PHYSIOLOGICAL REACTIONS TO A CHANGE IN LIGHT REGIME IN CULTURED SKELETONEMA-COSTATUM (BACILLARIOPHYTA) - IMPLICATIONS FOR ESTIMATION OF PHYTOPLANKTON BIOMASS

The marine planktonic diatom Skeletonema costatum (Cleve) was grown in batch culture under a 12 h light:12 h dark (LD) regime for several ge nerations before the experiment. At Time 0, half of the culture was tr ansferred to continuous light (CL). Particulate organic carbon (POC), nitrogen (PON), and photosynthetic pigments (measured by spectrophotom etry and high performance liquid chromatography) were monitored for 7 to 9 days in the two cultures. Under CL, POC and PON production were a lways lower. In addition, we measured a much higher proportion of chlo rophyll degradation products (as chlorophyllid a, phaeophytin a and ph aeophorbid a (PrD)). These indicate different physiological conditions between the two cultures, as reflected also be the lower POC/PON and chlorophyll a/POC ratios. Under CL illumination cells appear stressed, probably due to the total quantity of light to which cells are expose d. In fact, a higher proportion of the two xanthophylls diadinoxanthin and diatoxanthin in the CL culture indicates a photoprotective reacti on of the cells. In contrast with the LD culture, parameters measured were not significantly inter-correlated in the CL culture. The lack of correlation between chlorophyll a and POC in CL does not encourage th e use of the chlorophyll a/POC ratio as a biomass estimator. On the op posite, the ratio of total pigment content to POC did not vary much as a function of the light condition of the culture, remaining near 0.04 0 in both light regimes. Although further observations are needed, thi s index seems to be a reliable indicator of phytoplankton carbon bioma ss.