Abundant reticulate mats and intricate vertical structures discovered in cyanobacterial communities in Lake Pavilion, British Columbia

Characterizing the complex morphological structures constructed by modern microbial communities will significantly inform our interpretations of fossil microbialite morphology, and thus our understanding of the evolution of the microbial biosphere. Recently discovered cyanobacterial mats in Pavilion Lake, British Columbia, exhibit a spectrum of intricate, predominantly reticulate morphologies, as well as complex vertically stacked structures that incorporate significant void space.
These complex microbial structures grow off of Willow Point on the western shore of Lake Pavilion at a depth of approximately 5 m. The mats occur in hollows within Chara macroalgae beds, in locations characterized by groundwater influx and higher than ambient hydrogen sulfide. The walls of the hollows are constructed of Chara blades coated with thick microbial mats, while the floors are entirely microbial mat. In places, the floor is suspended several centimeters above sublayers of mat that represent older floors. Additional void space is created by the vertical tiering and stacking of mesoscale structures such as terrace steps, bridges, depressions, domes, and pillars. All are dominantly constructed by microbial mats. Holes in the floor open onto underlying layers of mat, revealing some of the underlying void space and microbial architecture.
The microbial mats that construct these structures are also organized into mm to cm scale microstructures. The predominant mat morphology is reticulate, defined here as a series of intersecting microbial ridges that define polygons with lower topography. The reticulate structures have a spectrum of geometries with endmembers characterized by angular versus curvilinear ridges. Additional microstructures include pustular and hairy forms. These microstructures are all predominantly constructed by a suite of filamentous cyanobacteria.
When reticulate mats were sampled and brought to the surface, cyanobacteria invariably migrated out of the mat onto surrounding surfaces. Filaments were observed to move rapidly in clumps, preferentially following paths of previous filaments. The migrating filaments organized into new angular and ropey reticulate biofilms within hours of sampling, demonstrating that cell motility is responsible for the reticulate patterns. The reticulate morphologies and observed microbial behaviors are similar to those observed in laboratory studies of another filamentous cyanobacteria. In the laboratory Pseudanabaena system, experimental work has demonstrated that aggregation into reticulate biofilms occurs as a result of random gliding and colliding among filaments whenever the participating filaments are motile, but does not require the influence of chemo- or phototaxis.