Effects of temperature, nitrogen, and light limitation on the optical properties of the marine diatom Thalassiosira pseudonana

A series of laboratory experiments were conducted to examine the optical properties of the marine diatom Thalassiosira pseudonana for a broad range of growth rates (mu from 0.22 to 2 d (1)) under temperature-, nitrogen-, and light-limited conditions. The effects of temperature and nitrogen limitation on spectral absorption cross-sections expressed on a per cell basis, sigma(a)(lambda), were similar. With the reduction in growth rate, sigma(a), at a light wavelength lambda = 674 nm showed an equivalent decrease regardless of whether the cells were limited by temperature or nitrogen. The effect of growth irradiance was distinctively different, since sigma(a) increased with light limitation. The chlorophyll a (Chl a)-specific absorption coefficient showed the opposite trends than those of sigma(a). These patterns resulted primarily from the acclimative strategies of T pseudonana involving an increase in Chl a content in response to light limitation and a decrease in Chl a under temperature and nitrogen limitation. The scattering cross-sections, sigma(b)(lambda), and carbon-specific scattering coefficients, b(C)(*)(lambda), were generally not a strong function of growth rate with the exception of lower values of b(C)(*) for light-limited cells. While T pseudonana exhibited some changes in cell size, variations in the refractive index had the major influence on cell optical properties. The imaginary part of, the refractive index (at lambda = 674 nm) showed a strong correlation with intracellular Chl a concentration, and the real part of the refractive index (at lambda = 660 nm) was correlated with the intracellular carbon concentration. Our results indicate that integrated studies of the effects of various environmental factors Such as light, nutrients, and temperature are needed to adequately describe optical variability in phytoplankton.