47
Multiphasic transport systems for i-leucine persisted in an
assemblage of marine bacteria which had undergone 30 divisions in a
continuous culture fed with particle-free unenriched seawater
(Hagstrom et al., in press).
(Km + Sn) varied from 3 nM to 230 nM
(Sn = substrate concentration). The high Km (and high Vmax ) systems
are either constitutive or may be induced by high t-leucine concentrations which could occur in the periplasmic space of gram-negative
bacteria during protein hydrolysis (Hollibaugh and Azam, 1983).
Considering the high metabolic cost of maintaining a transport system,
it would appear that planktonic marine bacteria must frequently
experience high t-leucine concentrations in their microenvironment for
such high Km transport systems to be constitutive.
We asked whether the seawater DOM pool could support the growth
of planktonic bacteria in the absence of POM. Using 0.2 ~m Nuclepore
filtered seawater as growth medium we were able to grow mixed
assemblages of bacterioplankton in batch or continuous cultures at
doubling times from 6h to 39h (Ammerman et al., in press; Hagstrom et
al., in press). These growth rates are in the range observed for
natural populations of marine bacteria (Fuhrman and Azam, 1980, 1982).
It appears therefore that at least some bacteria in seawater are able
to grow at the expense of the bulk-phase concentrations of DOM (and
without exposure to high nutrient microzones around particulate
sources of DOM). Growth in enriched microzones could be more rapid.
It appears that we need to revise our views regarding seawater
DOM as a medium for bacterial growth. Clearly, seawater DOM is not too
dilute to support bacterial growth; it is the bacteria that maintain
the utilizable DOM (UDOM) components at the exceedingly low levels
found in seawater. New inputs of nutrients, possibly at high concentrations in a production microzone, are effectively utilized by high
Km' high V max uptake systems. The nutrients which do diffuse into the
bulk-phase are scavenged by the high affinity low capacity systems.
The result is a tight coupling between the production of UDOM and its
uptake by bacteria, so these compounds do not accumulate. This view
is supported by the observation that DFAA levels in the euphotic zone
vary within an order of magnitude during a die 1 cycle despite rapid
flux through the DFAA pool (Mopper and Lindroth, 1982). In another
study (Ammerman and Azam, 1981) die 1 excursions of the 3',5' -cyclic
AMP (cAMP) concentration in seawater (in the picomolar range) were
apprently kept in. check by an extremely high affinity transport system
with a Km of a few picomolar.
Multiphasic transport systems for i-leucine persisted in an
assemblage of marine bacteria which had undergone 30 divisions in a
continuous culture fed with particle-free unenriched seawater
(Hagstrom et al., in press).
(Km + Sn) varied from 3 nM to 230 nM
(Sn = substrate concentration). The high Km (and high Vmax ) systems
are either constitutive or may be induced by high t-leucine concentrations which could occur in the periplasmic space of gram-negative
bacteria during protein hydrolysis (Hollibaugh and Azam, 1983).
Considering the high metabolic cost of maintaining a transport system,
it would appear that planktonic marine bacteria must frequently
experience high t-leucine concentrations in their microenvironment for
such high Km transport systems to be constitutive.
We asked whether the seawater DOM pool could support the growth
of planktonic bacteria in the absence of POM. Using 0.2 ~m Nuclepore
filtered seawater as growth medium we were able to grow mixed
assemblages of bacterioplankton in batch or continuous cultures at
doubling times from 6h to 39h (Ammerman et al., in press; Hagstrom et
al., in press). These growth rates are in the range observed for
natural populations of marine bacteria (Fuhrman and Azam, 1980, 1982).
It appears therefore that at least some bacteria in seawater are able
to grow at the expense of the bulk-phase concentrations of DOM (and
without exposure to high nutrient microzones around particulate
sources of DOM). Growth in enriched microzones could be more rapid.
It appears that we need to revise our views regarding seawater
DOM as a medium for bacterial growth. Clearly, seawater DOM is not too
dilute to support bacterial growth; it is the bacteria that maintain
the utilizable DOM (UDOM) components at the exceedingly low levels
found in seawater. New inputs of nutrients, possibly at high concentrations in a production microzone, are effectively utilized by high
Km' high V max uptake systems. The nutrients which do diffuse into the
bulk-phase are scavenged by the high affinity low capacity systems.
The result is a tight coupling between the production of UDOM and its
uptake by bacteria, so these compounds do not accumulate. This view
is supported by the observation that DFAA levels in the euphotic zone
vary within an order of magnitude during a die 1 cycle despite rapid
flux through the DFAA pool (Mopper and Lindroth, 1982). In another
study (Ammerman and Azam, 1981) die 1 excursions of the 3',5' -cyclic
AMP (cAMP) concentration in seawater (in the picomolar range) were
apprently kept in. check by an extremely high affinity transport system
with a Km of a few picomolar.