Proteolytic activity in coastal oceanic waters: depth distribution and relationship to bacterial populations

Proteolytic activity and the role of bacteria in protein degradation in seawater were investigated during 2 cruises to the Santa Monica Basin.The rate of hydrolysis of L-leucyl-P-naphthylaniine was used as a measure of proteolytic activity, and was compared with bacterial numbers, production, and growth rates throughout the water column (905 m max.depth).Peak proteolytic activities in the upper 100 m of the water column were more than 10 times those of deeper waters, though the natural substrate concentrations (combined hydrolyzable amlno acids) in the Santa Monica Basin have prev~ously been found to vary by only 3 to 4-fold Sediment porewaters were assayed during one cruise and found to have 20 to 30 times more proteolytic activity than the peak water column samples from the same cruise, though the origin of this activity could not be determined.Most of the proteolytic activity in the water column was associated with particles the size of bacteria (0.2 to 0.8 pm) and, overall, total proteolytic activity was highly correlated with all bacterial variables tested.However, the average activities per cell were not related to average cell sizes, suggesting that proteolytic activity is not s~mply a function of bacterial biomass.In samples from one of the crulses, the average activities per cell covarled with bacterial growth rates (thymidme incorporation per cell), but this was not true for samples taken during the second cruise.Laboratory expenments using 'seawater cultures' indicated that neither changes in cell size nor growth rate alone could account for changes in the amount of proteolytic enzyme expressed by marine bacteria.These results suggest that proteolytic activity in the water column is primarily associated with bacteria and is likely to be controlled by a number of factors related to the bacterioplankton, including biomass and growth rates.