Preventing the Forward Contamination of Mars (2006) / Chapter Skim
Currently Skimming:
5 Expanding Our Knowledge of the Limits of Life on Earth
5 Expanding Our Knowledge of the Limits of Life on Earth
Pages 69-90
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 69... ...
Over the ensuing 300 years, biologists relied on microscopy, nutrients required for growth, and biochemical characterization to describe microbial diversity. Differences in morphology, staining characteristics, metabolic capabilities, and physiological properties defined boundaries between different kinds of microorganisms.2 Serial dilution assays3 and colony 1"Extreme environment" is an anthropocentric term that refers to physical and chemical conditions outside what was once perceived to be hospitable to life.
|
From page 70... ...
These modest assessments of microbial diversity and population size were not consistent with a ~3.5 billion-year evolutionary history, during which single-cell organisms have developed an enormous metabolic repertoire to cope with Earth's dynamic environment. Such underestimates of microbial diversity reflect how difficult it is to identify morphological and biochemical characteristics (phenotypic traits)
|
From page 71... ...
Barophile Obligate barophiles are unable to grow at 1 atmosphere of pressure; barotolerant bacteria grow at 1 atmosphere and higher pressures; all barophiles grow optimally under high pressure Physiological Diversity Aerobe Capable of using oxygen as terminal electron acceptor; can tolerate levels of oxygen at or greater than 21 percent and has a strictly respiratory-type metabolism Anaerobe Grows only in the absence of oxygen; most have fermentative-type metabolism, but some carry out anaerobic respiration using terminal electron acceptors other than oxygen Facultative anaerobe Can grow aerobically or anaerobically Microaerophile Capable of oxygen-dependent growth at oxygen levels well below 21 percent Autotroph Uses carbon dioxide as its sole source of carbon Heterotroph Unable to use carbon dioxide as a sole source of carbon and requires one or more organic compounds Chemoorganoheterotroph Derives energy from chemical compounds and uses organic compounds as a reductant Chemolithoautotroph Relies on reduced chemical compounds as a source of energy and carbon dioxide as a source of carbon; includes hydrogen bacteria, iron bacteria, sulfur bacteria, ammonia oxidizers, nitrite oxidizers obligate methane oxidizers, carbon monoxide oxidizers Mixotroph Capable of growing both chemoorganoheterotrophically and chemolithoautotrophically Oligotroph Capable of growth on minimal media (1 to 15 µg carbon per liter) Copiotroph Requires nutrients at levels 100 times those of oligotrophs SOURCE: Madigan et al.
|
From page 72... ...
Current molecular databases contain more than 120,000 reference rRNA sequences (phylotypes) from diverse microbial forms.6 This window on the microbial world has revealed new levels of largely unexplored microbial diversity not represented in laboratory cultures.
|
From page 73... ...
. Studies of microbial diversity in polar oceans have shown, in addition to psychrophilic bacteria (Wells and Deming, 2003; Huston et al., 2004)
|
From page 74... ...
If the inoculum consisted of 1,000 initial cells, the corresponding required generation time to reach 100 cells per milliliter in 50 years would be 1.6 years. Organisms with generation times shorter, or even much shorter, than a year under martian conditions -- should such combinations of terrestrial microorganisms and martian habitats exist -- could pose a more substantial threat to planetary protection over a period of decades.
|
From page 75... ...
freeze completely during winter when solar radiation is low or absent. As with permanent lake ice assemblages, photosynthesis and N2 fixation by algae and cyanobacteria supply sufficient reduced carbon and nutrients to support the development of complex microbial ecosystems (Christner et
|
From page 76... ...
a close-up of the surface of the ice cover showing aeolian sediment accumulation, (C) an ice core from 2 m beneath the surface of the ice cover showing sediment accumulation, and (D)
|
From page 77... ...
A phylogenetic survey of a cryoconite found in the McMurdo Dry Valleys of Antarctica showed that these ecosystems are inhabited by species quite similar to those in adjacent microbial mat and lake ice
|
From page 78... ...
As with the permanent lake ice in the polar deserts of Antarctica, cryoconites serve as biological refuges in an environment that would appear to be inhospitable for life. Glacial Ice Earth's expansive polar ice caps cover ~10 percent of the terrestrial surface with ice and contain ~70 percent of the freshwater on the planet (Patterson, 1994)
|
From page 79... ...
SB12k-2-2 (glacial ice -- Sajama, Bolivia) Exiguobacterium undae Friedmannella antarctica (sandstone -- McMurdo Dry Valleys, Antarctica)
|
From page 80... ...
. The psychrophilic and psychrotrophic isolates shown in Figure 5.3 originate from locations ranging from aquatic and marine ecosystems to terrestrial soils and glacial ice, with little in common between these environments except that all are permanently cold or frozen.
|
From page 81... ...
have not been detected in this spring, they are found throughout Yellowstone National Park and terrestrial thermal springs worldwide. All micrographs are 1000×.
|
From page 82... ...
and thus were among the first in which the impressive diversity of uncultivated microbial populations in nature was revealed. This has been a typical finding in 16S rRNA gene surveys of microbial diversity in numerous habitats (e.g., Bintrim et al., 1997; Borneman and Triplett, 1997)
|
From page 83... ...
. At first, the microbial diversity in hot environments was thought to be relatively low, owing to what was perceived as an extreme environment, a conclusion based primarily on the limited information provided by culturing methods.
|
From page 84... ...
For these reasons, 3 it seems unlikely that Pg for an arbitrary organism would exceed 10 , and in fact it could be much lower, given the 3 potential hostility of many martian environments. Given sufficient resources, molecular techniques could be used to assess microbial diversity D on the spacecraft (see Chapter 6)
|
From page 85... ...
However, if we were to gain detailed knowledge of the microbial populations on spacecraft (using the techniques described in Chapter 6) , and further knowledge of the martian surface and subsurface, most terms in Equation 5.1 might be confidently set to zero, because PiS and Pig may well be extremely close to zero for nearly all kinds i of microorganisms on spacecraft.
|
From page 86... ...
1997. Molecular microbial diversity in soils from eastern Amazonia: Evidence for unusual microorganisms and microbial population shifts associated with deforestation.
|
From page 87... ...
1998. Microbial diversity in a hydrocarbon- and chlorinated-solvent-contaminated aquifer undergoing intrinsic bioremediation.
|
From page 88... ...
1997. A molecular view of microbial diversity and the biosphere.
|
From page 89... ...
1998. A natural view of microbial diversity within hot spring cyanobacterial mat communities.
|
From page 90... ...
1981. Algae in cryoconite holes on Canada Glacier in southern Victoria Land, Antarctica.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.