A universal driver of macroevolutionary change in the size of marine phytoplankton over the Cenozoic

doi:10.1073/pnas.0709381104

A universal driver of macroevolutionary change in the size of marine phytoplankton over the Cenozoic

*Environmental Science Program and
^¶Mathematics and Computer Science Department, Mount Allison University, Sackville, NB, Canada E4L 1A7;
^‡Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, 71 Dudley Road, New Brunswick, NJ 08901;
^§Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom;
^‖Department of Earth and Planetary Sciences, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854; and
**Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180

Contributed by P. G. Falkowski, October 3, 2007 (received for review September 6, 2007)

Abstract

The size structure of phytoplankton assemblages strongly influences energy transfer through the food web and carbon cycling in the ocean. We determined the macroevolutionary trajectory in the median size of dinoflagellate cysts to compare with the macroevolutionary size change in other plankton groups. We found the median size of the dinoflagellate cysts generally decreases through the Cenozoic. Diatoms exhibit an extremely similar pattern in their median size over time, even though species diversity of the two groups has opposing trends, indicating that the macroevolutionary size change is an active response to selection pressure rather than a passive response to changes in diversity. The changes in the median size of dinoflagellate cysts are highly correlated with both deep ocean temperatures and the thermal gradient between the surface and deep waters, indicating the magnitude and frequency of nutrient availability may have acted as a selective factor in the macroevolution of cell size in the plankton. Our results suggest that climate, because it affects stratification in the ocean, is a universal abiotic driver that has been responsible for macroevolutionary changes in the size structure of marine planktonic communities over the past 65 million years of Earth's history.

Footnotes

^†To whom correspondence may be addressed. E-mail: zfinkel{at}mta.ca or falko{at}marine.rutgers.edu
Author contributions: Z.V.F., O.M.E.S., and P.G.F. designed research; Z.V.F., J.S., S.F.-B., and J.R.Y. performed research; Z.V.F., A.J.I., and M.E.K. analyzed data; and Z.V.F., S.F.-B., and P.G.F. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0709381104/DC1.