Using a two-layered sphere model to investigate the impact of gas vacuoles on the inherent optical properties of &amp;lt;i&amp;gt;Microcystis aeruginosa&amp;lt;/i&amp;gt;

M W Matthews,S Bernard

doi:10.5194/bg-10-8139-2013

Abstract

Abstract. A two-layered sphere model is used to investigate the impact of gas vacuoles on the inherent optical properties (IOPs) of the cyanophyte Microcystis aeruginosa. Enclosing a vacuole-like particle within a chromatoplasm shell layer significantly altered spectral scattering and increased backscattering. The two-layered sphere model reproduced features in the spectral attenuation and volume scattering function (VSF) that have previously been attributed to gas vacuoles. This suggests the model is good at least as a first approximation for investigating how gas vacuoles alter the IOPs. Measured Rrs was used to provide a range of values for the central value of the real refractive index, 1 + ε, for the shell layer using measured IOPs and a radiative transfer model. Sufficient optical closure was obtained for 1 + ε between 1.1 and 1.14, which had corresponding Chl a-specific phytoplankton backscattering, bbφ*, between 3.9 and 7.2 × 10−3 m2 mg−1 at 510 nm. The bbφ* values are in close agreement with the literature and in situ particulate backscattering measurements. Rrs simulated for a population of vacuolate cells was greatly enlarged relative to a homogeneous population. A sensitivity analysis of empirical algorithms for estimating Chl a in eutrophic/hypertrophic waters suggests these are robust under variable constituent concentrations and likely to be species-sensitive. The study confirms that gas vacuoles cause significant increase in backscattering and are responsible for the high Rrs values observed in buoyant cyanobacterial blooms. Gas vacuoles are therefore one of the most important bio-optical substructures influencing the IOPs in phytoplankton.

Highlights

IntroductionEspecially those exhibiting intracellular gas vacuoles, is poorly described
Light scattering by cyanobacteria, especially those exhibiting intracellular gas vacuoles, is poorly described
The aφ∗ value (Fig. 1a) is well within the range presented in the literature for cyanobacteria (e.g. Dupouy et al, 2008), almost identical to that for M. aeruginosa determined by Zhang et al (2012), and slightly higher than values obtained by Dekker (1993) for eutrophic blue-green dominant assemblages

Summary

Introduction

Especially those exhibiting intracellular gas vacuoles, is poorly described. Vacuolate cells consistently show a flattening of the VSF in the forward direction, which corresponds to the reduced forward light scatter measured by Dubelaar et al (1987) This phenomenon is only reproduced by Mie modelling using homogeneous spheres with very low refractive index (= 0.4) relative to water (Schreurs, 1996). This confirms that the reduced forward light scatter is caused by vacuoles, which reduce the overall real refractive index of the cell

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Conclusion