Fossil bacteria: Part I
We have always been curious to know about our planet Earth, its ancient flora and fauna, dinosaurs and our ancestors. Archeologists discover a mummy or fossilized leaf and make us more enthusiastic about our past. Have you anytime thought of ancient bacteria? I hope it would be wonderful to know about them. You might be familiar to the words such as fossil fuels, fossilized plants or Neanderthal's footprints, fossilized feces, eggs, burrows or whole insect of trapped inside amber gum 2 billion years ago etc. Similarly, bacteria of ancient time are found as fossils in the nature. Fossils (having been dung up in Latin) are remains of living things from past life generally found trapped in different rock strata. The fossils are of 2 types; Body fossils and Trace fossils. Body fossil is either entire body or a part of the body that had been fossilized. Body fossils such as petrified wood, casts, molds or compressions provide information about anatomy of fossil organism. Trace fossils are also called Ichnofossils and they represent fossilized activities such as trails, burrows or droppings or nesting/niches or byproducts of organism.
The scientific study of fossils is done in paleontology. Bacterial fossils are microfossils as they are microscopic (less than 1micrometer) and the oldest fossils in the world. Micropaleontologists are concerned to study the microfossils with respect to their sampling, observation, formation, preservation and importance. Paleomicrobiology is a special branch of micropaleontology, the finding of tubercule and lepare bacilli in fossilized mummies led to study role of fossil bacteria in pathogenesis and past human microbial infections are especially studied in paleomicrobiology.
Determination of fossil age:
The oldest fossils are of bacteria and they are 3.5 billion years old. The specimen older than 10,000 years is termed as fossil. It is very important to determine the age of fossil. Paleontologists determine fossil age by radioactive decay method (Radiometric dating) using radioactive isotope pairs such as U-235: Pb207, K-40: Ar-40 and C-14: N-14. Radioactive decay is correlated to geological time scale to decide period of the fossil. Geologic time is divided into time units; eons represent the unit of longest duration. Three major eons are: Archean, Proterozoic and Phanerozoic. Each eon is subdivided into era and eras into the periods and epochs. Archean eon is the time from the formation of the Earth until 2.5 billion years ago. Life appeared for the first time on the Earth in the form of prokaryotes during Archean eon. Therefore, fossils from this eon are considered as the oldest fossils. Proterozoic and Phanerozoic eon covers the time span from 2.5 billion to 544 million years and the past 544 million years ago respectively. The fossils from these eons are generally of prokaryotes and eukaryotic organisms. Phanerozoic eon is subdivided into Paleozoic, Mesozoic and Cenozoic eras; for the convenience the eras have been divided into periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian; Triassic, Jurassic, Cretaceous; Quaternary, Tertiary respectively. Since the fossils are usually found in rock strata or layers, determination of time of rock formation by radioactive dating has been the most appropriate way to find age of the fossil and also to build the fossil record.
Detection and Identification of fossils:
Microfossils are very difficult for detection and identification in fossilized locations. Cyanobacteria and bacteria associated with them are some of the fossils of bacteria ever found and characterized. The combination of microbiological, physical, chemical assays and techniques are used to identify the fossils of bacteria. Scanning electron microscopy (SEM), fluorescence microscopy and Transmission electron microscopy (TEM) techniques reveal structural detains of fossilized bacteria. Bacterial fossils appear as rods, filaments or cocci with diameter in microscale usually less than 1μm. Fossilized colonies look like bunch grapes while as fossil mat growth as wavy smooth or ropy textured chert. Cherts are also regarded as fossilized biofilms of bacteria. Cyanobacterial fossils occur as calcified shells or as mats in the form of stromatolites. Thin sections of stromatolites shows presence of cyanobacteria and bacteria trapped in layers. Since it is extremely difficult and rare to find fossilized bacterial bodies, indirect evidences such as past microbial ecosystems, biochemical activities or chemical remains such as cell wall polypeptide and lipids; botryoids, pleoids or the presence of bacteria specific mineral deposits like magnetite, apatite, calcite, halite, silicate, siderite, hematite and phosphorite in the rocks or ocean floor indicates fossilized bacterial activity. Infected ancient bones are also indicative of niche of fossil bacteria. The possible fossil samples are also analysed by molecular and biological tests to detect proteins, DNA or mutagenesis. Fossils are further characterized by powder X-ray diffraction, Sediment pore- water analysis, spectrophotometry, magnetometry and thermal analysis. Laser techniques are employed for cleaning and to avoid destruction of sample during the study. Biohazard testing is recommended for meteorite, lunar and Martian samples before checking them for microbial evidence. Photomicrography contributes additional details for identification of future samples and also preserves the data in the form of photographs. Fossil samples are stored and maintained carefully in fossil museums or collection centers for detail study.
Archaebacteria are also called as fossil bacteria as 'Archae' means ancient. But this is not true. They have been classified as prokaryotes of kingdom Monera similarly like Eubacteria. Archaebacteria and Eubacteria have independent evolutionary history and also exhibit different biochemical, ecological, physiological, genetics and taxonomical properties. So they are not the fossil forms of today's bacteria but distinct prokaryotic microorganisms.
World's fossiliferous sites:
Bacterial fossils are found worldwide in places like marine rocks such as limestone, sandstone; stromatolites, marginalized environments, endolithic sites or mid-ocean ridges. Bacterial fossils are abundant in fossilized locations such as Devonian Hunsruck slates, Jurassic Solnhofen limestone deposits of Germany, Carboniferous Mazon Creeks, Bitter Springs Chert, Bolivian Proterozoic Stromatolites, Burgess Shale, Cambrian Maotianshan Shales, Gunflint Chert, Warrawoona, Hamelin Pool Marine Nature Reserve, Shark Bay of Western Australia, Wadi Kharaza, Egypt, Precambrian rocks of Eastern Siberia, Exuma Cays of Bahamas islands and all precambrian sedimentary rock sites from Canada, Africa, Scotland, Russia, Greenland and USA.
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