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 (h) varying from 0.22 to 1.32 d-l. Two experiments were conducted at each growth rate in which sampling of the culture was done at either the dark-to-light (D-L) or light-to-dark (L-D) transition of a 12-h photoperiod. Optical cross sections and efficiency factors for absorption, scattering, and attenuation were calculated from measurements of the spectral absorption and beam attenuation coefficients, cell concentration, and the size distribution of cells in suspension. The refractive index of cells, relative to seawater, was calculated from these data using an inverse method. Nitrogen limitation strongly influences the optical properties of this species. Absorption cross sections increased more than twofold with increasing h, predominantly caused by increases in the imaginary part of the refractive index, ~1’. At the red absorption peak, n’(673) increased 140% and was strongly correlated with intracellular Chl a concentration, Par most visible light wavelengths, scattering cross sections were not a strong function of growth rate but were consistently largest in the L-D transition experiments. The difference between D-L and L-D experiments at any given b increased with increasing growth rate; for example, scattering cross sections at 660 nm in the L-D experiments were 25% (h = 0.22 d-l) and 90% (h = 1.32 d-l) larger than in the respective D-L experiments. The real part of the refractive index at A = 660 nm varied from 1.024 to 1.039 among all experiments and was positively correlated with intracellular carbon concentration. The Chl u-specific absorption coefficient decreased with increasing h (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 Chl u-specific scattering coefficient at 660 nm decreased from 0.55 to 0.09 m2 (mg Chl a)-’ with increases in h. Our results suggest that the optical properties of phytoplankton populations in nitrogen-limited surface waters may bc significantly altered from those in deeper portions of the water column. Advances in optical instrumentation, combined with improved modeling of radiative transfer in the ocean, allow the optical properties of seawater to be monitored over a broad range of spatial and temporal scales. For most oceanic waters, variability in the inherent optical properties of seawater can be attributed to suspended biogenic particles and associated dissolved materials. This link between the optical properties of seawater and the planktonic microbial community dictates that biological processes be included in models of radiative transfer within the ocean, and provides an opportunity to extract biological information from measurements of optical variability using appropriate inversion models. Two of the inherent optical properties of seawater are the spectral absorption, a(h), and scattering, b(h), coefficients (see notations). Phytoplankton are one constituent of the marine planktonic community that exert a strong influence on