Diel patterns in optical properties of the chlorophyte Nannochloris sp.: Relating individual-cell to bulk measurements

To investigate how cell growth and division affect the optical properties of phytoplankton, cultures of the chlorophyte Nannochloris were sampled over a diel cycle to measure cell size and concentration, light-beam attenuation and absorption, flow cytometric forward light scattering and chlorophyll fluorescence, and carbon content. Refractive index was calculated using the anomalous diffraction approximation. At six different light levels, ranging from 60 to 1,500 mu mol photons m(-2) s(-1), cell division was tightly phased to the light:dark cycle, occurring soon after dark. There were pronounced diel patterns, with minima near dawn and maxima near dusk, in cell size, cell-specific beam attenuation and absorption, flow cytometric forward light scatter, and carbon content. The diel variations in attenuation cross section were primarily caused by changes in cell size due to growth and division, although there was some influence from changes in refractive index. Because the major constituents of many phytoplankton communities are small eukaryotic cells similar to Nannochloris in cell size and division pattern, the results presented here have important consequences for the interpretation of diel variations in optical properties observed in the ocean.