Light adaptation of the phytoplankton diatom Phaeodactylum tricornutum under conditions of natural light climate

Cells of Phaeodactylum tricornutum were precultured under axenic conditions 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 xanthophyll cycle pigments. In order to test the resistance of the cells grown at different depths against photostress, the cells were illuminated with photoinhibitory light. The variable chlorophyll a-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 α-tocopherol were monitored during photostress to get molecular information about the physiological reasons of light-stress resistance. The data give evidence that cells grown close to the surface show a faster decline in photosynthetic performance and a more efficient recovery than cells from lower depths. There is clear indication that under natural conditions when the light is fluctuating between optimal, sub- and supraoptimal intensities the photostress resistance is much higher than under conditions of the absence of light stress. The molecular basis for light stress resistance 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 in-situ light adaptation is discussed with respect to the assessment of the potential of the primary production.