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. IMPORTANCEBacteria in the environment rarely, if ever, grow in isolation. Some phenotypic traits can only be understood in the context of interspecies interactions. Here, we examine interactions between two bacterial species with very different modes of metabolism. Synechocystis sp. PCC 6803 is a freshwater cyanobacterium that grows photosynthetically using light, water, CO2 and minerals. Pseudomonas aeruginosa is a soil bacterium and an opportunistic pathogen that depends on a supply of organic molecules for survival. When P. aeruginosa is co-cultured with Synechocystis in mineral medium, it lyses Synechocystis cells, using its Type VI secretion system to access nutrients via predation. The presence of P. aeruginosa (and other foreign bacteria) triggers a Synechocystis response in which it aggregates to form floating assemblages or flocs. Floc formation creates a defensive barrier to P. aeruginosa predation, and we suggest this complex behavior is best understood as a defense against microbial predation in the wild.