prochlorococcus. humbling to realize that we didn't know this cell existed until 15 years ago, and we had models of the ocean processes and models of the earth. we thought we understood this pretty well. we always think we understand it pretty well, and then along comes something that just completely changes the way we think about these systems. narrator: prochlorococcus was finally discovered when an instrument used for biomedical research, a flow cytometer, was brought onboard an oceanographic-research vessel to test its usefulness in the study of phytoplankton. the flow cytometer breaks an extremely narrow stream of water into individual drops -- each one only slightly larger than the cells under investigation. the drops are illuminated by a laser, and if a desired cell is present in one of the drops, it will give off a particular wavelength of light. a tiny force can then be applied to sort that droplet from the others. chisholm: we started working with this instrument, and we realized that it wou