Rheological characterization and modeling of freshwater and marine microalgae and cyanobacteria mixed cultures

Abstract

Rheology is crucial for algal cultivation, affecting mixing efficiency and biomass productivity. There is limited research on the rheological aspects of mixed cultures of algae and cyanobacteria. The study aimed to investigate the rheological properties of microalgae and cyanobacteria mixed cultures (Chlorella vulgaris and Microcystis aeruginosa; Nannochloropsis oceanica and Prochlorococcus sp.). While all species were spherical and non-motile with similar aspect ratios, Prochlorococcus sp. exhibited meaningful cell-cell repulsion, increasing its apparent effective volume fraction by 20–40 %. Mixed suspensions of C. vulgaris and M. aeruginosa demonstrated shear thinning at all concentrations, accurately described by a power law model across the explored shear-rate range. The conventional Einstein and Krieger-Dougherty models confirmed that viscosity increased with cell concentration, with Prochlorococcus sp. having the greatest impact. However, these models did not effectively account for mixed suspensions. A novel model, based on Pal (2023) [1] and incorporating two independent parameters, provided a greater reflection of the viscosity in mixed suspensions. Interestingly, parameters for this new mixed culture model could be derived from mono-culture rheological models within the studied concentration range. This first-of-its-kind study highlighted the significance of rheology in optimizing mixed cultures and identifies the need for further research to address rheological challenges, notably towards higher industrial-scale biomass densities.