The single-celled S.roeselli's complex behavior was first recorded by zoologist Herbert Jennings in 1906 (Credit:Dexter et al. & Current Biology)

With no brains or nerve cells, single-cellular microorganisms are often regarded to be simple, primitive beings with few capabilities. However, a new Harvard University study suggests that protozoans, like Stentor roeselii, have the smarts to make "complex" decisions when confronted with unpleasant situations.

The trumpet-shaped creature, found worldwide in freshwater ponds, lakes, rivers, and ditches, attaches itself to a firm surface such as algae or submerged waste. It then uses its large mouth-like opening, which contains thousands of tiny hairs called cilia, to suck in food.

The simple creature's decision-making prowess was first observed in 1906 by American zoologist Herbert Jennings. The researcher noted that when exposed to an environmental irritant, in this case carmine powder, the S. roeselii swayed away as though hoping it would pass and things would return to normal. If that failed to work, the microorganism tried several other clever tricks, such as reversing its direction or contracting, before finally detaching from its anchor and floating away in search of a more tranquil location. Unfortunately, Jennings' findings, which could not be replicated by other scientists, were met with skepticism and soon dismissed.

When faced with an irritant, S. roeselii will first bend away and alter the beating of its cilia to expel the particles from its oral cavity (Credit: Dexter et al, 2019)

Now, over a century later, a team led by Jeremy Gunawardena, an associate professor at Harvard Medical School, has finally verified the zoologist's research. However, while Jennings' observations indicated that all S. roeselii protozoa followed the same sequence to avoid the external irritant, the 2019 study found a variation in the order between individuals. While one specimen would start by swaying away before altering the direction of its cilia, another would switch between the two actions. Regardless of the order, they all contracted as the last resort, before finally detaching and fleeing.

“They do the simple things first, but if you keep stimulating, they ‘decide’ to try something else,” says Gunawardena. “S. roeselii has no brain, but there seems to be some mechanism that, in effect, lets it ‘change its mind’ once it feels like the irritation has gone on too long. This hierarchy gives a vivid sense of some form of relatively complex, decision-making calculation going on inside the organism, weighing whether it’s better to execute one behavior versus another.”

If bending and cilia alteration are insufficient, S. roeselii will contract onto its holdfast, or detach and swim away (Credit: Dexter et al, 2019)

The scientists, who published their findings in the journal Current Biology on December 5, 2019, believe the complex decision behavior makes evolutionary sense. "Organisms like S. roeselii were apex predators prior to multicellular life, and they are extremely widespread in many different aquatic environments," Gunawardena said, "They have to be 'clever' at figuring out what to avoid, where to eat and all the other things that organisms have to do to live. I think it's clear that they can have complex ways of doing so."

Besides vindicating Jennings, the Harvard University research may have broader implications for biology. For example, cancer researchers currently assume that diseased cells are "programmed" to do what they do. However, if single-celled organisms have a "mind" of their own, it may change the way we think about how cancer cells multiply and how the immune system works. Gunawardena said. "I think this experiment forces us to think about the existence of, very speculatively, some form of cellular 'cognition,' in which single cells can be capable of complex information processing and decision-making in response."