230
became "baby machines" , sloughing off the progeny of the attached bacteria into the surrounding
water. Thus the marine snow aggregates can be portrayed as permeant with diffusible hydrolytic
enzymes and as a source of DOM as well as bacteria in the environment.
Biochemical strategies of bacteria in POM utilization: the "enzyme pathway" hypothesis
The above observations and arguments have led us to propose the following hypothetical scenario
of bacteria-POM interactions:
Bacteria attached to pelagic particles secrete diffusible
endohydrolases into the particle. The hydrolase secretion dynamically reflects particle
composition. Enzyme secretion rapidly solubilizes the particle into polymers, a large fraction of
which diffuse out of the particle due to loose hydrolysis-uptake coupling imposed by the
diffusible nature of the enzymes. DOM thus becomes accessible to free-living bacteria,
particularly the progeny of the attached bacteria which is released into seawater but stays near
the particle. Free-living bacteria utilize the polymers emanating from the particle via tight
hydrolysis-uptake coupling by using their non-diffusable cell-bound hydrolases.
We express this hypothesis as a material flux pathway (the "enzyme pathway", Fig. 5),
structured by the hydrolase activities of attached and free-living bacteria, and by whether the
enzyme is a diffusible endohydrolase (El-type) or a non-diffusible exohydrolase (E2-type):
POM-(E1) -- > Polymers-(E2) -- > Monomers -- > attached and free bacteria.
As depicted in Fig. 5, the structure of the enzyme pathway is a necessary consequence of
assuming that attached bacteria secrete diffusible hydrolases but the free-living bacteria express
cell-associated exohydrolases. Free-living bacteria have repeatedly been shown to have
surface-bound hydrolases (Hollibaugh and Azam, 1983; Somville and Billen, 1985; Paul
et al., 1988). Attached bacteria may hydrolyse the particle more efficiently by secreting the
enzymes. An enzyme molecule affixed to the surface of an attached bacterium has a space of
activity merely of molecular dimensions, while the release of the enzyme could enormously
increase its space of activity. The ability to rapidly solubilize the particle would be critical in
the competition of bacteria with metazoa. Solubilization would be even faster if the E1-type
became "baby machines" , sloughing off the progeny of the attached bacteria into the surrounding
water. Thus the marine snow aggregates can be portrayed as permeant with diffusible hydrolytic
enzymes and as a source of DOM as well as bacteria in the environment.
Biochemical strategies of bacteria in POM utilization: the "enzyme pathway" hypothesis
The above observations and arguments have led us to propose the following hypothetical scenario
of bacteria-POM interactions:
Bacteria attached to pelagic particles secrete diffusible
endohydrolases into the particle. The hydrolase secretion dynamically reflects particle
composition. Enzyme secretion rapidly solubilizes the particle into polymers, a large fraction of
which diffuse out of the particle due to loose hydrolysis-uptake coupling imposed by the
diffusible nature of the enzymes. DOM thus becomes accessible to free-living bacteria,
particularly the progeny of the attached bacteria which is released into seawater but stays near
the particle. Free-living bacteria utilize the polymers emanating from the particle via tight
hydrolysis-uptake coupling by using their non-diffusable cell-bound hydrolases.
We express this hypothesis as a material flux pathway (the "enzyme pathway", Fig. 5),
structured by the hydrolase activities of attached and free-living bacteria, and by whether the
enzyme is a diffusible endohydrolase (El-type) or a non-diffusible exohydrolase (E2-type):
POM-(E1) -- > Polymers-(E2) -- > Monomers -- > attached and free bacteria.
As depicted in Fig. 5, the structure of the enzyme pathway is a necessary consequence of
assuming that attached bacteria secrete diffusible hydrolases but the free-living bacteria express
cell-associated exohydrolases. Free-living bacteria have repeatedly been shown to have
surface-bound hydrolases (Hollibaugh and Azam, 1983; Somville and Billen, 1985; Paul
et al., 1988). Attached bacteria may hydrolyse the particle more efficiently by secreting the
enzymes. An enzyme molecule affixed to the surface of an attached bacterium has a space of
activity merely of molecular dimensions, while the release of the enzyme could enormously
increase its space of activity. The ability to rapidly solubilize the particle would be critical in
the competition of bacteria with metazoa. Solubilization would be even faster if the E1-type