Zinc (Zn) is an essential trace element for growth and enzyme activities of heterotrophic bacteria. However, excess load of Zn shows toxicity and inhibition to microbial processes. The effect of Zn on marine microbial function has not been studied focusing on bacterial and enzymatic activities. In this study, we examined the short-term effect of Zn on bacterial abundance and respective proteolytic activity. Seawater sample was filtered through 0.6-µm filter to remove particle and grazers, varying concentrations of Zn (0.01 and 10 µM) were added to the filtrates. Aliquots were withdrawn at 0, 4, and 16-hr.

Bacteria were enumerated using SYBR Gold staining and epifluorescence microscopy. Extracellular protease activities (aminopeptidase-, trypsin-and chmotrypsin-type activities) were measured by methylcoumarylamide (MCA)-substrates. Results showed that, the addition of Zn did not cause any significant changes on the bacteria abundance. However, the addition of Zn clearly decreased the aminopeptidase activity compared to control. Trypsin activity was high at 0 hr and no change was observed along the 16 hr incubation period in conditions either with or without Zn. Chymotrypsin activity was also not affected by Zn. This suggests that Zn has direct impact on aminopeptidase. Overall, Zn contamination seems to have effect on the microbial activities and transformation of proteins in aquatic system.