5
Bacteria and Marine Biogeochemistry
180
biologically catalyzed process. Chemically catalyzed processes, in contrast, tend to have a continuous increase in rate with increasing temperature. Although the sulfate reduction was slow at
<0°C this does not mean that the bacteria do not
function well at low temperature. On the contrary,
the Arctic sulfate reducers had their highest
growth efficiency (i.e. highest biomass production
per amount of substrate consumed) at around 0°C,
in contrast to sulfate reducers from temperate
environments which have their highest growth
efficiency at the warm temperatures experienced
during summer (Isaksen and Jørgensen 1996;
Knoblauch and Jørgensen 1999).
The radiotracer method of measuring sulfate
reduction in sediments (see Sect. 5.6) is a
sensitive tool to demonstrate the temperature
strains among the environmental microorganisms.
In 54-71°C warm sediment from the hydrothermal
sediments of the Guaymas Basin, Gulf of
California, two main groups of sulfate reducers
could be discriminated from their temperature
optima: a) moderate thermophiles with an optimum
at 60°C, b) extreme thermophiles with optimum at
80-90°C (Fig. 5.8B). Moderately thermophilic
sulfate reducers are well known from pure cultures
of Desulfotomaculum, whereas extreme thermophiles with optimum at 85°C are known among the
genus, Archaeoglobus (Stetter 1988). Such
prokaryotic organisms have been isolated from
hydrothermal environments and have also been
found in oil reservoirs 3000 m below the seabed
where the temperature is up to 110°C and the
hydrostatic pressure up to 420 bar.
Surprisingly, sulfate reducers adapted to the
normal low temperatures of the main sea floor
were also unknown until recently. Such coldadapted (psychrophilic) bacteria are generally
scarce among the culture collections in spite of
their major biogeochemical significance. This is
partly because of their slow growth which makes
them difficult to isolate and cumbersome to study.
A number of psychrophilic sulfate reducers were
recently isolated which have temperature optima
down to 7°C and are unable to grow above 1015°C because it is too hot (Knoblauch et al. 1999).
These examples demonstrate a general problem
in marine microbiology, namely that the large
prokaryotic diversity comprises adaptations to
very diverse environments and that only a very
small fraction, maybe less than one percent, of the
bacterial species in the ocean is known to science
today. Many of the unknown microorganisms may
be among the biogeochemically very important
species. The estimate is based on recent
methodological advances in molecular biology
that have made it possible to analyze the diversity
of natural prokaryotic populations, including the
large majority that has still not been isolated or
studied.
5.3.4
Other Regulating Factors
The major area of the sea floor lies in the deep sea
below several thousand meters of water where an
enormous hydrostatic pressure prevails. Since the
pressure increases by ca 1 bar (1 atm) for every
10 meters, bacteria living at 5000 m depth must be
able to withstand a pressure of 500 bar (50 MPa).
Microorganisms isolated from sediments down to
3000-4000 m have been found to be preferentially
barotolerant, i.e. they grow equally well at sea
surface pressure as at their in situ pressure. At
depths exceeding 4000 m the isolated bacteria
become increasingly barophilic, i.e. they grow
optimally at high pressures, and bacteria isolated
from deep sea trenches at 10,000 m depth were
found to grow optimally at 700-1000 bar (Yayanos
1986). Many barophilic bacteria are also psychrophilic in accordance with the low temperature of
1-4°C prevailing in the deep sea (DeLong et al.
1997). They appear to grow relatively slowly
which may be an adaptation to low temperature
and low nutrient availability rather than a direct
effect of high pressure. The degradation of organic material appears to be just as efficient in
the deep sea as in shallower water, since only a
few percent of sedimenting detritus resist mineralization and are buried deep down into the
sediment.
5.4
Energy Metabolism of
Prokaryotes
Microorganisms can be considered 1 µm large
bags of enzymes in which the important biogeochemical processes are catalyzed. This analogy,
however, is too crude to understand how and why
these microscopic organisms drive and regulate
the major cycles of elements in the ocean. This
requires a basic knowledge of their energy
metabolism and physiology.
