THE EFFECT OF PRESSURE ON LEUCINE AND THYMIDINE INCORPORATION BY FREE-LIVING BACTERIA AND BY BACTERIA ATTACHED TO SINKING OCEANIC PARTICLES

The effect of pressure on upper ocean free-living bacteria and bacteri a attached to rapidly sinking particles was investigated through study ing their ability to synthesize DNA and protein by measuring their rat e of H-3-thymidine and H-3-leucine incorporation. Studies were carried out on samples from the NE Atlantic under the range of pressures (1-4 30 atm) encountered by sinking aggregates during their journey to the deep-sea bed. Thymidine and leucine incorporation rates per bacterium attached to sinking particles from 200 m were about six and ten times higher, respectively, than the free-living bacterial assemblage. The r atio of leucine incorporation rate per cell to thymidine incorporation rate per cell was significantly different between the larger attached (18.9:1) and smaller free-living (10.4:1) assemblages. The rates of l eucine and thymidine incorporation decreased exponentially with increa sing pressure for the free-living and linearly for attached bacteria, while there was no significant influence of pressure on cell numbers. At 100 atm leucine and thymidine incorporation rate per free-living ba cterium was reduced to 73 and 20%, respectively, relative to that meas ured at I atm. Pressure of 100 atm reduced leucine and thymidine incor poration per attached bacterium to 94 and 70%, and at 200 atm these ra tes were reduced to 34 and 51%, respectively, relative to those measur ed at 1 atm. There was no significant uncoupling of thymidine and leuc ine incorporation for either the free-living or attached bacterial ass emblages with increasing pressure. indicating that the processes of DN A and protein synthesis may be equally affected by increasing pressure . It is therefore unlikely that bacteria, originating from surface wat ers, attached to rapidly sinking particles play a role in particle rem ineralization below approximately 1000-2000 m. These results may help to explain the occurrence of relatively fresh aggregates on the deep-s ea bed that still contain sufficient organic carbon to fuel the rapid growth of benthic micro-organisms; they also indicate that the effect of pressure on microbial processes may be important in oceanic biogeoc hemical cycles.