Cyanobacterial flocculation as a defence against bacterial predation

Many cyanobacteria are capable of flocculation: the formation of floating linked assemblages of many thousands of cells. Flocculation is a highly regulated process requiring both type IV pilus activity and extracellular polysaccharide production. Under laboratory conditions, flocculation often slows culture growth, and its physiological advantages remain unclear. Here, we determine whether flocculation can serve as a method of defence against bacterial predation. Using the unicellular cyanobacterium Synechocystis sp. PCC 6803, we show that flocculation is strongly triggered by exposure to live cells of "foreign" bacteria, including the ubiquitous opportunistic pathogen Pseudomonas aeruginosa, whose habitat overlaps with that of Synechocystis. The established P. aeruginosa virulence arsenal includes bacterial warfare systems for competition for resources and acquisition of nutrients by way of interbacterial competition. Here, we establish the use of these strategies for direct predation, via the use of type VI secretion systems. Comparisons of P. aeruginosa co-cultures with either wild type Synechocystis or a non-flocculating mutant revealed that Synechocystis flocculation minimizes both growth of P. aeruginosa and cell lysis of Synechocystis. This in turn reduces the impact of P. aeruginosa on Synechocystis growth by mechanistically limiting the photosynthetic products that P. aeruginosa can access. From these data, we propose that the type VI secretion system of P. aeruginosa can be used for predation and that a major function of cyanobacterial flocculation is for defence against microbial predation.