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–µm size class represented by coccoid Chlorella‐like cells, with lower growth rates in both higher and lower size classes. This 2–3–µm 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.