46
transport systems.
Phytoplankton organic matter becomes available for bacterial
uptake by a number of mechanisms, either directly from algae or from
herbivore feces (Williams, 1981). Healthy algae apparently exude a
certain (variable) fraction of the photosynthate, although the extent
and the nature of algal exudation is somewhat controversial (Sharp,
1977). Metabolic pools of algae might at times be released into seawater during predation on the alga by a herbivore (Lampert, 1978).
Senescent algae release DOH into seawater via leakage and autolysis.
Finally, animal feces and excretions introduce DOM into seawater. The
relative contribution of these processes to DO!f input is not known with
certainty. It is important to note however that all DOM sources are
particulate, hence DOH production events would create microzones of
high DOM concentrations in the vicinity of the source. The non-random
distribution of nutrient molecules may have significant implications
for nutrient uptake kinetics of bacteria. Given the above scenario of
DOM production, how do bacteria in the sea metabolically interact with
the organic matter in their environment to optimize nutrient u?take
and growth?
Uptake Kinetic~ of Directly Utilizable Compounds
It has been generally thought that planktonic marine bacteria
have evolved very high affinity membrane transport systems for the
uptake of dissolved nutrients present in nanomolar concentrations in
the seawater (Wright and Burnison, 1979). Azam and Hodson (1981)
showed that assemblages of marine bacteria exhibited multiphasic
uptake kinetics for D-glucose uptake, with Km range of 10- 9 to 10- 4 M.
They reasoned that high Km (high V max ) uptake systems indicate the
presence of microzones of high substrate concentration in a
bacterium's microenvironment. Nissen et al. (in press) studied
glucose uptake kinetics in a marine bacterial isolate, LNB-155. They
found evidence of a single transport system for glucose uptake which
changed its Km (range: 10-a to 10- 3 !l) in an all-or-none fashion at
certain critical concentrations of glucose in the medium. They
suggest that a mUltiphasic transport system would provide metabolic
flexibility for a bacterium living in a microenvironment where very
low (nanomolar) to high substrate concentrations might be encountered.
Since low Km' low Vmax systems saturate at low substrate concentrations, the presence of high Km (and high V max ) transport systems
allows enhanced rates of substrate uptake over a broad range of
environmental substrate concentrations.
transport systems.
Phytoplankton organic matter becomes available for bacterial
uptake by a number of mechanisms, either directly from algae or from
herbivore feces (Williams, 1981). Healthy algae apparently exude a
certain (variable) fraction of the photosynthate, although the extent
and the nature of algal exudation is somewhat controversial (Sharp,
1977). Metabolic pools of algae might at times be released into seawater during predation on the alga by a herbivore (Lampert, 1978).
Senescent algae release DOH into seawater via leakage and autolysis.
Finally, animal feces and excretions introduce DOM into seawater. The
relative contribution of these processes to DO!f input is not known with
certainty. It is important to note however that all DOM sources are
particulate, hence DOH production events would create microzones of
high DOM concentrations in the vicinity of the source. The non-random
distribution of nutrient molecules may have significant implications
for nutrient uptake kinetics of bacteria. Given the above scenario of
DOM production, how do bacteria in the sea metabolically interact with
the organic matter in their environment to optimize nutrient u?take
and growth?
Uptake Kinetic~ of Directly Utilizable Compounds
It has been generally thought that planktonic marine bacteria
have evolved very high affinity membrane transport systems for the
uptake of dissolved nutrients present in nanomolar concentrations in
the seawater (Wright and Burnison, 1979). Azam and Hodson (1981)
showed that assemblages of marine bacteria exhibited multiphasic
uptake kinetics for D-glucose uptake, with Km range of 10- 9 to 10- 4 M.
They reasoned that high Km (high V max ) uptake systems indicate the
presence of microzones of high substrate concentration in a
bacterium's microenvironment. Nissen et al. (in press) studied
glucose uptake kinetics in a marine bacterial isolate, LNB-155. They
found evidence of a single transport system for glucose uptake which
changed its Km (range: 10-a to 10- 3 !l) in an all-or-none fashion at
certain critical concentrations of glucose in the medium. They
suggest that a mUltiphasic transport system would provide metabolic
flexibility for a bacterium living in a microenvironment where very
low (nanomolar) to high substrate concentrations might be encountered.
Since low Km' low Vmax systems saturate at low substrate concentrations, the presence of high Km (and high V max ) transport systems
allows enhanced rates of substrate uptake over a broad range of
environmental substrate concentrations.