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

A two-layered sphere model is used to inves- tigate 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 in- creased backscattering. The two-layered sphere model repro- duced features in the spectral attenuation and volume scat- tering 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 + , for the shell layer using measured IOPs and a radia- tive transfer model. Sufficient optical closure was obtained for 1 + between 1.1 and 1.14, which had corresponding Chl a-specific phytoplankton backscattering, b b , between 3.9 and 7.2◊ 10 3 m 2 mg 1 at 510 nm. The b b 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 homo- geneous population. A sensitivity analysis of empirical algo- rithms for estimating Chl a in eutrophic/hypertrophic waters suggests these are robust under variable constituent concen- trations 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 buoy- ant cyanobacterial blooms. Gas vacuoles are therefore one of the most important bio-optical substructures influencing the IOPs in phytoplankton.

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