AME 34:129-137 (2004)  -  doi:10.3354/ame034129

ABSTRACT: In order to examine the hypothesis that the enzyme levels of heterotrophic nanoflagellates (HNF) are influenced by qualitatively different food bacteria, the production of aminopeptidase by an isolated marine HNF Jakoba libera-5(2) fed on natural communities of bacteria and 6 bacterial strains of 5 different taxonomic groups (Aeromonas, Bacillus, Coryneforms, Flavobacterium and Pseudomonas) were compared. The aminopeptidase activity (total and free) and abundance of J. libera-5(2) significantly differed with the types of food bacteria. The total and free aminopeptidase activities and abundance of J. libera-5(2) were at a maximum with Pseudomonas spp., followed by Flavobacterium sp. and natural communities of bacteria as prey. The values of total aminopeptidase activity of J. libera-5(2) with Pseudomonas spp., Flavobacterium sp. and natural communities of bacteria were 140 ± 6.78 to 285 ± 12.36, 123 ± 11.17 and 38 ± 0.56 μmol h^-1 l^-1, respectively, and those of free aminopeptidase activity were 83 ± 6.15 to 137 ± 5.83, 82 ± 12.18 and 8 ± 0.14 μmol h^-1 l^-1, respectively. J. libera-5(2) did not produce any detectable amounts of total and free aminopeptidase while grazing on Aeromonas, Bacillus and Coryneforms. The chemical characterization of partially purified aminopeptidase of J. libera-5(2) produced during grazing on a strain of Pseudomonas sp. indicated the enzyme to be metal-chelater-sensitive alkaline serine aminopeptidase with optimal activity at pH 8.0 and 30°C; it was not affected by the major cations of seawater, such as Na⁺, Ca²⁺ and Mg²⁺. The results suggest that at least some marine HNF may significantly contribute to the enzyme pool in marine environments while selectively grazing on bacteria.

KEY WORDS: Aminopeptidase · Grazing · Heterotrophic nanoflagellates · Marine bacteria