Size Limits of Very Small Microorganisms Proceedings of a Workshop (1999) / Chapter Skim
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Pages 85-106
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From page 85... ...
This is straightforward in principle, but it requires that we understand the limits of pattern formation by biologi cal and physical processes.
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From page 86... ...
On the other hand, there is no assurance that terrestrial life exhausts the possibilities of biological pattern generation; therefore, knowing the limits of pattern formation by physical processes may provide the best yardstick for evaluating martian or other extraterrestrial samples.
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From page 87... ...
Martian samples may actually be better preserved than terrestrial sediments of comparable age, but lack both modern martian organisms for comparison and a more or less continuous fossil record that connects the present with early planetary history. · A better understanding of biological signatures in sedimentary rocks is needed, and it is needed before intelligently collected martian samples are returned to Earth.
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From page 88... ...
But the still-simmering controversy might have been avoided had three lessons now learned from the episode been heeded from the outset: First, the search for evidence of past life, whether in Earth rocks or extraterrestrial samples, must be multidisciplinary at a minimum, based on the techniques and findings of biologists, paleobiologists, geologists, and geochemists or, better still, interdisciplinary, carried out by researchers schooled in both the life and physical sciences. Second, the evidence sought should be positive evidence that affirms the biologic origin of the features detected.
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From page 89... ...
(For example, are the features embedded in the rock matrix rather than being surficial contaminants, a test that can be met for minerals and fossils by studies of petrographic thin sections and for biologic isotopic signatures by ion microprobe analyses in situ.)
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From page 90... ...
Transmission and scanning electron microsocopy have been used to characterize microfossils previously detected in macerations or thin sections, but have not proven to be reliable detection techniques. Optical Microscopy of Macerations.
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From page 91... ...
Optical Microscopy of Petrographic Thin Sections. In petrographic thin sections, the other type of preparation used for optical microscopic detection of ancient microfossils, fossils are detected within the rock, so indigenousness can be demonstrated and the possibility of laboratory contamination ruled out.
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From page 92... ...
Transmission electron microscopy of macerated microfossils has been used to elucidate the structure of cell walls, membranes, and internal organic contents (Schopf and Oehler, 1976~. But because macerations are susceptible to contamination, such studies are useful only of fossils previously detected in petrographic thin sections.
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From page 93... ...
1996. Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001.
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From page 94... ...
Distinguishing biominerals from their inorganic counterparts has also proven quite difficult in fossil materials, and the biological interpretation of isotopic and organic chemical evidence can also be inconclusive, especially for rocks that have undergone significant diagenesis. The challenge of establishing biogenicity in ancient materials is illustrated by recent debates over the origin of features in Martian meteorite ALH 84001 (McKay et al.
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From page 95... ...
In the present context, an understanding of common taphonomic modes in microbial fossilization has value in providing additional criteria for assessing biogenicity. Preservational Modes in Precambrian Marine and Lacustrine Environments In Precambrian paleontology, studies of organically preserved cellular remains have provided the most complete paleobiological picture of early microbial life.
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From page 96... ...
Sample from a silicified limestone of the Belcher Island Group, Canada. Early silicification of some laminae enhanced organic matter preservation (compare 1 and 2 on image)
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From page 97... ...
Taphonomic Trends in Siliceous Thermal Springs As illustrated by the Belcher Island example, the microbial fossil record is strongly biased toward organisms that possess degradation-resistant cell walls and extracellular envelopes (sheaths and capsules)
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From page 98... ...
But, in the absence of thick cell walls or extracellular sheaths, entombed organic matter is rapidly decayed away, leaving behind only external molds that are rapidly infilled with opaline silica. During recrystallization there is a tendency for grain size to increase.
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From page 99... ...
It is formed under anaerobic conditions by the reaction of iron-bearing detrital minerals in sediments and H2S produced by bacterial sulfate reduction (Canfield and Raiswell 1991~. Some of the most spectacular examples of microbial fossilization involve the permineralization of organic materials by phosphate minerals (e.g., Xiao et al.
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From page 100... ...
biases place different constraints on the lower size limit for fossil microbes. Comparative taphonom~c studies of the Precambrian fossil record and modern analogs indicate a strong preservational bias, favoring higher order biosedimentary structures and biofabrics.
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From page 101... ...
1996. Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH 84001.
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From page 102... ...
1990. Microbial control of silver mineralization at a seafloor hydrothermal site on the northern Gorda Ridge.
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From page 103... ...
The essential elements of an electron microscope are a high-vacuum column, an electron gun (a thermal or field emission electron emitter) , a system of magnetic lenses to focus the electrons before (and after)
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From page 104... ...
When imaging nanometer-sized features on a coated surface, great care must be taken to distinguish indigenous surface microstructures from those caused or accentuated by application of the conductive coating. The problem of conductive coating artifacts is particularly problematical with the new generation of field emission scanning electron microscope (FE SEM)
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From page 105... ...
, electron diffraction, and electron energy-loss spectroscopy. A STEM with a field emission electron gun (FE STEM)
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From page 106... ...
associated with potential biominerals may be indicators of past biogenic activity [11. Although molecular species cannot be directly detected using electron microscopy, it is possible to probe the local (atomic and molecular)
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