Sound Science Bite: August 19. The Old Man and the Sea
There is a puzzle about how some microorganisms in the ocean manage to get the nutrients they need. Take the bacterium Prochlorococcus marina for example. This photosynthetic microbe is responsible for producing about 25% of the oxygen released to the atmosphere – not an insignificant contribution to life on the planet. The problem: It needs to absorb 400 million molecules of ammonium ions a day to get the nitrogen necessary to divide and reproduce, but the volume of seawater that holds that many ions is hundreds of millions of times its size. Even if it were to swim at a rapid pace for a microorganism, it would never be able to encounter that many ions.
Enter Einstein. He proved the existence of atoms and molecules to the satisfaction of the majority of scientists by studying Brownian motion. Although he didn't discover this motion (three guesses for the name of the person who did and the last two don't count), he was able to characterize it statistically to show that it could be explained by the existence of molecules. Brownian motion involves how dust particles seen under a microscope can be jiggled and moved around by collisions with molecules. Dust particles are small enough that they normally get more collisions from one direction than from another.
An article in the 2017 August 18 issue of the journal Science by Jonathan Zehr, Joshua Weitz, and Ian Joint points out that this motion solves the problem of how microbes like P. marina can get the nitrogen they need. By being jostled around by molecular collisions, these little guys can encounter four times the amount of ammonium ions necessary for reproduction. Brownian motion is important for other microbial life in the ocean also. As Zeir, Weitz, and Joint write, "[Brownian motion] alone can fuel the growth and reproductivity of abundant, free-living unicellular microorganisms in the open ocean." (Note: Einstein wasn't old but actually quite young when he published his work on Brownian motion.)