A penalty on photosynthetic growth in fluctuating light.

Fluctuating light is the norm for photosynthetic organisms, with a wide range of frequencies (0.00001 to 10 Hz) owing to diurnal cycles, cloud cover, canopy shifting and mixing; with broad implications for climate change, agriculture and bioproduct production. Photosynthetic growth in fluctuating light is generally considered to improve with increasing fluctuation frequency. Here we demonstrate that the regulation of photosynthesis imposes a penalty on growth in fluctuating light for frequencies in the range of 0.01 to 0.1 Hz (organisms studied: Synechococcus elongatus and Chlamydomonas reinhardtii). We provide a comprehensive sweep of frequencies and duty cycles. In addition, we develop a 2nd order model that identifies the source of the penalty to be the regulation of the Calvin cycle - present at all frequencies but compensated at high frequencies by slow kinetics of RuBisCO.