SIZE DISTRIBUTION OF AMINOPEPTIDASE ACTIVITY AND BACTERIAL INCORPORATION OF DISSOLVED SUBSTRATES IN 3 AQUATIC ECOSYSTEMS

Size fractionation of aminopeptidase (Amp) activity and incorporation of dissolved substrates such as glucose and thymidine were analyzed in three aquatic ecosystems: the Salvaje Beach (Spain) with low levels o f nutrients, and two sampling stations of the Butron River (Spain) wit h higher levels of nutrients. Amp activity in the <0.2 mum size fracti on was significant, and ranged from 0 to 59% of the total Amp activity . Taking into account size considerations, the 0.2-3.0 mum size fracti on can be mainly associated to free-living bacteria and contributed to the total Amp activity with mean values of 45% in the Salvaje Beach, and 31% and 45% in the Butron River. The > 3.0 mum size fraction repre sented a high percentage of the total Amp activity with mean values of 41% in the Salvaje Beach and 35% and 34% in the Butron River. The act ivity in this fraction could be attributed to particle attached bacter ia. However, the attached bacteria represented a low percentage of the total abundance and moreover, Amp activity in the > 3.0 mum size frac tion was not significantly correlated with the attached bacterial abun dance, biomass, and incorporation of glucose and thymidine in this fra ction, in any of the ecosystems studied. These results indicate that b acteria should not be considered the only microorganisms responsible f or the Amp activity in these aquatic systems. Amp activity in the 0.2- 3.0 mum size fraction correlated with bacterial abundance, biomass, an d glucose and thymidine incorporation in this fraction, but only in th e Salvaje Beach with low concentration of nutrients.