A New Family of Tubular Pilus from Microcystis aeruginosa, a Harmful Algal 2 Bloom Forming Cyanobacterium

Cyanobacteria are photosynthetic prokaryotes vital to global biogeochemical cycles. However, some form harmful algal blooms (cyanoHABs) that disrupt ecosystems and produce toxins. The mechanisms by which these blooms form have yet to be fully understood, particularly the role of extracellular components. Here, we present a 2.4 Å cryo-EM structure of a pilus, termed the cyanobacterial tubular (CT) pilus, found in the cyanoHAB-forming species Microcystis aeruginosa. The pilin exhibits a novel protein fold, forming a tubular pilus structure with tight, double-layer anti-parallel β-sheet interactions. We show that CT pili are essential for buoyancy by facilitating the formation of micro-colonies, which increases drag force and prevents sinking. Furthermore, the CT pilus surface is heavily glycosylated with ten monosaccharide modifications per pilin. We also found that CT pili can enrich microcystin, potentially enhancing cellular resilience, and co-localize with iron-enriched extracellular matrix components. Thus, we propose that this pilus plays an important role in the proliferation of cyanoHABs. This newly discovered pilus family appears to be widely distributed across several cyanobacterial orders, including Chroococcales, Nostocales, and Synechococcales. Our structural and functional characterization of CT pili provide insights into cyanobacterial cell morphology, physiology, and toxin interactions, and identify potential targets for disrupting bloom formation.