Immobilization of a Mixed Culture of Oxygenic Phototrophic Microorganisms on a Chitosan-Based Sorbent for Nutrient Bioremoval

The immobilization of cells of a mixed culture of the microalgae (MA) Micractinium sp. NAMSU A-19 and cyanobacteria (CB) Synechococcus sp. 1Dp66E-1 on a chitosan-based polymer was studied. A polycationic sorbent based on a natural chitosan polymer with a molecular weight of 600 kDa was obtained from the crosslinking of chitosan with glutaraldehyde via cryopolymerization. It has a high affinity for the surface structures of oxygenic phototrophic microorganisms (OPMs) and enables strong cellular attachment to the sorbent surface. The study of the kinetics and evaluation of the effectiveness of mixed-culture immobilization showed a high sorption capacity of the chitosan sorbent. During the first hour of cultivation, the immobilization efficiency was on average 40–52%, and, almost all cells were immobilized after 48 h. The highly porous, nontoxic, and biodegradable sorbent provided steady cellular attachment for 7 days of cultivation and did not affect the growth of immobilized mixed culture, either on the surface or in the inner layers of the polymer. The study of mixed-culture immobilization via scanning electron microscopy showed that CB and MA cells are tightly attached to the surface of the chitosan sorbent. This is followed by the formation of strands of the extracellular polymer matrix and a biofilm consisting of cells of the mixed culture of MA and CB and heterotrophic bacteria associated with Micractinium sp. Immobilization on a chitosan sorbent contributes to an increase in the bioremoval of nitrates and phosphates by the tested mixed culture.