Oxygenic photosynthesis is the most complex energy transduction process in nature. The light reactions, containing four major protein complexes encoded by more than 140 genes, can operate at close to 70% efficiency under optimal conditions in the laboratory. That is about 70% of absorbed photons can be converted to chemical bond energy. Using a biophysical approach, we developed methods to assess the actual efficiency in nature. We use two instruments, designed in our laboratory: an amplitude based fluorometer and a picosecond lifetime based fluorometer, to quantitatively derive photosynthetic energy conversion efficiency in the world’s oceans in real time. Our analysis reveals that the actual photosynthetic energy conversion efficiency is only about 35%. In this talk I will examine what limits that efficiency at the molecular level, and how eukaryotic phytoplankton have adapted on ecological and evolutionary time.