Microbial Enzymatic Activities and Carbon Flux in the Adriatic Sea
491
1999). All respiratory rates were reckoned using a
R.Q. 1 better to compare them to our data.
Bacterial abundance was higher in area A
than in area B and, on the whole, was lower than
those expected from an estuarine area (Heip et
aI. 1995» but confirmed previous data reported
for the Northern Adriatic Sea by Puddu et al.
(1998). Estimates of biovolumes, instead, showed
the occurrence of smaller or larger bacterial cells
which alternated in cold and warm periods.
Previous investigations in the Adriatic Sea had
shown no seasonal differences in the bacterial
abundance, whereas greater variations had been
observed in the bacterial structure - cell size and
genera composition - and activity (Zaccone and
Caruso 1996; La Ferla et al. 1998; Puddu et al.
1998; Zaccone et al. 1998). The succession of different microbial communities may reasonably be
associated to different trophic conditions since
smaller size bacteria normally inhabit low nutdent waters while larger size bacteria live in
enriched environments (Fuhrmann et al. 1980;
Holligan et al. 1984). Moreover, larger bacteria
generally live attached to the particulate organic
matter while small free living bacteria depend on
dissolved organic matter (Unanue et al. 1992).
The viable fraction of heterotrophic bacteria on
the total bacteria, contributed in a significant
way to microbial activity; this population, ranging from 0.02 to 0.31% in area A and from 0.02 to
8% in area B, surely constituted a fraction of
active bacteria. However. in marine environments, another fraction of total bacteria - the
viable but not cultivable cells - can display some
biological activities such as enzyme production,
even though it has lost the ability to replicate
itself on media (Roszack and Colwell 1987).
Our observations showed the highest microbial activities in the upper water layers, within 10
m depth and sediment trap studies revealed that
less than 10% of the particulate organic matter,
deriving from primary production, reached the
bottom (S. Rabitti, personal communication). As
a consequence, we hypothesised that the organic
matter deriving from phytoplankton, inducting
enzyme production. constituted an available
source of carbon for the bacteria metabolism
(Middelboe et a1. 1995). The correlations with
POC, DOC and ChI supported this hypothesis
and the viable fraction of heterotrophic bacteria
contributed to decomposition of organic compounds by their enzymatic spectrum (Zaccone et
aI. 1998). However, also the fresh water supply
assumed a predominant role influencing the
microbial activities and distribution as underlined by the numerous negative correlations with
salinity. In conclusion, the course of biological
processes is affected both by coupling between
production and mineralisation and by salinity
gradients (Naush et a!. 1998; Puddu et al. 1998).
The analysis of AMP and R integrated data in the
fIrst 10 m has underlined that twice the quantity
of organic carbon is on average hydrolysed and
oxidised in area A with respect to area B. In fact
the integrated average AMP value calculated was
81.7 e41.5 mgCm- 2 h-1for areasAandB,respectively; the integrated average value of respiratory
activity was 11.5 and 6.6 mg C m- 2 h- 1 in areas A
and B, respectively.
With the aim of estimating the microbial
processes coupling, the AMP/BCP ratio was calculated on the averaged values (Fig. 3). In area A,
the ratio (>45) indicated a higher production of
enzymes (AMP) with respect to incorporation
rates (BCP) in June 96, Feb. 97, Feb. 98. In area B.
in Feb. 97, the ratio was reduced «30) indicating
a higher coupling of processes. In June 97, in
both the areas, the ratio further decreased «20)
and the two processes were more balanced and
the C released by enzymatic hydrolysis could be
available for new biomass production. Substrate
hydrolysis may exceed substrate uptake mainly
for particle associated bacteria (Hoppe et al.
·1988; Karner and Hernd11992; Smith et al. 1992,
1995; Middelboe et al. 1995; Unanue et a1. 1998).
The quantification of carbon flux into bacteria closely depends on the accuracy of the estimates of the BCP/R ratio which indicates the balance existing between production of new bacterial biomass and consumption by means of oxidative metabolism. Our estimates ranged from 0.2
to 0.6 in area A and from 0.15 to 0.78 in area B
(Fig. 3), In Feb. 97, in both the areas, the ratio
reached the maximum values suggesting that
most of the respiratory activity may be sustained
by bacterial production. Zweifel et al. (1995)
determined a Bep /R ratio of 0.36 from sea water
cultures of bacteria collected from the Northern
Baltic Sea; Amon and Benner (1998) determined
higher ratios of 0.6-1.7, taking into account BCP
calculated from changes in bacterial cell abundance and 2.5-3.7 from bacterial leucine incorporation in samples collected in the Mississippi
river plume.
