Cell specialization and quorum dependent intercellular communication underly Synechococcus elongatus biofilm development

Cyanobacterial biofilms are prevalent in the environment and are associated with economic loss; yet the mechanisms underlying formation of these assemblages were overlooked for many years. We revealed a biofilm self-suppression mechanism that operates in Synechococcus elongatus PCC7942 and which requires the type IV pilus (T4P) assembly apparatus. This complex is involved in deposition of a biofilm inhibitor to the extracellular milieu, which in turn suppresses expression of the EbfG-operon that comprises genes enabling biofilm formation. Further studies uncovered two components required for the biofilm self-suppression mechanism: The RNA-chaperone Hfq and a conserved protein annotated `hypothetical` which we denote EbsA (essential for biofilm self-suppression A). We revealed a tripartite complex of EbsA, Hfq and the ATPase homolog PilB, and demonstrated that each one of these components is required for pilus-assembly and for DNA competence, in addition to their role in biofilm self-suppression. Additionally, analysis of EbfG-operon expression in individual cells revealed that high expression is limited to a subpopulation in the culture. Further analysis of one protein encoded by this operon indicated cell surface as well as matrix localization. These data suggest a beneficial ‘division of labor’ during biofilm formation where only some of the cells allocate resources to produce matrix proteins – ‘public goods’ that support robust biofilm development by the majority of the cells. Moreover, we revealed gradual accumulation of the biofilm inhibitor in aging wild-type cultures, which implies a quorum sensing mechanism in S. elongatus biofilm regulation.