sentences:  The application of flow cytometry to environmental samples led Chisholm and her collaborators (most notably R.J. Olson and H.M. Sosik) to the discovery that small plankton (in particular "Prochlorococcus" and "Synechococcus") accounted for a much more substantial part of marine productivity than had previously been realized.Previously, biological oceanographers had focused on silicaceous diatoms as being the most important phytoplankton, accounting for 10-20 gigatons of carbon uptake each year.Chisholm's work showed that an even larger amount of carbon was cycled through these small algae, which may also play an important role in the global nitrogen cycle.