Heterologous expression of a Volvox cell adhesion molecule causes flocculation in Chlamydomonas reinhardtii

Abstract

Harvesting microalgae at large scales is a significant barrier to economically feasible production of biofuels and other low-cost commodities from microalgal biomass. We have tested a strategy for reducing the costs of microalgal harvest by generating transgenic strains of Chlamydomonas reinhardtii that express a heterologous cellular adhesion molecule, Algal-CAM, from the multicellular green alga Volvox carteri. Constitutive expression of heterologous Algal-CAM causes Chlamydomonas unicells to adhere together and settle out of suspension much more rapidly than controls. Immunoblotting shows the heterologous Algal-CAM to be present in the extracellular matrix of Chlamydomonas transformants and possibly cross-linked with native glycoproteins there. We define this form of cell–cell adhesion as genetically engineered (GE) flocculation to distinguish it from other cell flocculation strategies. Future development of this trait can include making expression of Algal-CAM inducible for controlled timing of GE flocculation and exploring regulated expression of additional cell adhesion molecules from the many other multicellular relatives of Chlamydomonas. Advanced forms of this technology could lead to production of novel biomaterials from single-celled algae by controlled expression of diverse cell adhesion molecules with different cross-linking properties.