Presenter: Christopher Follett (Massachusetts Institute of Technology)

Description:
  The world’s smallest and most abundant phytoplankton, Prochlorococcus, is ubiquitous in the warm oligotrophic gyres, but populations collapse rapidly at high latitudes. What causes this collapse? Current ideas have focused on temperature, noting that researchers have failed to grow this species below approximately 10 C. North Pacific transect data, however, suggests that the collapse occurs across a range of temperatures and an additional mechanism may be relevant. Recent work explores whether an ecological interaction between heterotrophic bacteria and Prochlorococcus could lead to the collapse. As nutrient supplies and primary production increase moving northward, heterotrophic bacteria are alleviated of carbon limitation and increase in abundance and size. This growth feeds a shared predator (classical Apparent Competition) which eventually consumes Prochlorococcus faster than its growth rate. We go on to use global datasets of Prochlorococcus and their sharp transitions to question statistical models for this species which predict an increasing range and abundance as the oceans increase in temperature. A thermal growth threshold is supported by data, suggesting that the viable habitat for this species will increase in a warming world. However, temperature appears to be a weak driver of populations where they do exist, and is a poor predictor of sharp transitions. This work helps reconcile the opposing predictions of statistical vs. dynamical models for this species under climate change.