Light absorption by Volvocaceae colonies consisting of equidistant optically soft photosynthetic cells in a transparent spherical extracellular matrix

This study aims to develop a methodology to predict the absorption cross-section of colonial microalgae such as those of the Volvocaceae family including Eudorina, Pleodorina, and Volvox consisting of an ordered assembly of large and optically soft absorbing cells embedded within a non-absorbing spherical extracellular matrix (ECM). The absorption cross-section of spherical colonies, such as Eudorina, containing 16, 32, and 64 equidistant photosynthetic cells distributed on the surface of a sphere within a concentric spherical ECM was predicted by the superposition T-matrix method for ECM size parameters as large as 500 and by the Monte Carlo ray-tracing (MCRT) method for ECM size parameters as large as 900. The predicted absorption cross-sections given by the two methods were in excellent agreement despite the fact that the conditions for geometric optics were not rigorously satisfied. The absorption cross-section of the microalgae colonies considered was found to increase with increasing cell radius, absorption index or cell pigment concentration, and/or number of cells. Shading among cells decreased the mass absorption cross-section and was increasingly important for colonies with strongly absorbing cells, large cell radius, and/or large number of cells. These results demonstrate that accounting for shading effects is necessary to accurately predict the absorption cross-section of microalgae colonies. Furthermore, the study demonstrated that the MCRT method is an accurate and efficient method for modeling light absorption by an ensemble of many large, ordered, and optically soft particles. Finally, the impact of colony formation as well as pigment and biomass concentrations on the local (LRPA) and mean (MRPA) rates of photon absorption within a microalgae culture was also assessed. At low biomass concentrations, the LRPA and MRPA decreased in the presence of colonies. This effect was more pronounced at higher pigment concentrations.