A common partitioning strategy for photosynthetic products in evolutionarily distinct phytoplankton species

Summary We compare the nutrient‐dependent photosynthetic efficiencies of the chlorophyte, Dunaliella tertiolecta , with those of the marine diatom, Thalassiosira weissflogii . Despite considerable evolutionary and physiological differences, these two species appear to use nearly identical growth strategies under a wide range of nutrient limitation. Using a variety of physiological measurements, we find that, for both species and across all growth rates, 75% of the gross photosynthetic electron flow is invested in carbon fixation and only 30% is retained as net carbon accumulation. A majority of gross photosynthesis (70%) is ultimately used as reductant for biosynthetic pathways and for the generation of ATP . In both species, newly formed carbon products exhibit much shorter half‐lives at slow growth rates than at fast growth rates. We show that this growth rate dependence is a result of increased polysaccharide storage during the S phase of the cell cycle. We present a model of carbon utilization that incorporates this growth rate‐dependent carbon allocation and accurately captures ( r 2  = 0.94) the observed time‐resolved carbon retention. Together, our findings suggest a common photosynthetic optimization strategy in evolutionarily distinct phytoplankton species and contribute towards a systems‐level understanding of carbon flow in photoautotrophs.