This topic submitted by Melissa Driessnack (
Driessmk@miamioh.edu) at 12:53 AM on 6/10/06.
The back of our truck was home on San Salvador in the
3.8 billion years ago the environment of Earth became suitable for life in the form of bacteria. The earliest record of fossiliferous rock is 300 million years younger at 3.5 billion years old and contained a rich biota of bacteria. Since the onset of life there has been a dynamic relationship between the biota and abiota (Mayr 42-44). Cyanobacteria, previously called blue-green algae, are no exception; in fact they played a role in the conversion of the atmosphere from oxygen-free to the oxygen containing atmosphere that supports life today. These microbial mats in shallow marine environments formed Stromatolites the oldest fossil record that still continues today.
Stromatolites are organosedimentary structures formed predominantly by sediment trapping, binding or the in situ precipitation due to growth and metabolic activity of the microorganisms. The oldest stromatolite fossils are 3.5 billion years old, with the fossils being found in Western Australia and in Africa (Papineau et al.2005). Stromatolites are a very useful tool in evaluating the nature of early Earth as well as microbial metabolism and the development of the biogeochemical cycles. They indicate what past environments were like in an area, such as the Lake Turkana stromatolites that what is now an area desert was once a lake (Leakey 14). Not only do these fossils give us a view into the past they can also give us a view of microbial life.
Modern stromatolites are located in Hamelin Pool in Western Australia, Highborne Cay in the Bahamas, a fringing reef in Belize, Tiekehau Atoll in the French Polynesia, karst sink holes in South Africa and riverine waterfalls. In today’s world stromatolites are rare compared to their relative abundance in fossil records, but they continue to exist in hostile environments such as Shark Bay in Western Australia (Leakey 14). In Shark Bay the Stromatolites live in water with a high salinity and rise to a height of about 0.5meters and can be in a variety of shapes including columnar, spheroidal, nodular or irregular (Papineau et al. 2005). They have a characteristic layering of a soft and hard layers, which can be seen in the Exuma Cays in the Bahamas that is also another location for living stromatolites. In Highborne Cay, part of Exuma Bay, the Stromatolites there can range from a few centimeters tall to up to 2 meters (Visscher et al. 2005 ). Stromatolites form by trapping and binding sediment or by the precipitation of carbonate from the microorganisms. They form as a thin layer of microorganisms that may start centrally on a grain of sand. As they grow a thin layer of sediment accumulates and is bound to the microorganism by sticky mucus that the cells secrete. The binding of sediment is a result of using calcium carbonate that is separated from the water. Once the living layer is covered the microbes once again move to the surface and the layering continues (Leaky 14). Calcium carbonate is deposited into the microbial communities forming a layer of sediment which the microbes grow through and form a new surface referred to as lamina. The rate at which lamina is formed is determined by environmental factors and lithification occurs when cyanobacteria growth slows or ceases (Smith et al. 2005).
Research has found that there are three different types of carbonate lamina; thick, thin micritic, and thicker hard. The thick lamina layer is 200 micrometers to several millimeters and composed primarily of unconsolidated carbonate ooids. The thin micritic layer is 10-30 micrometers mainly formed by microcrystalline calcium carbonate whose diameter is less than four micrometers. The layer referred to as thicker hard is 100-300 micrometers are made from fused ooid grains. The three layers are a result of three different microbial communities that are associated with the deposition of a particular form of carbonate (Visscher et al. 2005).
The surface community that is involved in carbonate ooid deposition is a filamentous cyanobacteria, Schizothrix gebeleinii, and are characteristic of rapid growth. This community is responsible for providing the organic carbon for the stromatolite during photosynthesis. It has a characteristic caramel-green layer, but at depth the color is lost and a white lithified layer of unconsolidated ooids results. The thin micritic crust is a result of heterotrophic bacteria embedded in an extracellular secretion and has an underlying layer of filamentous cyanobacteria. This thin layer is seen during periods of slow deposition and can continue at depth. When this condition persists a different community is formed that has the greatest microbial diversity, with carbonate deposition occurring within the ooids creating the fused layer. The color persists for several centimeters below the surface before eventually becoming a gray-green color. (Visscher et al. 2005).
