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East—west gradient in ectoenzyme activities in the subarctic Pacific: Possible regulation by zinc

We determined depth profiles of potential activities of leucine aminopeptidase (LAPase) and β—glucosidase (BGase) in the subarctic Pacific between July and September 1997. The data revealed a systematic shift in vertical patterns of the LAPase:BGase ratios along the east‐west axis of the investigated area. The ratio increased by over 10‐fold with depth in the eastern region (139°W‐159°W), while it varied little (<3‐fold) in the central region (169°W‐179°E, including the Bering Sea). It decreased with depth by about six‐fold in the western region (169°E–161°E). The decrease of LAPase:BGase ratio with depth is consistent with a general pattern in the decomposition of sinking particles, i.e., protein is degraded faster than polysaccharides, but the increase of the ratio with depth suggests that export of proteinaceous materials to the mesopelagic zone is high. Cell specific LAPase activity in the upper layer (0–100 m) was positively correlated (r = 0.92, P < 0.05) with the concentration of dissolved zinc, a catalytic element of proteases. We hypothesize that the low level of dissolved zinc stresses LAPase activity in the euphotic zone, leading to high export of protein to the mesopelagic zone in the eastern subarctic Pacific. These results have important implications for modeling the carbon and nitrogen pump in open oceans where surface concentrations of trace metals are extremely low.