The cells use the chemical energy of organic or
inorganic compounds for cell functions which
Bacteria and Marine Biogeochemistry
180
biologically catalyzed process. Chemically catalyzed processes, in contrast, tend to have a continuous increase in rate with increasing temperature. Although the sulfate reduction was slow at
<0°C this does not mean that the bacteria do not
function well at low temperature. On the contrary,
the Arctic sulfate reducers had their highest
growth efficiency (i.e. highest biomass production
per amount of substrate consumed) at around 0°C,
in contrast to sulfate reducers from temperate
environments which have their highest growth
efficiency at the warm temperatures experienced
during summer (Isaksen and Jørgensen 1996;
Knoblauch and Jørgensen 1999).
The radiotracer method of measuring sulfate
reduction in sediments (see Sect. 5.6) is a
sensitive tool to demonstrate the temperature
strains among the environmental microorganisms.
In 54-71°C warm sediment from the hydrothermal
sediments of the Guaymas Basin, Gulf of
California, two main groups of sulfate reducers
could be discriminated from their temperature
optima: a) moderate thermophiles with an optimum
at 60°C, b) extreme thermophiles with optimum at
80-90°C (Fig. 5.8B). Moderately thermophilic
sulfate reducers are well known from pure cultures
of Desulfotomaculum, whereas extreme thermophiles with optimum at 85°C are known among the
genus, Archaeoglobus (Stetter 1988). Such
prokaryotic organisms have been isolated from
hydrothermal environments and have also been
found in oil reservoirs 3000 m below the seabed
where the temperature is up to 110°C and the
hydrostatic pressure up to 420 bar.
Surprisingly, sulfate reducers adapted to the
normal low temperatures of the main sea floor
were also unknown until recently. Such coldadapted (psychrophilic) bacteria are generally
scarce among the culture collections in spite of
their major biogeochemical significance. This is
partly because of their slow growth which makes
them difficult to isolate and cumbersome to study.
A number of psychrophilic sulfate reducers were
recently isolated which have temperature optima
down to 7°C and are unable to grow above 1015°C because it is too hot (Knoblauch et al. 1999).
These examples demonstrate a general problem
in marine microbiology, namely that the large
prokaryotic diversity comprises adaptations to
very diverse environments and that only a very
small fraction, maybe less than one percent, of the
bacterial species in the ocean is known to science
today. Many of the unknown microorganisms may
be among the biogeochemically very important
species. The estimate is based on recent
methodological advances in molecular biology
that have made it possible to analyze the diversity
of natural prokaryotic populations, including the
large majority that has still not been isolated or
studied.
5.3.4
Other Regulating Factors
The major area of the sea floor lies in the deep sea
below several thousand meters of water where an
enormous hydrostatic pressure prevails. Since the
pressure increases by ca 1 bar (1 atm) for every
10 meters, bacteria living at 5000 m depth must be
able to withstand a pressure of 500 bar (50 MPa).
Microorganisms isolated from sediments down to
3000-4000 m have been found to be preferentially
barotolerant, i.e. they grow equally well at sea
surface pressure as at their in situ pressure. At
depths exceeding 4000 m the isolated bacteria
become increasingly barophilic, i.e. they grow
optimally at high pressures, and bacteria isolated
from deep sea trenches at 10,000 m depth were
found to grow optimally at 700-1000 bar (Yayanos
1986). Many barophilic bacteria are also psychrophilic in accordance with the low temperature of
1-4°C prevailing in the deep sea (DeLong et al.
1997). They appear to grow relatively slowly
which may be an adaptation to low temperature
and low nutrient availability rather than a direct
effect of high pressure. The degradation of organic material appears to be just as efficient in
the deep sea as in shallower water, since only a
few percent of sedimenting detritus resist mineralization and are buried deep down into the
sediment.
5.4
Energy Metabolism of
Prokaryotes
Microorganisms can be considered 1 µm large
bags of enzymes in which the important biogeochemical processes are catalyzed. This analogy,
however, is too crude to understand how and why
these microscopic organisms drive and regulate
the major cycles of elements in the ocean. This
requires a basic knowledge of their energy
metabolism and physiology.
The cells use the chemical energy of organic or
inorganic compounds for cell functions which