Visualization of the "Brazil nut effect," an interesting phenomenon of microbial origin

The research group comprised of Dr. Atul Srivastava, Graduate School of Biomedical Engineering, Tohoku University (at the time of research); Professor Takuji Ishikawa and Associate Professor Kenji Kikuchi, School of Engineering, Tohoku University; and others discovered that objects, such as plastic debris, that have settled in fine particles float in water against the force of gravity owing to the foaming of carbon dioxide during yeast fermentation. The research group is investigating how plastic waste, which is an environmental problem, affects the natural ecosystem by mixing artificial substances (such as silicon rubber) that are difficult to biodegrade in the fermentation vessel. The artificial matter mixed in the fermentation vessel rises from the bottom to the water surface, owing to the adhesion of fermentation bubbles, then descends repeatedly, causing agitating in the inside of the vessel. As a result, nutrients are distributed throughout, and yeast growth is promoted.

At first glance, this was a phenomenon that seemed to be an obstacle to the yeast; however, it has become clear that it plays a role in promoting yeast growth. In addition, the research group discovered the "microbial Brazil nut effect ", in which foaming owing to the fermentation of microorganisms moves an object that is one billion times larger than the microorganism itself under the condition that the artificial object is buried in the surrounding fine particles. In addition, the group was able to clarify the non-equilibrium dissipative physical phenomenon mediated by living beings in a wet environment' based on imaging methods using X-rays and theoretical analysis based on stochastic differential equations.

According to Associate Professor Kikuchi, "It is a remarkable phenomenon that can be called the microbial brazil nut effect. We have succeeded in visualizing and evaluating the physical phenomenon in which soil is fluidized in an environment where soil is deposited, such as the bottom of a lake, riverbed, or seabed, and an object buried by the infiltration of particles rises to the surface of the water. With this result, we would like to contribute to the revival of microorganisms and viruses dormant in the soil and elucidate the mechanism by which unknown pathogens appear."

■ Brazil nut effect: A physical phenomenon in which large particles float and small particles settle when vibration is applied to a mixture of powders and granules of different sizes. The name is derived from the phenomenon in which big Brazil nuts settle above small peanuts among mixed nuts.