THE EFFECT OF NITROGEN LIMITATION ON THE ABSORPTION AND SCATTERING PROPERTIES OF THE MARINE DIATOM THALASSIOSIRA-PSEUDONANA

Optical properties of the marine diatom Thalassiosira pseudonana were examined in nitrate-limited semicontinuous cultures for growth rates ( mu(N)) varying from 0.22 to 1.32 d(-1). Two experiments were conducted at each growth rate in which sampling of the culture was done at eith er the dark-to-light (D-L) or light-to-dark (GD) transition of a 12-h photoperiod. Optical cross sections and efficiency factors for absorpt ion, scattering, and attenuation were calculated from measurements of the spectral absorption and beam attenuation coefficients, cell concen tration, and the size distribution of cells in suspension. The refract ive index of cells, relative to seawater, was calculated from these da ta using an inverse method. Nitrogen limitation strongly influences th e optical properties of this species. Absorption cross sections increa sed more than twofold with increasing mu(N), predominantly caused by i ncreases in the imaginary part of the refractive index, n'. At the red absorption peak, n'(673) increased 140% and was strongly correlated w ith intracellular Chi a concentration. For most visible light waveleng ths, scattering cross sections were not a strong function of growth ra te but were consistently largest in the GD transition experiments. The difference between D-L and GD experiments at any given mu(N) increase d with increasing growth rate; for example, scattering cross sections at 660 nm in the GD experiments were 25% (mu(N) = 0.22 d(-1)) and 90% (mu(N) = 1.32 d(-1)) larger than in the respective D-L experiments. Th e real part of the refractive index at lambda = 660 nn varied from 1.0 24 to 1.039 among all experiments and was positively correlated with i ntracellular carbon concentration. The Chl a-specific absorption coeff icient decreased with increasing mu(N) (e.g. 43% decrease in the blue Soret band) in response to decreases in the carotenoid-to-Chl a ratio and increased pigment packaging. The Chi a-specific scattering coeffic ient at 660 nm decreased from 0.55 to 0.09 m(2) (mg Chi a)(-1) with in creases in mu(N). Our results suggest that the optical properties of p hytoplankton populations in nitrogen-limited surface waters may be sig nificantly altered from those in deeper portions of the water column.