Dissolved organic phosphorus regeneration by bacterioplankton: 5'-nucleotid-ase activity and subsequent phosphate uptake in a mesocosm enrichment experiment

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Abstract
Bacterial 5''-nucleotidase activity was studied during a mesocosm enrichment experiment on the SW coast of Finland, Baltic Sea. 5''-nucleotidase activity and uptake of 32PO4 hydrolyzed from 5''-nucleotide was compared to direct 32PO4 uptake measurements and other nutrient data. The enzyme was not repressed by elevated PO4 concentrations, and it hydrolyzed orthophosphate from dissolved organic phosphorus in amounts which were quantitatively significant in the phosphorus dynamics of the community. Bacteria partly assimilated the hydrolyzed PO4, but depending on nutritional conditions, 0 to 80% of the hydrolyzed PO4 was released to the environment. Temporal uncoupling between nutrient assimilation and growth was evident for both bacteria and algae. 5''-nucleotidase represented a mechanism of phosphorus regeneration which supports the concept of closely integrated microaggregates having an important role in nutrient cycles of nutrient-depleted surface waters.