Immobilization on chitosan triggers structural rearrangements and boosts phosphorus accumulation by the cells of the chlorophyte Lobosphaera sp. IPPAS C-2047

Microalgal biotechnology is a promising solution for bioremoval of nutrients, especially phosphorus (P). Microalgae can accumulate large quantities of P and the P-enriched algal biomass can be used as bio-fertilizer. Immobilization of microalgae on cheap, biocompatible, and biodegradable chitosan-based carriers facilitates the biomass harvesting, increases the stress resilience of their cells and improves the economic viability of the technology. Here, we demonstrated that immobilization of the green microalga Lobosphaera sp. IPPAS C-2047 on a chitosan-based carrier increased the bioremoval of P from the cultivation media. This effect was accompanied by the build-up of P-rich deposits in the cell wall and an expansion of the vacuolar compartment harboring the bulk P of the cell. An increase in the abundance of P-rich intracellular inclusions as compared to the cells of suspended culture was revealed by ultrastructure analysis and confirmed by energy-dispersive spectroscopy (EDS). The immobilization of the Lobosphaera sp. cells resulted in reorganization of their cell wall, increase in its electron density and thickness, probably due to the penetration of the chitosan nanoparticles into cells. This hypothesis was supported by the pattern of the changes in elemental composition of the Lobosphaera sp. cell wall which revealed, in addition to the anions (phosphate, nitrate, sulfate and chloride) adsorbed to the positively charged nanoparticles of chitosan, a characteristic presence of cationic counterions (potassium, sodium, magnesium, calcium). The presence of the putative chitosan particles was also noted in the vacuoles. Overall, the rearrangements in the cell of the Lobosphaera sp. triggered by its immobilization on chitosan seem to be among the factors increasing its cell quota for P accumulation and hence the removal efficiency for this nutrient as compared to the suspended cell.