MODELING OF LIGHT-DEPENDENT ALGAL PHOTOSYNTHESIS AND GROWTH - EXPERIMENTS WITH THE BARENTS SEA DIATOMS THALASSIOSIRA-NORDENSKIOELDII AND CHAETOCEROS-FURCELLATUS

The models by SAKSHAUG et al. (1989, Limnology and Oceanography, 34, 198-205) and WEBB et al. (1974, Oecologia, 17, 281-291), for prediction of the gross growth rate of phytoplankton and short-term photosynthesis, respectively, have been modified on the basis of experiments with cultures of the centric diatoms Thalassiosira nordenskioeldii and Chaetoceros furcellatus grown at 0.5-degrees-C at combinations of two irradiances (25 and 400-mu-mol m-2 s-1) and two day-lengths (12 and 24 h). The models have one spectrum, degrees-sigma, which represents chlorophyll a (Chla) specific absorption of photosynthetically usable light, and introduces a factor q which represents Chla per PSU, functionally defined. The models describe phytoplankton growth in terms of physiologically relevant coefficients. A properly scaled fluorescence excitation spectrum (degrees-F) represents a more appropriate estimate for degrees-sigma than the Chla-specific absorption spectrum degrees-a(c) judging from calculations of PHI-max (= alpha-B/degrees-sigma). On the basis of degrees-F, PHI-max is 0.04 g-at C(mol photons)-1 for gross growth and about 0.05-0.08 for short-term carbon uptake (unfiltered samples). Calculations based on degrees-a(c) yield values for PHI-max which on average are 44% lower. P vs I (photosynthesis vs irradiance) parameters are relatively independent of day-length and highly dependent on growth irradiance. The product of q [mg Chla (mol PSU)-1] and tau (the minimum turnover time of the photosynthetic unit, h) increases 2-3-fold from high to low irradiance, thus P(m)B (= PHI-max/q-tau) and I(k) (= 1/q-tau-degrees-sigma) decreased. degrees-F decreases from high to low irradiance. Carbon-specific dark respiration rates are < 0.09 day-1. Pigment ratios vary inversely with irradiance and day-length. The Chla:C ratio is particularly low under high, strong continuous light; Chlc:Chla ratios are higher for shade- than for light-adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chla. The fucoxanthin:Chla ratio is virtually independent of the light regime. The two species are similar with respect to variations in growth rate (0.09-0.33 day-1) and I(k) (31-36 vs 49-100-mu-mol m-2 s-1 at low and high irradiance respectively). P(m)B and alpha-B for growth as well as degrees-F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product q-tau is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition.