Light absorption and scattering by high light-tolerant, fast-growing Chlorella vulgaris IPPAS C-1 cells

Algal cells are highly complex optical systems that can dynamically change their structure. Consequently, absorption and scattering properties of algae change, while the cells are acclimating to different light conditions or during growth and division in a cell cycle. This may be particularly important in algal species that can grow rapidly under very high-light such as Chlorella vulgaris IPPAS C-1 that is studied here. From cell transmittance measured conventionally and using integrating sphere, we evaluated absorption and scattering coefficients and cross sections per cell dry weight and chlorophyll content. This was done for asynchronous cell culture grown in low-light (LL; 220 μmol (photons) m⁻² s⁻¹) or high-light (HL; 1760 μmol (photons) m⁻² s⁻¹) light, as well as during cell cycle of synchronous culture grown in HL. During the cell cycle, we also determined cell ultrastructural organization by transmission electron microscopy, and correlated its parameters with absorption and scattering cross sections per cell dry weight. We found that the IPPAS C-1 cells of asynchronous culture scatter light more than other cells, however, internal organization of the cells that is decisive for scattering is less sensitive to HL and LL treatment than the cell pigment content that controls absorption. The light scattering and absorption were dynamically changed during cell cycle of synchronous cells grown in the HL. Changes in ratio of chloroplast to protoplast area, reflecting amount of scattering chloroplast membrane (outer, inner) interfaces, best correlated with changes in light scattering. We suggest that the increased light scattering by the HL-acclimated IPPAS C-1 cells might be responsible for increased HL resilience reported in the literature. Biotechnological aspect of this study is that the scattering and absorption properties of phytoplankton cells ought to be calibrated for each particular growth phase or irradiance to which the cells are acclimated.