Downloaded By: [University of Alabama at Birmingham] At: 16:09 17 July 2007
Critical Reviews in Microbiology, 33:183–209, 2007 c Informa Healthcare Copyright ISSN: 1040-841X print / 1549-7828 online DOI: 10.1080/10408410701451948
Microbial Extremophiles at the Limits of Life Elena V. Pikuta and Richard B. Hoover National Space Sciences and Technology Center/NASA, Astrobiology Laboratory, Huntsville, Alabama, USA
Jane Tang Noblis, Falls Church, Virginia, USA
Prokaryotic extremophiles were the first representatives of life on Earth and they are responsible for the genesis of geological structures during the evolution and creation of all currently known ecosystems. Flexibility of the genome probably allowed life to adapt to a wide spectrum of extreme environments. As a result, modern prokaryotic diversity formed in a framework of physico-chemical factors, and it is composed of: thermophilic, psychrophilic, acidophilic, alkaliphilic, halophilic, barophilic, and radioresistant species. This artificial systematics cannot reflect the multiple actions of different environmental factors since one organism could unite characteristics of several extreme-groups. In this review we show the current status of studies in all fields of extremophiles and summarize the limits of life for different species of microbial extremophiles. We also discuss the finding of extremophiles from unusual places such as soils, and briefly review recent studies of microfossils in meteorites in the context of the significance of microbial extremophiles to Astrobiology. Keywords
Extremophiles; Microbial diversity; Astrobiology; Ecology of Microorganisms; Limits of Life
INTRODUCTION During Earth’s evolution, accompanied by geophysical and climatic changes, a number of ecosystems have been formed. These ecosystems differ by the broad variety of physicochemical and biological factors composing our environment. Traditionally, pH and salinity are considered as geochemical extremes, as opposed to temperature, pressure, and radiation that are referred to as physical extremes (Van den Burg 2003). Life inhabits all
Received 2 February 2007; accepted 10 May 2007. We want to thank the reviewers for their helpful comments and the NASA/MSFC Center Director’s Discretionary Fund and the NASA/JSC Center for Biomarkers in Astromaterials for support of this research. Address correspondence to Elena V. Pikuta, Richard B. Hoover National Space Sciences and Technology Center/NASA, VP-62, 320 Sparkman Dr., Astrobiology Laboratory, Huntsville, AL 35805. E-mail:
[email protected] or
[email protected]
possible places on Earth interacting with the environment and within itself (cross species relations). In nature it is very rare when an ecotope is inhabited by a single species. As a rule, most ecosystems contain the functionally related and evolutionarily adjusted communities (consortia and populations). In contrast to the multicellular structure of eukaryotes (tissues, organs, systems of organs, whole organism), the highest organized form of prokaryotic life in nature is presented by the benthic colonization in biofilms and microbial mats. In these complex structures all microbial cells of different species are distributed in space and time according to their functions and to physicochemical gradients that allow more effective system support, self-protection, and energy distribution. In vitro, of course, the most primitive organized structure for bacterial and archaeal cultures is the colony, the size, shape, color, consistency, and other specific characteristics which differ on the species or subspecies levels. In Table 1 all known types of microbial communities are shown (Pikuta et al. 2005b). Additional factors could be added to this classification Table 1: in deep-sea ecosystems (pressure), and in deep underground lithospheric ecosystems (pressure and radiation). Currently the best-studied ecosystems are: human body (due to the medical importance), and freshwater and marine ecosystems (because of environmental concerns). For a long time, extremophiles were terra incognita, since the environments with aggressive parameters (compared to the human body temperature, pH, mineralization, and pressure) were considered a priori as a dead zone. It took time to find out that the environments with extreme physico-chemical and climatic parameters are inhabited by a wide spectrum of different microorganisms. Extremophiles were discovered in the following chronological order: Long ago it was known that many fungi could grow in slightly acidic (pH 4–6) conditions, but the first obligately acidophilic bacterium to be described was Acidithiobacillus ferrooxidans (formally Thiobacillus ferrooxidans). Subsequently thermophilic lithotrophic acidophiles were found, and the hyperacidophilic species of the genus Picrophilus growing at negative pH values were described in 1996 (Schleper et al. 1996).
183
Downloaded By: [University of Alabama at Birmingham] At: 16:09 17 July 2007
184
E. V. PIKUTA ET AL.
TABLE 1 Known types of microbial communities Types of communities 1. Freshwater psychrophilic 2. Freshwater, meso-thermal 3. Freshwater moderately thermophilic 4. Freshwater thermophilic 5. Marine psychrophilic 6. Marine, meso-thermal 7. Marine moderately thermophilic 8. Marine thermophilic
NaCl, Temperature, ◦ C pH %(w/v) 5–7
0–1
