hatena was found in a sample from a sandy beach in Wakayama Prefecture,” said co-discoverer Isao Inouye, a biologist at the University of Tsukuba in Japan.

It was a routine sampling, he said. But when they studied the organism under an electron microscope, they discovered that the green body inside the microbe had all the parts of an independent living algae cell.

The discovery of Hatena, which means "enigmatic" in Japanese, was described in a recent issue of the journal Science.

The presence of the algae inside Hatena meant one of two things: either Hatena had just eaten the algae — which would be followed by digestion — or that there was some sort of symbiotic relationship.

Since the algae was not digested, and even stayed with one of the daughter cells when Hatena reproduced (by splitting in two), it was clear that there was some sort of cooperation going on.

There are other cases, Inouye said, where an algae is stolen and used for a lifestyle change (from predator to plant lifestyle). But those cases show no evidence the algae is cooperating.

In Hatena, on the other hand, the algae involved was a very specific kind and once inside the microbe, it appeared to collaborate by always lining up its pigmented "eyespot" with the top of the Hatena cell.

This is probably the most advantageous alignment for both organisms.

"This indicates host and symbiont (the algae) have already established certain relationships," Inouye said. "We do not know how many (life) cycles Hatena retains the symbiont," but the consistent alignment of algae cells within Hatena suggests that it's more than another case of microbial theft.

There is a longer-term relationship developing here, Inouye said.

It would be even more compelling if the algae also divided while inside Hatena so that one of the two daughter cells did not have to go and find another algae cell to engulf, explained Ralph Lewin of the Scripps Institution of Oceanography at the University of California at San Diego.

"The fact that the algae doesn't divide is significant," said Lewin. It might mean the symbiotic relationship is temporary, or that the algae just takes longer to divide.

Either case would be easy to determine, if Hatena was more amenable to being cultivated in a laboratory, which it unfortunately is not, said Inouye.