Growth rate peaks at intermediate cell size in marine photosynthetic picoeukaryotes

We have performed an in situ test of Raven's prediction that there is a reversal of the relationship between cell size and maximum achievable growth rate in unicellular algae at the low end of size classes. In a natural population of marine phytoplankton, including the smallest picoeukaryote known to date, and under both nutrient sufficiency and deficiency, we find a maximum in growth rate (4.8 and 3.3 divisions d(-1), respectively) in the 2-3-mu m size class represented by coccoid Chlorella-like cells, with lower growth rates in both higher and lower size classes. This 2-3-mu m size class is also the most robust under nutrient deficiency, reducing its growth rate by 14% only relative to nutrient-sufficient conditions, versus 50-60% for the lowermost and uppermost size classes, respectively.