Regulation of seasonal variability of aminopeptidase activities in surface and bottom waters of Uranouchi Inlet, Japan

Abstract

The potential aminopeptidase activity (APA) vis-a-vis important ecological parameters of surface and bottom waters of Uranouchi Inlet (Japan), a semi-enclosed eutrophic coastal ecosystem, were studied on a weekly to biweekly basis from mid-August 1996 to early November 1997. The aim of this study was to investigate the main factors regulating the annual and seasonal dynamics of APA in the inlet. APA was estimated in short-term incubations as the V, , of the rate of enzymatic hydrolysis of a fluorophore 7-L-leucyl-4-methyl couomarinylarnide under conditions close to those of in situ. The water column structure at the study site changed considerably during the study period as the ecological parameters such as temperature, dissolved oxygen (DO), nutrient contents, and bacterial abundance exhibited pronounced seasonal stratification (May to September). When the water column was mixed (October to March), surface and bottom waters possessed similar APA . But when the water column became stratified, pronounced differences became apparent, with surface water showing considerably higher APA. On extreme days, surface water possessed as much as -20 times higher APA than that of bottom water. Thus, APA of surface water demonstrated strong seasonality and varied by almost 40-fold compared to that of bottom water, which varied only about 12-fold during the study period. Notably, bacterial abundances of both waters showed limited seasonality and varied only -3-fold during the same period. These results imply that changes in APA are not merely a function of bacterial abundance but rather more of per-cell activity. On an annual basis, APA in both waters were positively correlated (p < 0.001) with temperature and bacterial abundance, and negatively correlated with s M i t y (p c 0.001 for surface and p < 0.01 for bottom water) but were not correlated (p > 0.05) with DO, chlorophyll a (p < 0.05; only for surface water), inorganic or organic forms of nitrogen (DIN, DON) or inorganic phosphorous (DIP) contents. Furthermore, when the water column was mixed, APA correlated strongly with temperature (p < 0.001; r = 0.87), and marginally (p < 0.05) with DIN (r = 0.43) and DIP (r = 0.40) content but not with DO (p > 0.05) content. In contrast, when the water column became stratified, APA correlated only marginally with temperature (0.01 c p < 0.05; r = 0.33) but strongly with DO (p c 0.001; r = 0.73), and negatively (p c 0.001) with DIN (r = -0.61) and DIP (r = -0.70) content. DON content did not show any correlation (p > 0.05) with APA regardless of whether the water column was mixed or stratified. These correlations suggest that temperature played a critical role in regulating the APA when the water column was mixed (October to March), and DO or inorganic nutrient contents (May to September) when the water colun~n was stratified.

Extracellular Proteolytic Activity in the Surface Sediment of a Eutrophic Inlet.

“…5-14 mM), and phosphorous (DIP) (ca. 1-3 mM) concentrations were obtained when the water column was stratified in the summer, but values were considerably lower (DIN; 1-3 mM, and DIP; 0.1-0.8 mM) during the winter (Patel et al 2000). Chlorophyll a in bottom water was rather low especially during summer (usually < 1 mg L -1 ), and ranged 0.2-7.9 mg L -1 during the study period.…”

“…Notably however, the APA in surface sediment showed limited seasonality and varied only by 5 to 6-fold during the study period. Interestingly, the APA in the water column had exhibited a much more pronounced seasonality during the same period; varying in surface water (2 m) about 40-fold, and in bottom water about 12-fold 37) . This may indicate that heterotrophic activities in bottom environments are less affected by the seasonal changes in ecological variables than those in surface environments.…”

“…A description of the study site can be found in earlier publications 14,15,37) . In brief, the site is a semi-enclosed shallow water (depth~16-17 m) coastal ecosystem which witnesses only limited water exchange 15) , and experiences pronounced density stratification 14,37) during an annual cycle. Furthermore, the inlet contains a large number of fish rearing cages from which a considerable amount of nutrients is discharged leading to eutrophication of the inlet.…”

“…At least 3 time points in addition to the zero-time point were obtained at each substrate concentration. To obtain the zero-time point and desired time points, we prepared supernatant for reading the fluorescence in a spectrofluorometer (Shimadzu RF-500 calibrated against the standard chemical compound 7-amino-4-methylcoumarin, AMC), and calculated net fluorescence and APA in the manner described earlier 37) . The APA in the sediment component was calculated on a per unit fresh weight basis by subtracting the mean APA in bottom water from that in slurry, and multiplying by the dilution factor (nmole AMC released g -1 wet weight h -1 ), and was reported on a dry weight basis after taking the water content of the sediment into account.…”

“…We evaluated aminopeptidase activity (APA) in surface and bottom waters 37) , and surface sediment (0-1 cm) of Uranouchi Inlet, Japan, simultaneously, as a measure of the extracellular hydrolysis of protein. Uranouchi Inlet is a shallow water eutrophic coastal ecosystem in which a great deal of intensive fish rearing in cages is practiced.…”