Estimates of bacterial growth efficiency
(BGE). as the percentage ratio between BCP and
491
1999). All respiratory rates were reckoned using a
R.Q. 1 better to compare them to our data.
Bacterial abundance was higher in area A
than in area B and, on the whole, was lower than
those expected from an estuarine area (Heip et
aI. 1995» but confirmed previous data reported
for the Northern Adriatic Sea by Puddu et al.
(1998). Estimates of biovolumes, instead, showed
the occurrence of smaller or larger bacterial cells
which alternated in cold and warm periods.
Previous investigations in the Adriatic Sea had
shown no seasonal differences in the bacterial
abundance, whereas greater variations had been
observed in the bacterial structure - cell size and
genera composition - and activity (Zaccone and
Caruso 1996; La Ferla et al. 1998; Puddu et al.
1998; Zaccone et al. 1998). The succession of different microbial communities may reasonably be
associated to different trophic conditions since
smaller size bacteria normally inhabit low nutdent waters while larger size bacteria live in
enriched environments (Fuhrmann et al. 1980;
Holligan et al. 1984). Moreover, larger bacteria
generally live attached to the particulate organic
matter while small free living bacteria depend on
dissolved organic matter (Unanue et al. 1992).
The viable fraction of heterotrophic bacteria on
the total bacteria, contributed in a significant
way to microbial activity; this population, ranging from 0.02 to 0.31% in area A and from 0.02 to
8% in area B, surely constituted a fraction of
active bacteria. However. in marine environments, another fraction of total bacteria - the
viable but not cultivable cells - can display some
biological activities such as enzyme production,
even though it has lost the ability to replicate
itself on media (Roszack and Colwell 1987).
Our observations showed the highest microbial activities in the upper water layers, within 10
m depth and sediment trap studies revealed that
less than 10% of the particulate organic matter,
deriving from primary production, reached the
bottom (S. Rabitti, personal communication). As
a consequence, we hypothesised that the organic
matter deriving from phytoplankton, inducting
enzyme production. constituted an available
source of carbon for the bacteria metabolism
(Middelboe et a1. 1995). The correlations with
POC, DOC and ChI supported this hypothesis
and the viable fraction of heterotrophic bacteria
contributed to decomposition of organic compounds by their enzymatic spectrum (Zaccone et
aI. 1998). However, also the fresh water supply
assumed a predominant role influencing the
microbial activities and distribution as underlined by the numerous negative correlations with
salinity. In conclusion, the course of biological
processes is affected both by coupling between
production and mineralisation and by salinity
gradients (Naush et a!. 1998; Puddu et al. 1998).
The analysis of AMP and R integrated data in the
fIrst 10 m has underlined that twice the quantity
of organic carbon is on average hydrolysed and
oxidised in area A with respect to area B. In fact
the integrated average AMP value calculated was
81.7 e41.5 mgCm- 2 h-1for areasAandB,respectively; the integrated average value of respiratory
activity was 11.5 and 6.6 mg C m- 2 h- 1 in areas A
and B, respectively.
With the aim of estimating the microbial
processes coupling, the AMP/BCP ratio was calculated on the averaged values (Fig. 3). In area A,
the ratio (>45) indicated a higher production of
enzymes (AMP) with respect to incorporation
rates (BCP) in June 96, Feb. 97, Feb. 98. In area B.
in Feb. 97, the ratio was reduced «30) indicating
a higher coupling of processes. In June 97, in
both the areas, the ratio further decreased «20)
and the two processes were more balanced and
the C released by enzymatic hydrolysis could be
available for new biomass production. Substrate
hydrolysis may exceed substrate uptake mainly
for particle associated bacteria (Hoppe et al.
·1988; Karner and Hernd11992; Smith et al. 1992,
1995; Middelboe et al. 1995; Unanue et a1. 1998).
The quantification of carbon flux into bacteria closely depends on the accuracy of the estimates of the BCP/R ratio which indicates the balance existing between production of new bacterial biomass and consumption by means of oxidative metabolism. Our estimates ranged from 0.2
to 0.6 in area A and from 0.15 to 0.78 in area B
(Fig. 3), In Feb. 97, in both the areas, the ratio
reached the maximum values suggesting that
most of the respiratory activity may be sustained
by bacterial production. Zweifel et al. (1995)
determined a Bep /R ratio of 0.36 from sea water
cultures of bacteria collected from the Northern
Baltic Sea; Amon and Benner (1998) determined
higher ratios of 0.6-1.7, taking into account BCP
calculated from changes in bacterial cell abundance and 2.5-3.7 from bacterial leucine incorporation in samples collected in the Mississippi
river plume.
Estimates of bacterial growth efficiency
(BGE). as the percentage ratio between BCP and