Photoacclimation in the marine diatom Skeletonema costatum

Photoacclimation was examined in the marine diatom Skeletonema costatum, which was subjected to reciprocal shifts between irradiances of 50 (low-light) and 1,200 (high-light) μmol photons m^-2 s^-1. Cell chlorophyll a and fucoxanthin contents were higher but diadinoxanthin and diatoxanthin contents lower in cells grown at 50 μmol photons m^-2 s^-1 than in cells shifted to 1200 μmol photons m^-2 s^-1. Cell carbon contents measured at the start of the light period were similar in both high-light and low-light treatments. However, by 6 h into the light period, the carbon contents in the high-light cells were about twofold higher than in the low-light cells. Dark respiration rates, dark Chl a synthesis rates, and dark cell-division rates were greater in the high-light acclimated cells than in the low-light cells. Thus, there was a greater uncoupling of carbon assimilation from cell division during the day in the high-light cells, but pigment synthesis and cell division continued in darkness. Cell-specific, light saturated photosynthesis rates, and chlorophyll a specific light-limited photosynthesis rates were constant during reciprocal shifts between growth irradiances of 50 and 1200 μmol photons m^-2s^-1. Thus, differences of photosynthesis versus irradiance curves between cells acclimated to high-light versus low-light could be accounted for largely in terms of changes in cell chlorophyll a contents. Although the chlorophyll a-specific initial slope, α^chl, was constant, the chlorophyll a-specific light absorbtion coeffecient, a^chl, increased and the maximum quantum efficiency of photosynthesis (φ_m) declined following the shift to high light. The increase of a^chl was most likely due to a decreased package effect. The decline of φ_m was most likely due to accumulation of xanthophyll cycle pigments. Carbon-specific, light-saturated photosynthesis rates were lower in high-light than in low-light cells; this observation may indicate that control of light-saturated photosynthesis shifts from enzymes of the carbon dioxide reduction cycle (Calvin cycle) in low-light cells to the photosynthetic electron transfer chain in high-light cells.