Turbulent motions may control phytoplankton photosynthesis in the upper ocean

In their natural environment, algal cells may experience large amplitude variations in incident light on time scales shorter than those associated with the reproductive rate. A major source of variability is due to vertical mixing of water masses: turbulence in the upper mixing layer of the ocean transports algae through a light field that decreases exponentially from the sea surface. Several attempts have been made to discover how the physiological adaptation of algae to fluctuating light 1–9 might affect the magnitude of marine primary production 10–16,17 , but it has proved difficult to estimate the vertical mixing rate at sea, and hence the time scales of irradiance variability 18,19 . We recently succeeded, for the first time to our knowledge, in making direct measurement of rates of turbulent kinetic energy (TKE) dissipation, simultaneously with those of photoadaptation properties of natural phytoplankton populations. Dissipation of TKE in these experiments was a strong determinant of algal photosynthetic rate in the upper mixing layer; we conclude that the vertical mixing induced by turbulence to a large degree controls the photosynthetic performance of algae in nature.