Modern marine stromatolites are living examples of one of the earth's oldest and most persistent widespread ecosystems. Although rare today, these layered deposits of calcium carbonate are found in shallow marine seas throughout 3.4 billion-year-old geologic records. Ancient stromatolites represent a mineral record of carbonate chemistry and the evolution of early life. In the Geology paper, Dr. Andres and colleagues point out that incorrect assumptions have been made in interpreting stromatolite data: phototrophs, or oxygen-producers, were actually dominated by heterotrophs, or oxygen-consumers, in their contribution to stromatolite formation.

"The motivation for this study is that in ancient stromatolites, direct evidence of microbial activity is lacking," Dr. Andres explained. "Stable isotopes have provided a powerful tool to 'fingerprint' microbial pathways and better understand the sedimentary structures we see in the geologic record. Surprisingly, no study to date has documented this process for modern marine stromatolites."

Stromatolites are the oldest known macrofossils, dating back over three billion years. Dominating the fossil record for 80 percent of our planet's history, stromatolites formed massive reefs in this plane's primitive oceans. While stromatolites look much like coral reefs, they are actually formed from living microorganisms, both animal and plant-like. These microorganisms trap and bind sand grains together and/or produce calcium carbonate to form laminated limestone mounds.

"We knew that the stromatolite ecosystem was dominated by photosynthetic cyanobacteria, and expected to see this reflected in a positive carbon isotopic value. However, we saw the exact opposite." Andres said.

"We still don't understand how stromatolites calicify," Dr. Andres said, referring to her research plans. "This information will be key to understanding how organisms form skeletons and when this process - leaving lasting impressions of historical biological data - first began."

More information on stromatolites can be found at http://stromatolites.info. Rosenstiel School is part of the University of Miami and, since its founding in the 1940s, has grown into one of the world's premier marine and atmospheric research institutions.  See http://www.rsmas.miami.edu

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Rosenstiel School of Marine & Atmospheric Science
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