Modeling of light-dependent algal photosynthesis and growth: experiments with the Barents sea diatoms Thalassiosira nordenskioldii and Chaetoceros furcellatus

Abstract

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°C at combinations of two irradiances (25 and 400μmol m −2s −1) and two day-lengths (12 and 24 h). The models have one spectrum, °σ, which represents chlorophyll a (Chl a) 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 (° F) represents a more appropriate estimate for °σ than the Chl a-specific absorption spectrum ° a c judging from calculations of Φ max (= α B /° σ ). On the basis of ° F, Φ 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 ° a c yield values for Φ 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 Chl a (mol PSU) −1] and τ (the minimum turnover time of the photosynthetic unit, h) increases 2–3-fold from high to low irradiance, thus P m B (= Φ max / qτ) and I k(=1/ qτ° σ)decreased. ° 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 Chl a:C ratio is particularly low under high, strong continuous light; Chl c: Cha a ratios are higher for shalde- than for light-adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chl a. The fucoxanthin: Chl a 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 μmol m −2 s −1 at low and high irradiance, respectively). P m B and a B for growth as well as ° F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product qτ is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition.