Flow

Prochlorococcus community circled in red.

Flow cytometry is a laser based biophysical technology used for cell counting and sorting.  It is used widely in the medical field to analyze blood samples but over the past few decades it has become an important instrument used in marine biology to determine microbial community structure and abundances.  Flow cytometry suspends cells in a stream of fluid that then passes by an electronic detector.  The analysis can characterize the physical and chemical attributes of up to thousands of particles per second.  At Station Aloha and other low nutrient regions of the ocean, a genus of photosynthetic microorganisms called prochlorococcus are abundant and can be identified and quantified using flow cytometry (see picture).  Prochlorococcus are incredibly small (0.6um) cyanobacteria that are the most abundant photosynthetic organisms on earth.  One milliliter of seawater may contain up to 100,000 cells of these organisms.  Due to their small size Prochlorococcus have a high surface to volume ratio which gives them a competitive advantage in low nutrient waters.  They can also grow at very low light conditions.  Despite their ecological importance they were not discovered until 1986 by Sallie W. (Penny) Chisholm (Massachusetts Institute of Technology) and other collaborators.  The discovery was made using flow cytometry methods in the Sargasso Sea. 

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Flow cytometer onboard R/V Kilo Moana