Cyanobacteria are most commonly associated with stromatolites because they are easily identified and visible, but they are not the only microbe in a stromatolite. Recent studies using rRNA from stromatolites in Shark Bay and comparing with clone libraries has revealed more information on the microbial composition. It was found that the microbial communities are diverse and that the surface and interior communities are different. The Shark Bay stromatolites are about 90% Bacteria and about 10% Archaea with no indication of eukaryotic rRNA being found. 19 of the 52 identified bacteria divisions were found in the communities. The most abundant was proteobacteria while cyanobacteria consisted of only a small percentage of the microbial community. When the interior and exteriors were compared the communities in the interior were bacteria and archaea. For the 19 divisions of bacteria identified 12 of them were found on the surface. This is not seen only in Shark Bay but other studies have shown similar percentages. Even though cyanobacteria are not found as high in number as other bacteria they are functionally very important being the main source of primary production (Papineau et al. 2005).
Stromatolites are capable of providing valuable information about past events in geological history. It has been noted that after many of the extinctions that have occurred there appears to be increase in the amount of stromatolites in the fossil record. This trend has been seen in the late Ordovician (Sheehan et al. 2004), early Triassic and early Jurassic (Pruss et al. 2004) Periods among others. During the Ordovician Period stromatolites were confined to more hostile environments such as very salty or acidic waters. Then later in the period there was an extinction event in western North America leading to a microbial resurgence. The resurgence was due to the loss of microbial mat inhibiting animals that had forced the stromatolites into their hostile environments. The Ordovician microbial resurgence that continued into the Silurian lasted for about five million years and corresponded with a loss in diversity of megafauna (Pruss et al. 2004).
Some recent research has been with hypersaline stromatolite environments that serve as elevated hurricane activity and climate change indicators. In periods of high hurricane activity the hypersaline environment, which is about five times normal seawater salinity, experiences more frequent freshening periods. These freshening periods lead to an increase of nutrient cycling and photosynthesis and in turn a proliferation of stromatolites (Paerl et al. 2003). Stromatolite fossils are not only being used to research hurricanes and climate but also in space exploration. These bacterial fossils serve as a template for the search for life on other plants.
Stromatolites persist in our modern world as they did 3.5 billion years ago. By learning about stromatolites that still survive in places in the Bahamas and Shark Bay in Australia we can learn our world’s geological history. Knowing how the stromatolites are formed and how climate affects them can allow for interpretation of the fossils. We already know that a resurgence of stromatolites in an area could be due to increased hurricanes or an indication of extinctions on a global and local scale. While the bacteria may be microscopic when looked at more closely they give a view of the history of the planet Earth.
Leakey, Richard and Lewin Roger. The 6th Extinction patterns of Life and The Future of Humankind. New York. Anchor Books. 1995
Mayr, Ernest, What Evolution is. New York, Basic Books. 2001
Paerl, H.W., Steppe, T.F., Buchan, K.C., and Potts, M., 2003. Hypersaline cyanobacterial mats as indictors of elevated Tropical Hurricane Activity and Associated Climate Change. Ambio. 32, 87-90.
Papineau, D., Walker, J.J., Mojzsis, S.J., and Pace, N.R., 2005. Composition and Structure of Microbial Communities from Stromatolites of Hamelin Pool in Shark Bay, Western Australia. Applied and Environmental Microbiology. 71, 4822-4832.
Pruss, S.B., and Bottjer, D.J., 2004. Late Early Triassic microbial reefs of the Western US: a description and model for their deposition the aftermath of the end-Permian mass extinction. Paleogeography, Paleclimatology, Paleoecology. 211, 127-137.
Sheehan, P.M., and Harris, M.T., 2004. Microbialite resurgence after the late Ordovician extinction. Nature. 430, 75-78.
Smith, A.M., Uken, R., and Thackeray, Z., 2005. Cape Morgan peritidal stromatolites: the origin of lamination. South African Journal of Science 101,107-108.
Visscher, P.T., and Stolz, J.F., 2005. Microbial Mats as bioreactors: populations, processes, and products. Paleogeography, Paleclimatology, Paleoecology. 219, 87-100.
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