substrates compete with naturally occurring dimers and in some cases also with polymers
(Hoppe, 1983 ; Somville, 1984). Furthermore, there was nearly no qualitative discrepancy
in the growth of pure bacteria cultures with polymers and corresponding MUFsubstrates. It is, however, clear that a much better working knowledge of the MUFsubstrate properties is needed when extrapolation of results such as turnover rate or
velocity of hydrolysis (V
m
) in the natural pool of soluble polymers is desired. Nevertheless, such extrapolation is occasionally attempted here in a cautious manner, because
correlation between extracellular enzymatic activity (EEA) and heterotrophic activity
was convincing. Usually EEA on MUF-substrates followed first order enzyme kinetics as
is also frequently reported for heterotrophic substrate uptake mechanisms. It may be
suggested that application of enzyme kinetic approaches is even more suitable for studies
on EEA than for heterotrophic uptake, because the function of extracellular enzymes
seems to be more independent than the function of transport enzymes which are more
closely linked to the cell metabolism (Krambeck, 1979). All the theoretical objections to
the use of simple enzyme kinetics for multienzyme systems inherent with substrate uptake
by different bacteria (Williams, 1973) may also be valid for EEA.
The results from culture experiments (Kim and Hoppe, this volume) were confirmed by
EEA measurements of size fractions from natural waters. Size fractions dominated by
free-living bacteria contributed most to EEA and there was little activity recorded from
fractions >3 µm. Only phosphatases were more abundant in larger size classes (8 - 150
µm), where their activity was about twice as high as in the bacterial size classes. It may be
that attached bacteria possess disproportionally higher polymer-degrading abilities than
free-living ones, as suggested by Hollibaugh and Azam (1983) and Kim (personal discussion). However, in the investigated waters of the Kiel Bight (Baltic Sea) the fraction of
attached bacteria is steadily less than 10 % of total bacteria and, thus, most EEA
originates from free-living bacteria.
The existence of active free dissolved enzymes no longer closely associated with their sites
(cells) of origin would be of considerable biochemical and ecological significance, since
these enzymes could condition macromolecular DOC and organic surfaces for subsequent bacterial growth. Size fractionation experiments described here revealed different
patterns for the abundance of free dissolved enzymes (found in 0.2µm filtrates) dependent
upon different MUF-substrates and substrate concentrations. Free proteases are generally believed to be scarce in marine waters. Our results suggest that the activity of free
proteases may climb from 14 to 29 % of total activity of a water sample inversely
depending on substrate concentration. The opposite was true for hydrolytic activities of
β-glucosidase and N-acetyl-glucosaminidase, whereas no clear dependency on substrate
concentrations could be observed for α-glucosidase and phosphatase.
From field measurements of EEA distribution it can be concluded that the turnover of
α-glucosides and glucosaminides is much faster in polluted inshore areas than in offshore
waters. Because V for a-glucosidase was also relatively high at the inshore station (c.f.
Tab. 4) it may be suggested that this is due to a high abundance of bacteria specialized in
the decomposition of α-glucosidic compounds. In the case of protein and organic phosphorous a much faster turnover was measured in the offshore region. This may be
attributed to a smaller pool of these substrates and a relatively higher abundance of
specialized bacteria in offshore waters.
The relation between the hydrolyzation rate (HR) of the available polymer pool and the
microbial turnover rate (TR) of the corresponding monomers may provide some information on the coupling of these pools under natural conditions. For this it has to be
accepted that the concentration of monomers in the water is kept rather constant over
time and that monomers resulting from extracellular enzymatic decomposition of poly126
(Hoppe, 1983 ; Somville, 1984). Furthermore, there was nearly no qualitative discrepancy
in the growth of pure bacteria cultures with polymers and corresponding MUFsubstrates. It is, however, clear that a much better working knowledge of the MUFsubstrate properties is needed when extrapolation of results such as turnover rate or
velocity of hydrolysis (V
m
) in the natural pool of soluble polymers is desired. Nevertheless, such extrapolation is occasionally attempted here in a cautious manner, because
correlation between extracellular enzymatic activity (EEA) and heterotrophic activity
was convincing. Usually EEA on MUF-substrates followed first order enzyme kinetics as
is also frequently reported for heterotrophic substrate uptake mechanisms. It may be
suggested that application of enzyme kinetic approaches is even more suitable for studies
on EEA than for heterotrophic uptake, because the function of extracellular enzymes
seems to be more independent than the function of transport enzymes which are more
closely linked to the cell metabolism (Krambeck, 1979). All the theoretical objections to
the use of simple enzyme kinetics for multienzyme systems inherent with substrate uptake
by different bacteria (Williams, 1973) may also be valid for EEA.
The results from culture experiments (Kim and Hoppe, this volume) were confirmed by
EEA measurements of size fractions from natural waters. Size fractions dominated by
free-living bacteria contributed most to EEA and there was little activity recorded from
fractions >3 µm. Only phosphatases were more abundant in larger size classes (8 - 150
µm), where their activity was about twice as high as in the bacterial size classes. It may be
that attached bacteria possess disproportionally higher polymer-degrading abilities than
free-living ones, as suggested by Hollibaugh and Azam (1983) and Kim (personal discussion). However, in the investigated waters of the Kiel Bight (Baltic Sea) the fraction of
attached bacteria is steadily less than 10 % of total bacteria and, thus, most EEA
originates from free-living bacteria.
The existence of active free dissolved enzymes no longer closely associated with their sites
(cells) of origin would be of considerable biochemical and ecological significance, since
these enzymes could condition macromolecular DOC and organic surfaces for subsequent bacterial growth. Size fractionation experiments described here revealed different
patterns for the abundance of free dissolved enzymes (found in 0.2µm filtrates) dependent
upon different MUF-substrates and substrate concentrations. Free proteases are generally believed to be scarce in marine waters. Our results suggest that the activity of free
proteases may climb from 14 to 29 % of total activity of a water sample inversely
depending on substrate concentration. The opposite was true for hydrolytic activities of
β-glucosidase and N-acetyl-glucosaminidase, whereas no clear dependency on substrate
concentrations could be observed for α-glucosidase and phosphatase.
From field measurements of EEA distribution it can be concluded that the turnover of
α-glucosides and glucosaminides is much faster in polluted inshore areas than in offshore
waters. Because V for a-glucosidase was also relatively high at the inshore station (c.f.
Tab. 4) it may be suggested that this is due to a high abundance of bacteria specialized in
the decomposition of α-glucosidic compounds. In the case of protein and organic phosphorous a much faster turnover was measured in the offshore region. This may be
attributed to a smaller pool of these substrates and a relatively higher abundance of
specialized bacteria in offshore waters.
The relation between the hydrolyzation rate (HR) of the available polymer pool and the
microbial turnover rate (TR) of the corresponding monomers may provide some information on the coupling of these pools under natural conditions. For this it has to be
accepted that the concentration of monomers in the water is kept rather constant over
time and that monomers resulting from extracellular enzymatic decomposition of poly126