PHOTOSYNTHETIC PARAMETERS, PIGMENT COMPOSITION AND RESPIRATION RATES OF THE MARINE DIATOM SKELETONEMA-COSTATUM GROWN IN CONTINUOUS LIGHT AND A 12/12 H LIGHT-DARK CYCLE

Nutrient-sufficient cultures of a Trondheimsfjord (Norway) clone of the marine centric diatom Skeletonema costatum (Grev.) Cleve were grown at 75 mumol m-2 s-1 and 15-degrees-C at 24 and 12 h daylength to study diurnal variations and the effect of daylength on pigment and chemical composition, photosynthetic parameters, dark respiration rates and scaled fluorescence excitation spectra (degrees-FBAR), the latter used as estimates for the absorption of energy available to Photosystem II. Specific growth rates were 1.06 and 0.56 day-1 in 24 and 12 h daylength, respectively, while dark respiration rates were generally 85% of the net growth rate. The Chla-normalized photosynthetic coefficients P(m)B and alpha(B) were almost-equal-to 20-25% higher in continuous light than at 12 h daylength, while the Chla:C ratio was approximately 15% lower (0.051 versus 0.061 w:w). Thus, the carbon-normalized coefficients P(m)C and alpha(C) were < 11 % lower at 24 h than at 12 h daylength. The maximum quantum yield phi(max), the Chla:C ratio and -degrees-FBAR differed negligibly, as did the light saturation index I(k), the N:C ratio and the ratios Chlc:Chla and Fucoxanthin:Chla. P(m)B and I(k) did not exhibit diurnal variations at 24 h daylength, and varied within 23% of the daily mean at 12 h daylength. Predictions of the daily gross photosynthetic rate based on data for a given time of the day should thus not be >10% in error relative to an integrated value based on several data sets collected through 24 h. phi(max) was 0.084-0.117 mol O2 (mol photons)-1 for gross oxygen evolution. However, if used in mathematical models for predicting the gross and net growth rates (i.e. the gross and net carbon turnover rates), 'practical' phi(max) values of 0.076 and 0.040 g-at C (mol photons)-1, respectively, should be employed. Correspondingly, values for alpha(B) and P(m)B should be adjusted pro rata.