BACTERIAL-COLONIZATION AND ECTOENZYMATIC ACTIVITY IN PHYTOPLANKTON-DERIVED MODEL PARTICLES - CLEAVAGE OF PEPTIDES AND UPTAKE OF AMINO-ACIDS

Phytoplankton-derived model particles were created in laboratory from a mixture of autoclaved diatom cultures. These particles were colonize d by a marine bacterial community and incubated in rolling tanks in or der to examine the relationship between aminopeptidase activity and le ucine uptake. Bacteria inhabiting particles and ambient water were cha racterized for abundance, biovolume, aminopeptidase activity, leucine uptake, and growth rate. Particles were a less favorable habitat than ambient water for bacterial growth since growth rates of particle-atta ched bacteria were similar or even lower than those of free-living bac teria. During the first similar to 100 h of the particle decomposition process, there were not statistically significant differences in the aminopeptidase activity:leucine uptake ratio between attached and free -living bacteria. From similar to 100 h to similar to 200 h, this rati o was higher for attached bacteria than for free-living bacteria. This indicates an uncoupling of aminopeptidase activity and leucine uptake . During this period, attached and free-living bacteria showed similar hydrolytic activities on a cell-specific basis. In the free-living ba cterial community, variations in aminopeptidase activity per cell were associated with variations in leucine uptake per cell and growth rate s. However, in the attached bacterial community, when leucine uptake a nd growth rates decreased, aminopeptidase activity remained constant. Thus, after similar to 100 h, particle-attached bacteria were not taki ng advantage of their high aminopeptidase activity; consequently the h ydrolysed amino acids were released into the ambient water, supporting the growth of free-living bacteria. These results demonstrate that ov er the particle decomposition process, the relationship between hydrol ysis and uptake of the protein fraction shows different patterns of va riation for attached and free-living bacterial communities. However, i n our experiments, this uncoupling was not based on a hyperproduction of enzymes by attached bacteria, but on lower uptake rates when compar ed to the free-living bacteria.