Sallie Watson Chisholm
|Residence||Massachusetts, United States|
|Alma mater||Skidmore College|
University at Albany, SUNY
|Known for||Study of phytoplankton, especially Prochlorococcus|
|Awards||National Medal of Science|
Alexander Agassiz Medal (2010)
Crafoord Prize (2019)
|Institutions||Massachusetts Institute of Technology|
Sallie Watson (Penny) Chisholm (born in 1947 in Marquette, USA) is a U.S. biological oceanographer at the Massachusetts Institute of Technology. She is an expert in the ecology and evolution of ocean microbes.
Chisholm has been a faculty member at the Massachusetts Institute of Technology since 1976. Her research has focused on the ecology of marine phytoplankton. Chisholm's early work focused on the processes by which such plankton take up nutrients and the manner in which this affects their life cycle on diurnal time scales. This led her to begin using flow cytometry which can be used to measure the properties of individual cells.
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.
In January 2010, she was awarded the Alexander Agassiz Medal, for "pioneering studies of the dominant photosynthetic organisms in the sea and for integrating her results into a new understanding of the global ocean." 
She was a co-recipient in 2012 of the Ruth Patrick Award from the Association for the Sciences of Limnology and Oceanography.