LIGHT ADAPTATION OF THE PHYTOPLANKTON DIATOM PHAEODACTYLUM-TRICORNUTUM UNDER CONDITIONS OF NATURAL LIGHT CLIMATE

Cells of Phaeodactylum tricornutum were precultured under axenic condi tions in a full medium and then exposed to natural light conditions at various depths in the eutrophic lake ''Meerfelder Maar'' (Eifel, FRG) for several days. After exposition the cells were characterized with respect to growth parameters, photosynthetic performance and xanthophy ll cycle pigments. In order to test the resistance of the cells grown at different depths against photostress, the cells were illuminated wi th photoinhibitory light. The variable chlorophyll alpha-fluorescence and the oxygen quantum yield at a non-saturating light intensity were simultaneously measured after photostress and subsequent recovery. The xanthophyll cycle pigments and the content in alpha-tocopherol were m onitored during photostress to get molecular information about the phy siological reasons of light-stress resistance. The data give evidence that cells grown close to the surface show a faster decline in photosy nthetic performance and a more efficient recovery than cells from lowe r depths. There is clear indication that under natural conditions when the light is fluctuating between optimal, sub-and supraoptimal intens ities the photostress resistance is much higher than under conditions of the absence of light stress. The molecular basis for light stress r esistance seems to be the pool size and the conversion kinetics of the xanthophyll cycle pigments and the capacity of the oxygen-scavenging system. The effect of ia situ light adaptation is discussed with respe ct to the assessment of the potential of the primary production.