DEVELOPMENT OF COMPLEX MORPHOLOGY IN A CYANOBACTERIAL LABORATORY MODEL SYSTEM: IMPLICATIONS FOR THE INTERPRETATION OF FOSSIL MICROBIALITES

The microbialite fossil record provides a unique window into the history of microbial communities and the evolution of life on early Earth. Morphology is one of the defining characteristics of microbialites, but the biological, physical, and chemical processes influencing microbial mat morphogenesis remain poorly understood. A new cyanobacterial laboratory model system rapidly and reliably generates complex structures in response to biological and physical processes. Pillars and peaks form perpendicular to the colonized substrate including culture vessel walls or down from the air-water interface. Complex topography develops in stagnant waters, under flow, and in shaken flasks. Motility and growth in response to light has been observed, but growth is not solely upwards or directly towards the light source. Once a critical cell density is reached, topography develops. Vertical structures form preferentially along the edges of glass slides, on topographic highs of disrupted mat, and on tops of bubbles. Bubbles produced by the cyanobacteria nucleate both above and below the mat. They can either be released freely into the water column, cause mat to float off the substrate, or be trapped to varying degrees by the mat. Physical disruption due to shaking, in situ bubble production, or human interference often results in the formation of vertical planar structures. The disruption rips up sheets of mat that either remains attached at the base or are deposited elsewhere. These sheets maintain a new vertical orientation due to their neutral or slightly positive buoyancy. Vertical planar structures are reinforced by the migration of cells to the new edges. Mats grown on smooth impermeable plastic tend to form regularly spaced narrow pillars, whereas mats on porous quartz sand form peaks and ridges that are frequently lifted off the substrate by gas bubbles.

This research provides several constraints for a model of peak formation in cyanobacterial mats. Peak development is not due to diffusion gradients within the culture media, nor is it due solely to phototaxic behavior. Peak formation may be in response to nutrient excess or depletion or may involve cellular communication such as quorum sensing.