Graduation Year


Document Type




Degree Granting Department

Marine Science

Major Professor

Mya Breitbart, Ph.D.

Co-Major Professor

David Hollander, Ph.D.

Committee Member

Lisa Robbins, Ph.D.


Stromatolite, Cyanobacteria, Proteobacteria, Carbonate, Lipid


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. !