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defined) free-living bacteria. Since bacteria can use only DOM ,our results indicate that a large
fraction of POM somehow becomes DOM (presumably via hydro lases of attached bacteria) but
paradoxically a large fraction of the DOM produced is utilized by free-living bacteria. These
results point to a loose hydrolysis-uptake coupling on particles.
This conclusion is supported by a study of marine snow solubilization (Simon, Alldredge, and
Azam,unpublished). Hand-collected marine snow from surface waters which was suspended in
0.2 I'm filtered seawater from 500 m depth, released an order of magnitude more dissolved
combined amino acids (DCAA) into seawater than the carbon demand of attached bacteria (a
maximum estimate of DCAA utilization by attached bacteria). These observations also suggest
loose hydrolysis-uptake coupling on particles.
In another study of marine snow aggregates (D. C. Smith, A. L. Alldredge, and F. Azam
unpubl.), we asked whether colonized particles harbor intense enzyme activities and whether the
enzymes were cell-bound there as they are known to be in free-living bacteria. Measurement of
enzymatic activity on individual marine snow particles, collected by SCUBA, exhibited activities
up to three orders of magnitude higher than in the surrounding bulk water for the enzymes
detected (protease, phosphatase, lysozyme, B-glucosidase, chitosidase, and N-acetyl
B-glucosarninidase). Not all enzyme activities were elevated to the same degree, suggesting that
they were not merely the result of higher bacterial densities on the particles but possibly due to
the expression of enzymes in response to the particle composition. Interestingly, the particles had
very high soluble activity which was released into the dissolved phase; 41.8 ± 22.7% (± S.D.)
was found in the supernatant after high speed centrifugation. This soluble activity was 2-3 orders
of magnitude higher than the soluble activity in the bulk water. A significant portion of the
particle associated activity diffused into the water in approximately one hour. This suggests that
these marine snow particles could be a source of enzyme activity into the bulk water. It also
implies the existence of within-particle mechanisms for sustained production of soluble enzyme
activities. Marine snow aggregates appeared to be a source of bacteria into the surrounding
water, since during the incubation large bacteria morphologically similar to those on the
aggregate appeared in the surrounding water. This is consistent with the observations ofJacobsen
and Azam (1984) who found that colonized fecal pellets of the copepod Calanus pacijicus
defined) free-living bacteria. Since bacteria can use only DOM ,our results indicate that a large
fraction of POM somehow becomes DOM (presumably via hydro lases of attached bacteria) but
paradoxically a large fraction of the DOM produced is utilized by free-living bacteria. These
results point to a loose hydrolysis-uptake coupling on particles.
This conclusion is supported by a study of marine snow solubilization (Simon, Alldredge, and
Azam,unpublished). Hand-collected marine snow from surface waters which was suspended in
0.2 I'm filtered seawater from 500 m depth, released an order of magnitude more dissolved
combined amino acids (DCAA) into seawater than the carbon demand of attached bacteria (a
maximum estimate of DCAA utilization by attached bacteria). These observations also suggest
loose hydrolysis-uptake coupling on particles.
In another study of marine snow aggregates (D. C. Smith, A. L. Alldredge, and F. Azam
unpubl.), we asked whether colonized particles harbor intense enzyme activities and whether the
enzymes were cell-bound there as they are known to be in free-living bacteria. Measurement of
enzymatic activity on individual marine snow particles, collected by SCUBA, exhibited activities
up to three orders of magnitude higher than in the surrounding bulk water for the enzymes
detected (protease, phosphatase, lysozyme, B-glucosidase, chitosidase, and N-acetyl
B-glucosarninidase). Not all enzyme activities were elevated to the same degree, suggesting that
they were not merely the result of higher bacterial densities on the particles but possibly due to
the expression of enzymes in response to the particle composition. Interestingly, the particles had
very high soluble activity which was released into the dissolved phase; 41.8 ± 22.7% (± S.D.)
was found in the supernatant after high speed centrifugation. This soluble activity was 2-3 orders
of magnitude higher than the soluble activity in the bulk water. A significant portion of the
particle associated activity diffused into the water in approximately one hour. This suggests that
these marine snow particles could be a source of enzyme activity into the bulk water. It also
implies the existence of within-particle mechanisms for sustained production of soluble enzyme
activities. Marine snow aggregates appeared to be a source of bacteria into the surrounding
water, since during the incubation large bacteria morphologically similar to those on the
aggregate appeared in the surrounding water. This is consistent with the observations ofJacobsen
and Azam (1984) who found that colonized fecal pellets of the copepod Calanus pacijicus