Fossil bacteria: Part II
Since bacteria do not possess skeleton or internal structures and being microscopic; the fossil formation or fossilization of bacteria is naturally a very complex process. It is difficult to detect as well as to understand fossil formation and conservation in bacteria. The fossilization in resin (amber) and bones of larger animals like whales and their presence in mummified human remains have been well understood. Among the different types of fossil formation and preservation like permineralization, cast and molding, authigenic mineralization, replacement and crystallization, adpression and bioimmuration; the fossilization in bacteria take place by authigenic mineralization and replacement-crystallization methods. In authigenic mineralization process, the bacterial cell acts as nucleus for the precipitation of minerals like hematite, siderite or pyrite resulting in formation of 3-dimensional cast or nodule. Siderite deposits from Mazon creek were formed by authigenic mineralization. The gradual replacement of bone or any other tissue with another mineral like calcium phosphate (bacterial secretion) followed by crystallization forms the fossilized tissue. Such crystal or structure is called calcite; it also maintains the original internal structure of the fossilized tissue. In vitro fossilization experiments have shown that bacteria are fossilized mostly by precipitation. It involves dense growth of bacterial colonies or biofilm inside the tissue. The organic matter inside the tissue is eaten up by bacteria and forms replica of the tissue. Reducing or anoxic conditions are required to avoid autolysis and form the replica of the tissue. Densely populated bacteria secrete calcium phosphate crystal which provides support to internal organization of tissue and bacteria along with the tissue are fossilized. Another method suggests that bacteria and cyanobacteria are able to trap sediments and metals from the surrounding atmosphere. The metals are deposited inside exopolysaccharide layer of cell wall or the capsular polysaccharide network. These carbohydrate structures are suitable sites for precipitation or deposition of metal ions. Siderites, magnetites or pyrites of fossilized bacteria are thought to be formed by similar way. The formation of stromatolites requires the trapping of aquatic shallow sediments by bacterial and cyanobacterial growth. Calcium phosphate excretions of cyanobacteria are deposited on sediment layer after layer. The layered porous dome like structures is stromatolite; fossilized colonies of bacteria and cyanobacteria can be observed inside the layers of stromatolites.
The living fossils:
The term 'Living Fossil' was coined by Charles Darwin. They are living organisms whose fossils are same as their living specimens. Almost all genera of Cyanobacteria (including Oscillatoria) living today are considered as living fossils. They have not changed morphologically or functionally since past many eras. Even cyanobacteria-bacteria associations are the oldest form of microbial interactions. Stromatolites or a layered dome shaped structure in shallow water formed by cyanobacteria in association with algae is also the best example of living fossil bacterial association. Stromatolites are formed when sediment layer in aquatic environment is trapped by mat growth of cyanobacteria followed by secretion of calcium carbonate; they are called as oncolites if they acquire round shape during their formation. Stromatolites are the oldest living fossils and are abundant in Western Australia.
Examples of fossil bacteria: Usually, whenever a new fossil is unearthed, paleontologists look for presence of bacteria or any signs of microbial activity. Many bacterial fossils have been discovered in association with macrofossils like animals or plants. Bacteria have been found in fossils of whale, dinosaurs, tissues and bones of human mummies or animals and fossilized tree resins or amber. Many scientists have claimed the revival of fossil bacteria trapped inside the resin. They have also been discovered in fossilized animal embryos and soft tissues; actually these bacteria have prevented soft tissues from destruction and preserved them for billion years. Bacterial products such as magnetites, siderites have been found in carboniferous sediments of Lake Chiemsee of Germany, Mazon creek of Illinois and green river from Utah. Prehistoric fossils of cyanobacterial associations were discovered in Massanutten sandstones of Virginia, USA. Scientists from Ohio State University discovered the bacterial clumps in arthropod fossils which they found in Antarctica. Near this site, their team also discovered the rarest of the fossils of Archaebacteria.
Importance of bacterial fossils:
• Tiny bacterial fossils are very important to study Earth's formation, early atmosphere, ancient environment and related evolutionary processes.
• Living bacterial fossils provides a basis for creation or genesis of the species.
• Bacterial fossils are indicators of rhythmic climatic oscillations of past. They show linkage of climatic and evolutionary changes. This is very evident from fossils found in rock strata; alternate arrangement of different layers of sedimentary rocks and their bacterial fossil contents represents response to cold and hot climate changes during particular time.
• Detection of magnetite, pyrite, silicate or phosphorite indicates biological activity and atmospheric conditions of the past. This is because these minerals could be deposited under certain climatic conditions and required for the survival of organisms.
• Presence of bacterial fossils in moon, Martian rock or meteorite is the evidence of extraterrestrial life. The comparative study of early life on the Earth and the Mars would be very significant to know Earth's evolutionary history.
• As the organism die, bacteria carry out degradation of organic matter until its complete decay. This decomposition was once thought to be hazardous for fossilization process; in fact bacteria were enemies of the fossils. Recently, it has been proved that bacteria help in the preservation of large fossil structures. Bacteria aid in the fossilization and preservation of soft tissues (known as Lagerstatten) by secreting calcium phosphate in reducing atmosphere. Bacterial mineral precipitation also helps in the preservation of bony structures. The fossil of clam like creature, about 180 million years old unearthed in Antarctica by geologist of Ohio State University is best example of fossil preserved by bacteria.
• Paleomicrobiology, a sub-branch of paleontology is very important field of research work regarding microbiological diseases of the past, their epidemics, evolution of human pathogens and anthropology.
• Fossils like that of whales or dinosaurs provide evidence of microbial activities from variety of geologic ages and environments.
• The stromatolites from Archean and Proterozoic eon contain massive mat growth of cyanobacteria and it is speculated that their photosynthetic process was responsible for oxygenation of Earth's early atmosphere.
World's Fossil Museums: Enlisted below are some of the world's famous fossil museums and fossil collection centers where you are able to see bacterial fossils.
o Natural History Museum, London
o Mt. Blanco Fossil Museum, Texas
o Fossil Collection, Western Australian Museum
o Museum of Paleontology, University of Michigan
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