AME 40:151-161 (2005)  -  doi:10.3354/ame040151

ABSTRACT: Extra-cellular enzyme activity was investigated during a cyanobacterial bloom dominated by Nodularia spumigena on the SW coast of Finland, Baltic Sea. When filaments of N. spumigena were abundant in surface waters, alkaline phophatase (AP) activity was elevated, and ~37% of the total activity was associated with the >2 µm fraction. About 30% of the total leucine aminopeptidase (LAP) activity was also associated with the >2 µm size fraction. When spherical aggregates containing a mixture of moribund and decaying N. spumigena strands mixed with other cells were abundant in surface waters, ~50% of the total AP activity and ~22% or less of the total LAP activity was associated with the >2 µm fraction. Both total LAP and AP activity were positively correlated with chlorophyll a. LAP activity in the >2 µm fraction was positively correlated with thymidine uptake, suggesting that much of the observed LAP activity in the >2 µm fraction was due to heterotrophic bacteria associated with cyanobacteria. In contrast, AP activity in this size fraction was not significantly associated with bacterial numbers or activity. Experiments with isolated cyanobacterial aggregates demonstrated that they were sites of enhanced enzymatic activity. Cultured, axenic strands of N. spumigena displayed LAP and AP activity. Our data indicate that colonies and aggregates of cyanobacteria in the Baltic Sea can be important sites of hydrolytic activity and therefore of nutrient regeneration.

KEY WORDS: Baltic Sea · Nodularia spumigena · Leucine aminopeptidase · Alkaline phosphatase