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

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 R_rs 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, b_bφ^*, between 3.9 and 7.2 × 10⁻³ m² mg⁻¹ at 510 nm. The b_bφ^* values are in close agreement with the literature and in situ particulate backscattering measurements. R_rs 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 R_rs values observed in buoyant cyanobacterial blooms. Gas vacuoles are therefore one of the most important bio-optical substructures influencing the IOPs in phytoplankton.