Photoacclimation in the marine diatom Skeletonema costatum

Photoacclimation was examined in the marine diatom Skeletonema costatum, wh ich was subjected to reciprocal shifts between irradiances of 50 (low-light ) and 1,200 (high-light) mu mol photons m(-2) s(-1). Cell chlorophyll a and fucoxanthin contents were higher but diadinoxanthin and diatoxanthin conte nts lower in cells grown at 50 mu mol photons m(-2) s(-1) than in cells shi fted to 1200 mu mol photons m(-2) s(-1). Cell carbon contents measured at t he 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 t he high-light cells were about twofold higher than in the low-light cells. Dark respiration rates, dark Chi 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 ce ll division during the day in the high-light cells, but pigment synthesis a nd cell division continued in darkness. Cell-specific, light saturated phot osynthesis rates, and chlorophyll a specific light-limited photosynthesis r ates were constant during reciprocal shifts between growth irradiances of 5 0 and 1200 mu mol photons m(-2)s(-1). Thus, differences of photosynthesis v ersus irradiance curves between cells acclimated to high-light versus low-l ight could be accounted for largely in terms of changes in cell chlorophyll a contents. Although the chlorophyll a-specific initial slope, alpha (chl) , was constant, the chlorophyll a-specific light absorbtion coeffecient, a( chl), increased and the maximum quantum efficiency of photosynthesis (phi ( 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 phi (m) was m ost likely due to accumulation of xanthophyll cycle pigments. Carbon-specif ic, light-saturated photosynthesis rates were lower in high-light than in l ow-light cells; this observation may indicate that control of Light-saturat ed 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.