Size distribution of aminopeptidase activity and bacterial incorporation of dissolved substrates in three 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 of nutrients, and two sampling stations of the Butron River (Spain) with higher levels of nutrients. Amp activity in the 3.0 μm size fraction represented 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 activity in this fraction could be attributed to particle attached bacteria. However, the attached bacteria represented a low percentage of the total abundance and moreover, Amp activity in the > 3.0 μm size fraction was not significantly correlated with the attached bacterial abundance, biomass, and incorporation of glucose and thymidine in this fraction, in any of the ecosystems studied. These results indicate that bacteria should not be considered the only microorganisms responsible for the Amp activity in these aquatic systems. Amp activity in the 0.2–3.0 μm size fraction correlated with bacterial abundance, biomass, and glucose and thymidine incorporation in this fraction, but only in the Salvaje Beach with low concentration of nutrients.