Extreme Environments and Biotopes
Research during the past thirty years has demonstrated that life's
boundaries reach far beyond the conditions for human existence (Rusterholtz
and Pohlschroder, 1999; Rothschild and Mancinelli, 2001). In human
view of life, microorganisms inhabit "harsh" environments, defined
by a wide range of environmental factors: temperature, pressure, pH,
water availability, salinity, radioactivity, and nutrient source.
These habitats exhibit extremes (Horneck, 2000) in temperature (-20oC
to 113oC), pH (1-11), salinity (>25% NaCl), high pressure (up to
110 MPa, or megapascal), etc. Microorganisms thriving in these environments
are called extremophiles (Madigan and Marrs, 1997), which encompasses
thermo-, psychro-, alkali-, acido-, halo-, and piezophiles (Cowan,
1998; Stetter, 1999). Indeed, extremophiles do not just tolerate environmental
extremes, but actually require the extreme conditions for growth (Madigan
and Marrs, 1997; Madigan and Oren, 1999).
The hydrostatic pressure is unique to the ocean and is a function
of the weight of water above a surface at a given depth, and increases
1 atm or 0.103 MPa (0.1 MPa = 0.9869 atm = 14.7 PSI) for every 10
m depth. The deep-sea is generally defined as waters of 1,000 m. Thus,
the deep-sea constitutes 75% of the ocean's total volume and comprises
~62% of the biosphere (Jannasch and Taylor, 1984). Therefore, much
of the ocean is dominated by the high hydrostatic pressure which could
have a profound impact on deep-sea microbiology.
Piezophilic bacteria are defined as those shown optimal growth at
a pressure higher than 0.1 MPa or showing a requirement for increased
pressure for growth (Yayanos, 1995). DeLong et al. (1997) reported
that eleven cultivated psychrophilic and piezophilic deep-sea bacteria
are affiliated with one of five genera within the gamma-subgroup:
Shewanella, Photobacterium, Colwellia, Moritella, and an
unidentified genus.
One of the major constituents in bacterial membrane is the bilayers
of phospholipid molecules, which constitute some 40-70% of the total
membrane dry weight in bacteria. The glycerol phosphates represent
the predominant structural lipid class in bacterial cell membranes
and consist of fatty acid esters of sn-glycerol-3-phosphate. The dynamic
states of lipids (the fluidity and the order) are closely related
to the functions of biological membranes. Pressure effects on lipid-based
cell membrane systems. Increasing pressure, like a reduction in temperature,
tends to solidify or "freeze-out" phospholipids, which leads
to a disruption of biological functions of lipid-based cell membrane.
Fang et al. (2000, 2002) reported the detection of unique phospholipids
and polyunsaturated fatty acids (PUFA) C20:5 fatty acid in extremely
piezophilic bacteria DB21MT-2 and both C20:5 and C22:6 in extremely
piezophilic bacteria DB21MT-5. These extremely piezophilic bacteria
were isolated from sediments from the Mariana Trench (Kato et al.,
1998). The vital roles of unsaturated fatty acids in regulating membrane
fluidity under high pressures have been implicated in a number of
studies.
Go to current research in (a) fatty
acid biosynthesis and stable carbon isotope fractionation; (b)
Biosynthesis
and dietary uptake of PUFAs; and (c) microbial
diversity and biogeochemistry of piezophilic bacteria in deep-sea
sediments.
References Cited:
DeLong, E. F., Franks, D. G., and Yayanos, A. A., 1997. Appl.
Environ. Microbiol. 63, 2105-2108.
Jannasch H.W. and Taylor C.D., 1984. Ann. Rev. Microbiol.
38, 487-514.
Kato C., Li L., Nogi Y., Nakamura Y., Tamaoka J. and Horikoshi K.,
1998. Appl. Environ. Microbiol. 64, 1510-1513.
Fang, J., Barcelona, M.J., Kato, C. and Nogi, Y., 200. Deep-Sea
Research I 47, 1173-1182.
Fang, J., Barcelona, M. J., Abrajano, T. A., Jr., Kato, C., Nogi,
Y. 2002. Mar. Chem. 80, 1-9.
Yayanos, A. A., 1995. Ann. Rev. Microbiol. 49, 777-805
