Aggregation with clay causes sedimentation of the buoyant cyanobacteria Microcystis spp.

We investigated whether the attachment of clay particles can result in sedimentation of the buoyant cyanobacteria Microcystis spp. For this purpose, we measured aggregation of clay to Microcystis spp. in field samples as well as in laboratory cultures of isolated strains. We focused on how the aggregation of clay to Microcystis spp. is affected by (1) the concentration of suspended clay particles, (2) the type of clay and (3) the stickiness and extracellular polysaccharide composition of these cyanobacteria. Our results show that aggregation of clay to Microcystis spp. is largely driven by mass action. The aggregation rate is proportional to the concentration of suspended clay particles. Experiments in a Couette chamber showed that aggregation of bentonite clay to Microcystis spp. is 15 to 22 times more efficient than aggregation of kaolinite clay. This reflects the difference in cation exchange capacity between these 2 clay minerals. Stickiness varied by more than 1 order of magnitude among different Microcystis strains, and showed a significant positive correlation with the amount of extracellular uronic acids in the slime layer of Microcystis spp. This indicates that uronic acids play an important role in aggregation, presumably through the formation of cation bridges. Our results imply that these buoyant cyanobacteria may undergo high sedimentation rates in lakes with high concentrations of suspended clay.