Screening Microalgal Cultures in Search of Microbial Exopolysaccharides with Potential as Adhesives
Nearly 800 cultures from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Collection of Living Microalgae (CCLM) were screened for exopolysaccharide (EPS) production by examining the viscosity of conditioned media supernatants. We first established dose-response relationships for the viscosity of reference polysaccharides dissolved in microalgal growth media. Then, using the 40 most viscous CCLM cultures, we confirmed that the viscosity of culture supernatants correlated well with their total sugar (and thus EPS) content. The conditioned medium with the highest viscosity (6.55 cP, equivalent to 1.16 g/L xanthan gum) was produced by a non-axenic isolate of the cyanobacterium Microcystis aeruginosa f. flos-aquae. Two types of bacteria, designated CSIRO501 (Gram-positive) and CSIRO505 (Gram-negative), were subcultured from it. At 20 dry wt% sugar, an exopolymer preparation from CSIRO505 contained substantially more EPS than its counterparts from CSIRO501 or from axenic Microcystis cultures, and it resembled a well-known bacterial EPS (xanthan gum) in being much more effective at bonding wood than PVC. It bonded wooden lap joints with a dry shear strength of 1.5 MPa, four times the value obtained in equivalent tests of a commercial polyvinyl acetate (PVA) glue.