Shade and chromatic adaptation of phytoplankton photosynthesis in a thermally stratified sea

Abstract

Spectral absorption of photosynthetically active radiation (PAR) by natural phytoplankton and photosynthetic rates under blue, green, red and white lights with various intensities were determined in the thermally stratified open sea off Shikoku Island, Japan, between 9 June and 8 July 1982. Two characteristic phytoplankton populations, sun- and shade-types, were recognized above and below the 10% light depth, respectively, from photosynthetic responses. The shade-type is characterized by a small photosynthesis maximum at low optimum light intensity, severe photo-inhibition and steep initial slope of photosynthetic rate in comparison with the sun-type. Photosynthetic rates of both types were highest in blue light among three other lights at a given intensity; this “effective use of blue light” is expected to be advantageous mostly for the shade-type because the blue light field predominated at greater depths. Together with these photosynthetic responses and the blue light environment, the subsurface population was expected to maintain effective utilization of light in its habitat. An increase of cell pigment concentration in a low-light environment presumably induced phytoplankton to harvest light energy effectively. Quantum yields determined varied between 0.02 to 0.12 mole C E^-1, although no obvious difference was noticed in different light sources and growth habitats.