Simulating the optical properties of phytoplankton cells using a two-layered spherical geometry

Abstract

Abstract. Effective use of ocean colour and other bio-optical observations is dependent upon an ability to understand and characterise the angular scattering properties of phytoplankton populations. The two-layered sphere appears to offer the simplest heterogeneous geometry capable of simulating the observed angular scattering of phytoplankton cells. A study is made of the twolayered spherical model for the simulation of the inherent optical properties of algal populations, with a particular focus on backscattering as causal to ocean colour. Homogenous and two-layered volume-equivalent single particle models are used to examine the effects of varying cellular geometry, chloroplast volume, and complex refractive index on optical efficiency factors. A morphology with a chloroplast layer surrounding the cytoplasm is shown to be optimal for algal cell simulation. Appropriate chloroplast volume and refractive index ranges, and means of determining complex refractive indices for cellular chloroplast and cytoplasm material, are discussed with regard to available literature. The approach is expanded to polydispersed populations using equivalent size distribution models: to demonstrate variability in simulated inherent optical properties for phytoplankton assemblages of changing dominant cell size and functional type. Finally, a preliminary validation is conducted of inherent optical properties determined for natural phytoplankton populations with the two-layered model, using the reflectance approximation. The study demonstrates the validity of the two-layered geometry and refractive index structure, and indicates that the combined use of equivalent size distributions with the heterogeneous geometry can be used to establish a quantitative formulation between single particle optics, size and assemblage-specific inherent optical properties, and ocean colour.

Using a two-layered sphere model to investigate the impact of gas vacuoles on the inherent optical properties of <i>Microcystis aeruginosa</i>

“…Two-layered and three-layered sphere models which simulate internal structures of the cell wall, the cytoplasm, and or the chloroplast can more adequately simulate phytoplankton IOPs (e.g. Quinby-Hunt et al, 1989;Kitchen and Zaneveld, 1992;Bernard et al, 2009). In particular the AdenKerker two-layered sphere is probably the simplest geometrical arrangement capable of reproducing experimental IOPs (Quirantes and Bernard, 2006).…”

“…The traditional assignment of the two spherical layers of the model to chloroplast and cytoplasm, respectively (e.g. Bernard et al, 2009) is less suitable to prokaryotic cyanobacteria. The thylakoids in cyanobacteria are not arranged in strict membrane-bound chloroplasts but rather occur in the intracytoplasmic membrane towards the periphery of the cell (the so-called chromatoplasm).…”

“…The details of Mie and Aden-Kerker theory of light scattering with small particles may be found in Morel and Bricaud (1986) and Bernard et al (2009). Briefly, the complex refractive index (m) is composed of real (n) and imaginary parts (n ) according to m = n − in .…”

“…This effect must be accounted for in calculations when altering the relative volumes of core and shell layers. The equation relevant to a two-layered geometry relating n to c i and the relative shell volume, V s , is given by Bernard et al (2009):…”

“…A two-layered sphere model using the Aden and Kerker (1951) formulation and the code of Toon and Ackerman (1981) were used to calculate the IOPs (after Bernard et al, 2009) in a Fortran/Matlab environment (The MathWorks ™ ). Inputs to the model are the radius of the core and shell layers, m for the core and shell layers, the wave number (wavelength), and angular resolution (which is 0.1 • ).…”