Margalef’s mandala, Prochlorococcus, and geoengineering*

Ocean phytoplankton played a central role in oxygena ng our planet’s atmosphere billions of years ago. Hence these early “geoengineers” were crucial for the evolu on of life on Earth. Their modern-day ancestor, the marine cyanobacterium Prochlorococcus, is the most abundant photosynthe c cell on the planet. Its discovery 30 years ago served as a reminder of how li le we understand about the complexi es of marine food webs. Yet proposals to fer lize the oceans, either to mi gate climate or enhance fi sheries, con nue to gain momentum both within the scien fi c community and in the commercial sector. If implemented, the unintended consequences of these and other geoengineering proposals are likely to be enormous, and impossible to an cipate. Correspondence: Sallie W. Chisholm Dept. of Civil and Environ. Engineering Massachuse s Ins tute of Technology 77 Massachuse s Ave. Cambridge, MA 02139, USA