Profiles of ectoenzymes in the Indian Ocean: phenomena of phosphatase activity in the mesopelagic zone

Phosphatase (P-ase) activity was determined together with other extracellul ar enzyme activities, bacterial abundance and production rates during the 2 SW Monsoon process studies of the German JGOFS Arabian Sea Program. Water samples were collected along the cruise tracks from the equator to the upwe lling region at the shelf edge off Oman. Depth profiles of P-ase activity w ere strikingly different from those of the other enzymes. While values of a minopeptidase and beta-glucosidase generally decreased below the euphotic z one, P-ase increased by factors of 1 to 7. The relation between peptidase- and P-ase activity was from 4 to 21 at the surface and from 3 to 5 at 800 m depth. Because P-ase production (dissolved and cell-bound) in deep waters is mainly dependent on bacteria, P-ase activities per bacterial cell were c alculated: these were, on average, 37 times higher at 800 m than at the sur face. We also observed a positive correlation of P-ase activity with phosph ate concentrations in the depth profiles below the euphotic zone, while thi s relationship was much more variable in the mixed surface layer. These obs ervations suggest that C-limited bacteria in the deep strata did not primar ily focus on the phosphate generated by their P-ase activity but on the org anic C compounds which were simultaneously produced and which could probabl y not be taken up prior to the hydrolytic detachment of phosphate. It is hy pothesised that a considerable part of the measured P-ase activity was diss olved (though it might have originated from bacteria). These enzymes may be important for the slow, but steady regeneration of phosphate and organic C in mesopelagic waters.