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

Phosphatase (P-ase) actlvity was determined together with other extracellular 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 upwelling region at the shelf edge off Oman. Depth profiles of P-ase activity were striklngly different from those of the other enzymes. While values of aminopeptidase and P-glucosidase generally decreased below the euphotic zone. P-ase increased by factors of 1 to 7. The relation between peptidase-and P-ase actlvity 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 calculated: these were, on average, 37 times higher at 800 m than at the surface. We also observed a positive correlation of P-ase activlty with phosphate concentrations in the depth profiles below the euphotic zone, while this relationship was much more varlable in the muted surface layer. These observations suggest that C-limited bacteria in the deep strata did not primarily focus on the phosphate generated by the11 P-ase activity but on the organic C compounds which were simultaneously produced and whlch could probably not be taken u p prior to the hydrolytic detachment of phosphate. It is hypothesised that a considerable part of the measured P-ase activlty was dissolved (though it mlght have o n g ~n a t e d from bacteria). These enzymes may b e important for the slow, but steady regeneration of phosphate and organic C in mesopelagic waters.