Flocculation of a cyanobacterium confers defense against bacterial predation

Many cyanobacteria, including unicellular species, 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 the production of extracellular polysaccharides. Under standard laboratory conditions flocculation often slows culture growth, and its physiological advantages remain unclear. Proposed benefits include self-shading as protection from excess light exposure and flotation in the water column. Here, we determine whether flocculation can serve as a method of defense 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 opportunistic pathogen Pseudomonas aeruginosa . 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 the primary function of cyanobacterial flocculation is for defense against microbial predation.