Depth variation of bacterial extracellular enzyme activity and population diversity in the northeastern North Atlantic Ocean

Distinct profiles of extracellular proteolytic enzyme activity were observe d in the water column of the North Atlantic, with maximum potential proteol ytic activity occurring in the top 35 m. The proteolytic enzyme V-max value s varied significantly and decreased from 1.46 nM min(-1) in surface waters to 0.365 nM min(-1) at 100 m. In contrast, K-m values increased with depth from about 70 to 360 muM. Cell-associated enzymes accounted for the majori ty of the observed proteolytic activity. Dissolved enzymes comprised only 3 0-40% of the total extracellular enzyme activity and exhibited a low substr ate affinity (K-m = similar to 1000 muM). These observations indicate clear stratification of bacterial associated extracellular enzyme activity, with the maximum activity in surface waters. This is consistent with some envir onmental changes in the water column, especially algal biomass and nitrate concentration. Bacterial mediated nitrogen remineralization in surface wate rs was approximately three times the total nitrogen demand of phytoplankton and bacteria. We determined bacterial population diversity using 16S rRNA sequence analysis and found evidence for stratification, with a higher repr esentation of the Cytophaga/Flexibacter/Bacteriodes group at 5 m compared t o 100 m. No similar stratification was observed among the alpha -proteobact erial SAR11 cluster, which were especially prevalent in the PRIME eddy. How ever, sequences phylogenetically related to another marine cluster, SAR122, were only observed at 100 m. We suggest that stratification of proteolytic activity within the water column may be explained at least in part, by dif ferences in the composition of the bacterial community. (C) 2001 Elsevier S cience Ltd. All rights reserved.