Infrared light can help hungry, water-dwelling protozoa adopt a sleeker, faster-swimming form, allowing them to disperse in search of new food sources. The tactic could be an evolutionary adaptation that helps populations of Tetrahymena thermophila to weather lean times, a team of Japanese and US scientists report.

The researchers spotted the behaviour while investigating the biological effects of infrared (IR) light. 'IR rays from sunlight permeate the Earth's atmosphere, yet little is known about their interactions with living organisms,' said Robert Shiurba of Waseda University. IR radiation has a wavelength between that of light and radio waves and, while its energy is too low to disrupt cell structures and molecules, it can interact with chemical bonds. 'The IR region is among the last to be studied biologically,' said Shiurba.

To investigate how IR affects single cells, Shiurba and his colleagues studied the protozoan T. thermophila. In its freshwater environment it tends to swim upwards, gathering near the water surface in a teeming swarm where there is more oxygen, edible debris - and sunlight.

Shiurba compared two cultures of T. thermophila in near-starvation conditions, one of which was bathed in IR radiation. The irradiated cultures grew normally, but major changes occurred in their cell physiology when the nutrients began to run out. The metabolic activity of the cells increased, and they elongated into spindle shapes.

These cells swam almost half again as fast as those in the control culture that was not exposed to IR light. They also followed much straighter trajectories. When competition for food reaches a maximum, transformation greatly increases a cell's odds of survival, suggested Shiurba. In future work, the group hopes to work out the molecular basis of this phenomenon.

Michael Spencelayh