Formation of Physical Sedimentary Structure in the Presence of Microbial Communities
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Microbial life has been fundamental in the evolution of the Earth throughout geologic time. Although insignificant as individuals, collectively as communities microorganisms impact sedimentary environments by producing physically resilient structures. Many of these sedimentary structures, when understood in specific contexts, may serve as unique records for ancient life. Conical mats are some of the most distinctive fossil microbial communities in the geologic record. However, much is debated about how they form. We here show with experimental evidence that cones constructed by Leptolyngbya sp. occurs by repeated aggregation of mobile filaments likely coordinated by physical contact. Cone-forming cultures also rolled underlying sand grains into small piles beneath each biological cone. Repeated rolling over multiple day/night cycles gradually sorted sand into regularly spaced coarse piles with finer-grained lags in between. Significantly, statistically identical sorting patterns were discovered in 3.22-billion-year-old fossil conical mats that grew in sandy tidal environments of the Moodies Group, South Africa. These results demonstrate that group movement coordinated by touch-sensing systems could have structured populations of filamentous, photosynthetic microorganisms since at least the Paleoarchean. Microbial streamers are surface-attached microbial communities that paradoxically seem to roughen mats under rapid, high shear flows, potentially exposing the community to greater risk of erosion. They are common structures found in fast flow environments yet the mechanism of their formation and effects on mat erosion are poorly understood. We here show evidence that streamers are initiated by shear-induced viscoplastic deformation, and they locally re-attach boundary layers and therefore smooth rough bedding surfaces hydraulically, reducing shear experienced by near-surface mat communities. These results suggest a novel set of feedbacks that could reduce net mat erosion in energetic flows, and could help guide the evaluation of biosignatures in sedimentary rocks deposited in the presence of microbial mats. The presence of microbial communities have long been suggested to cause the increase of sediment cohesive strength, which is responsible for forming a wide range of microbial-sedimentary structures. Step-wise increase of mat strength towards the end of Archean has been documented, but it is uncertain what caused this change. We here suggest that the mechanical strength of mats increased as a direct product of the metabolic switch from an anoxygenic to oxygenic benthic microbial ecosystem. Support for this hypothesis is provided by examining the strength of experimental mats with productivity limited by various nutrients. In addition, we also expand the record of estimated mat strength beyond the Archean eon. These results add to a growing body of evidence how one single metabolic innovation – oxygenic photosynthesis forever altered the face of our planet.
Gong, Jian (2015). Formation of Physical Sedimentary Structure in the Presence of Microbial Communities. Doctoral dissertation, Texas A & M University. Available electronically from