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

The potential aminopeptidase activity (APA) vis-h-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(max) of the rate of enzymatic hydrolysis of a fluorophore 7-L-leucyl-4-methyl couomarinylamide 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 similar to 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 similar to 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 salinity (p < 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 < p < 0.05; r = 0.33) but strongly with DO (p < 0.001; r = 0.73), and negatively (p < 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 column was stratified.