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 generations before the experiment. At Time 0, half of the culture was transferred to continuous light (CL). Particulate organic carbon (POC), nitrogen (PON), and photosynthetic pigments (measured by spectrophotometry and high performance liquid chromatography) were monitored for 7 to 9 days in the two cultures. Under CL, POC and PON production were always lower. In addition, we measured a much higher proportion of chlorophyll degradation products (as chlorophyllid a, phaeophytin a and phaeophorbid 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 exposed. In fact, a higher proportion of the two xanthophylls diadinoxanthin and diatoxanthin in the CL culture indicates a photoprotective reaction 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 the use of the chlorophyll a/POC ratio as a biomass estimator. On the opposite, the ratio of total pigment content to POC did not vary much as a function of the light condition of the culture, remaining near 0.040 in both light regimes. Although further observations are needed, this index seems to be a reliable indicator of phytoplankton carbon biomass.