Polarized‐light scattering studies of marine Chlorella

Abstract

This paper compares results of rigorous calculations of light scattering by a distribution of coated spheres with the measured light scattering of marine Chlorella. The elastic scattering properties of any organism are described completely by the 16-element scattering matrix that gives all the intensity and polarization information as a function of angle. Fitting the angular dependence of several matrix elements affords a much more stringent test of scattering calculations than does fitting only one, such as the phase function or linear polarization. The added requirements of this test significantly narrow the range of acceptable optical models and thereby permit better characterization of the scattering medium. Measurements of several elements of the scattering matrix of laboratory cultures of Chlorella were obtained with a scanning polarization-modulation nephelometer. The results for these elements were best fit by calculations based on a model of a Gaussian distribution of spheres with a relative complex refractive index of l.OS(t-0.005)-0.05i(~0.005) and a 60-nm coating of index1.13(+0.005)-0.04i(f0.005) to approximate the cell membrane. Good agreement was obtained for only a very narrow range of particle parameters. Experimental results were broken into spherical and nonspherical contributions to evaluate the effects of nonsphericity.Particles in the oceanic water column scatter and absorb light to alter its intensity, spectrum, and polarization state. The polarization of light in the ocean has long been known to affect the behavior of marine organisms (Waterman 1954). Measurement of the intensity and polarization of light scattered by particles in the water as a function of angle can be used to investigate the nature of the particles and to predict the propagation of light in the ocean. To better understand light scattering in the ocean, we AcknowledgmentsWe thank Richard Spinrad, Eric Hartwig, and Pat Wilde for discussions and support.

Significant contribution of large particles to optical backscattering in the open ocean

“…Similarly, Volten et al (1998) concluded that theoretical predictions could not achieve a "good agreement" with their measured volume scattering functions because these predictions are "too steep" in the forward direction (i.e., the backscattering ratio b bp :b p predicted by Mie theory is too small). Quinby-Hunt et al (1989) demonstrated that the elements of the scattering matrix measured on a Chlorella culture could be satisfyingly reproduced only by a coatedsphere model. When the volume-average of the refractive indices fitted for the coating and for the core of the modeled cell was used as input to the homogeneous spherical model, backscatter was overestimated by a factor 3.…”

“…It is noteworthy, however, that these experiments were all based on the same technique and hypotheses: conversion of the measured volume scattering function at a narrow angular range into b bp . On the other hand, many independent experimental (Zielinski et al, 1986(Zielinski et al, , 1987Quinby-Hunt et al, 1989;Volten et al, 1998;Stramski and Piskozub, 2003;Vaillancourt et al, 2004) and theoretical (Meyer, 1979;Kitchen and Zaneveld, 1992 at contributes most of the forward scattering, and by the scattered intensity generated by its outer shell (dashed ue line) that generates most of the backscattering (λvac = 520 nm). Scattered intensities were multiplied by e corresponding scattering cross-sections relative to the scattering cross-section of the coated-sphere.…”

Scale closure in upper ocean optical properties : from single particles to ocean color

“…The method initially developed by Ackleson and Spinrad (1988) for the application ofMie theory to flow cytometry was improved in this study to describe the deviation of phytoplanton cells from homogenous spheres and to determine phytoplankton n', knowledge of which is necessary for quantifying the contribution of phytoplankton to absorption. Validation of the individual particle approach with independent measurements was important, because it has previously been shown that phytoplanon scattering deviates from that estimated by Mie theory (Quinby-Hunt et al 1989;Volten et aL. 1998).…”

“…Mie theory (Quinby-Hunt et al 1989;Volten et al 1998), and modeling work has focused on better understanding the effects of non-sphericity and inhomogeneities on theoretical determinations of angular scattering (e.g. Quinby-Hunt et al 1989;Kitchen and Zaneveld 1992;Herrng and Boss 2000).…”

“…Mie theory (Quinby-Hunt et al 1989;Volten et al 1998), and modeling work has focused on better understanding the effects of non-sphericity and inhomogeneities on theoretical determinations of angular scattering (e.g. Quinby-Hunt et al 1989;Kitchen and Zaneveld 1992;Herrng and Boss 2000). With its assumptions in mind, Mie theory can provide a framework for calculating scattering at all angles from the fOlWard and side angle scattering measurements of flow cytometry Mie theory (or an approximation there of) has been applied to flow cytometric measurements of particles to determine paricle diameter and real refractive index (n) (Ackleson andSpinrad 1988, Ackleson andRobins 1990) and the contribution of different phytoplanton species to beam attenuation ).…”