2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 12
Presentation Time: 8:00 AM-12:00 PM


SHEPARD, Rebekah, ALEXANDER, Kathryn M. and SUMNER, Dawn Y., Geology, Univ of California, Davis, 1 Shields Ave, Davis, CA 95616, shepard@geology.ucdavis.edu

Investigation of peak and ridge development in microbial mats within the laboratory will allow us to glean evidence of microbial behavior from fossil microbialites. The surprising frequency of structure development in a diversity of mats and lab conditions suggests that the growth of mats into complex topographies may be a common behavior of microbial mats. Therefore, an understanding of the processes governing this behavior is crucial to the interpretation of ancient microbial communities.

Natural mats and single-species cultures incubated within the lab produced a variety of complex morphological structures. We have observed the formation of topographic peaks and ridges by both physical processes associated with the subaerial exposure of mats, and by purely biological processes within subaqueous mats. When exposed, vertically oriented filaments collapse into peaks and ridges due to surface tension. Exposed wet mats form ridges through the temporary capture of gas bubbles by the network of filaments. The subsequent escape of bubbles results in collapse of the mat into ridge structures.

Subaqueous mats form peaks and ridges through the upward migration and vertical orientation of cyanobacterial filaments from the sediment into the water column. If topographically elevated features such as sediment ridges or algal filaments were initially present, the cyanobacteria migrated up them, commonly forming rosette structures. On flat surfaces, cyanobacteria migrate into low-relief ridges of high cell concentration. This migration is inferred from observations of cyanobacteria organizing themselves into ridges across the tops of vertically-oriented algal filaments in other experiments. Ridge relief may also be enhanced by localized cell reproduction. In fact, the longer time taken for cyanobacterial self-organization into ridges when they are colonizing a new surface also suggests that a minimum biomass is required for structure formation.

Results to date suggest that both filamentous algae and cyanobacteria form peak and ridge structures in exposed mats. Subaqueous ridge formation appears to require conditions allowing cyanobacteria to out-compete algae. Elucidation of conditions promoting processes of peak formation will provide insight into the behaviors responsible for microbialite morphology.