Page 3 - Caruso_Zaccone_2000

LAP ACTIVITY IN AQUATIC ECOSYSTEMS 953

100
(a)
80
ng Cl –1 h –1 60
40
20
0
Northern area Central area Southern area

10
(b)
8
µg N l –1 6
4
2
0
Northern area Central area Southern area

Fig. 2 Egadi Archipelago: distribution of (a) leucine aminopeptidase (LAP) and (b) particulate organic carbon (PON) in the photic zone
(*) and at the bottom (&) in the northern, central and southern sectors of the area examined. Error bars indicate S.D. from the mean
value (n ˆ 3)

In this ecosystem, the leucine aminopeptidase activity showed the highest signi®cant positive correlations of LAP
was ®rst evaluated in 1995, within the framework of the with viable heterotrophic bacterial density and activity, par-
Prisma 1 research programme, in an area receiving Po and ticularly in the surface waters. This suggested the possible
Adige river out¯ows. Two coastal sites (S1 and S2, 20 m coupling between the processes of extracellular hydrolysis
deep) differently affected by river run-off, and one off- and substrate uptake through the utilization of hydrolysis
shore station (S3, 27 m deep) were investigated during products by bacteria for their growth.
three seasonal cruises carried out in April, July and In the framework of the second phase of the Prisma pro-
October. Microbial aminopeptidase values were between gramme (Prisma 2), two other different areas of the
ÿ1
003 and 16 mg C l ÿ1 h , while viable plate counts ranged Adriatic basin, area A in the north in front of the Po delta,
2 4 ÿ1
from 198 10 to 340 10 cfu ml . The enzyme pat- and area B in the south near the Ancona coast, were exam-
tern, reported in Fig. 3, showed a decreasing trend from ined during four oceanographic cruises carried out in sum-
coastal to off-shore stations; the highest Vmax values were mer (June 96 and 97) and winter (February 97 and 98).
reached at the stations directly in¯uenced by ¯uvial inputs, The intense input of organic matter from the Po river, and
and a peak of 506 mg C l ÿ1 h ÿ1 was observed in July in the the anticyclonic circulation, strongly affected the amino-
surface layer of station S1, located in front of the Po delta. peptidase distribution, causing differences in the enzyme
At the pelagic station S3, the aminopeptidase activity levels levels in relation to the area, depth, site or season. Values
were constantly lower and decreased with depth. This dis- of LAP recorded during the 2-year study ranged from 49
tribution was closely related to the trophic supply of the ‹ 86 to 99 ‹ 107 mg C l ÿ1 h ÿ1 (mean ‹S.D.) and from
rivers, which enriched the surface layers with new organic 31 ‹ 26 to 76 ‹ 91 mg C l ÿ1 h ÿ1 (mean ‹S.D.) in the
matter available for microbial decomposition. The strati®- northern and southern area, respectively.
cation of the `plume' waters over the underlying marine Vertical pro®les of enzyme (Fig. 4) showed, in general,
waters was responsible for the local con®nement of the the occurrence of higher activity levels in the upper layers
trophic resources in the upper layer, and for the differences in association with the greater organic matter load, and a
in the enzyme levels recorded between surface and deeper gradual decrease with increasing depth. As an exception,
waters. Enzyme data also showed a seasonal variability, two peaks of activity were recorded near the bottom in
doubling from spring to summer in the surface layer in winter 97 in the northern area and in summer 96 in the
relation to the higher temperatures. southern one, due to the breakdown of the thermocline,
Pearson correlation coef®cients calculated separately for which caused a rapid sinking of organic matter, and to
the surface layer and the whole water column (Table 1) down-welled freshwater input (salinity 352±357). Moving

= 2000 The Society for Applied Microbiology, Journal of Applied Microbiology, 89, 951ÿ959