Modern microbialites are carbonate-precipitating microbial mats and
represent the closest living analogues to ancient stromatolites. These ancient
carbonate formations are the oldest fossil evidence of life on Earth; however, our
comprehension of their relationship to early earth ecosystems relies heavily on
understanding the formation of modern microbialites. Research regarding these
formation processes has suggested that chemical constraints of CaCO 3
precipitation vary on sub-millimeter spatial scales within the living microbial
community. In an attempt to shed light on the importance of these chemical
microenvironments, this study focused on understanding the spatial distribution
of the organisms and processes involved in the formation of modern
microbialites. This was accomplished by isolating five visually distinct layers from
the upper 2 – 3 cm of an actively forming microbialite found in the freshwater
system of Cuatro Ciénegas, Mexico. Each layer was analyzed using genomic,
molecular organic, and stable isotopic techniques. Bacterial diversity was
determined by 16S rRNA gene analyses, lipid biomarker content was detected by
GC-MS, and carbon isotope composition of organic matter and CaCO 3 were
used as indicators of specific microbial processes. Results of the 16S rRNA gene
analysis showed that there is little overlap in the community composition of
individual layers. Approximately 90% of the ribotypes identified in the microbialite
were unique to a single layer. Furthermore, the relative accretion of CaCO 3 at
each layer was used to connect the distribution of organisms and processes with
two specific zones of CaCO 3 precipitation. The first zone of CaCO3 accretion,
which accounted for approximately 55% of total CaCO 3 accumulation, is found in
the surface two layers of the microbialites and dominated by photoautotrophic
cyanobacteria and algae. The second zone of CaCO 3 precipitation, found at the
interior (layers 4 and 5), is composed primarily of heterotrophic proteobacteria
and dominated by sulfate-reducing !-proteobacteria. The lipid content of the
microbialite reflected the community structure as determined by genomics.
Numerous photosynthetic biomarkers were detected and decreased in
abundance with depth, indicating the important function of heterotrophic
degradation. Additionally, the detection of sulfurized phytol compounds in layer 5
highlighted an important mechanism for the preservation of biogenic signatures,
and reflected both the abundance of phototrophic organisms and sulfatereducing
bacteria. In combination, these interdisciplinary analyses provided an
understanding of microbial community composition and metabolism while
indicating the spatial relationship to CaCO 3 formation and the preservation of
distinct biochemical signatures. !
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-4831 |
| Date | 27 September 2010 |
| Creators | Nitti, Anthony G. |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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