Investigation on the optical properties of mixed-species microalgae aggregation accounting for the microstructure

This paper takes two typical CO₂-fixing microalgae-Chlorella sp. and Chlamydomonas reinhardtii-as the research subjects, aiming to comprehensively analyze the radiative characteristics of mixed-species aggregates. The models of mixed-species microalgae aggregates considering internal microstructures were constructed. The absorption cross section (C_abs), scattering cross section (C_sca), and scattering phase function of aggregates in the wavelength range of 400-750 nm were calculated using the multi-sphere T-matrix method. The theoretical calculations of the T-matrix and the experimental measurement results are highly consistent, confirming the accuracy of the theoretical method. The effects of the number of cell aggregates (N = 2-8) and species mixing ratio were systematically examined. The results show that the absorption and scattering cross-sections of the mixed-species aggregates increase with increasing N, but the shielding effect causes the rate of increase to slow down. In the N = 8 fixed co-culture system, increasing the proportion of Chlamydomonas reinhardtii enhanced light absorption, while a higher proportion of Chlorella increased light scattering. The scattering phase function is primarily determined by the optical structure of the cells themselves and is less influenced by the aggregation state. Comprehensive evaluation showed that a mixture of 75% Chlorella sp. and 25% Chlamydomonas reinhardtii was optimal, achieving synergistic optimization of high light absorption and low scattering loss across the entire visible spectrum.