Bacterial activity along a trophic gradient

Bacterial biomass, secondary production, and extracellular enzymatic activity [α-glucosidase and leucine-aminopeptidase, measured as cleavage of artificial fluorogenic substrates 4-methyl umbelliferyl (MVF) α-D-glucopyranoside and L-leucine 7-amido-4-methyl coumarin (MCA)] were measured along a trophic gradient in the Northern Adriatic Sea in four ecologically different situations. Bacterial parameters were compared with chlorophyll a and inorganic and organic nutrient concentrations. Bacterial secondary production and extracellular enzymatic activity markedly changed among different seasons and along the trophic gradient. Average bacterial secondary production increased from 0.61 to 2.09 μg Cl^-1 hour^-1 preceding a bloom, to 2.09 μg Cl^-1 hour^-1 during the bloom, decreasing again to 0.81 and 0.83 μg Cl^-1 hour^-1 in the post-bloom and summer periods, respectively (values from 0.5 m depth). Leucine-aminopeptidase activity showed more consistent trends than α-glucosidase activity. Average values of leucine-aminopeptidase activity, measured by enzymatic release of MCA, increased from a pre-bloom value of 164.0 to 1,712.0 (nM MCA) hour^-1 released during a bloom, decreasing to 298.5 and 133.7 (nM MCA) hour^-1 released for the post-bloom and summer situation, respectively (values from 0.5 m depth). Average growth rates decreased during the bloom, whereas average extracellular enzymatic activity levels expressed on a cell basis increased by an average factor of 2. Along the trophic gradient, a consistent increase in bacterial secondary production could be observed in all but the summer situation (values from 0.5 m depth). Leucine-aminopeptidase activity also showed positive trends along the gradient, while α-glucosidase activity did not exhibit such a clear trend. Bacterial biomass trends were less obvious considering both seasonal changes and the tropic gradient. Highly significant interrelations were detected between bacterial proteolytic activity, secondary production, chlorophyll a content, and nitrate concentrations, especially in the surface horizon.