BACTERIAL BIOMASS AND ACTIVITY IN THE DEEP WATERS OF THE EASTERN ATLANTIC - EVIDENCE OF A BAROPHILIC COMMUNITY

Bacterial biomass and activity were investigated in deep waters at two sites in the eastern Atlantic, of similar depth (4560-4800 m), but va rying in their nutritional status. The Northern (N) site was eutrophic and subject to a strong seasonal input of surface derived organic mat ter (phytodetritus) to the sediment. The Southern (S) site was oligotr ophic. Deep water at this site does not appear to receive any strong s easonal input. Bacterial numbers in the deep water column at the N sit e showed no significant seasonal variation but were greater than those at the S site. Deep water bacteria were typically small and free-livi ng. From biovolume determinations, if was estimated that mean concentr ations of bacterial organic carbon at depths greater than 500 m were 0 .12 (0.03-0.29) mu g Cl-1 and 0.02 (0.01-0.04) mu g Cl-1 at the N and S sites, respectively. Rates of thymidine and leucine incorporation we re used as indicators of bacterial activity. Bacterial communities in water in contact with the sediment (SCW; sediment contact water) at bo th sites (but especially at the S site) were strongly barophilic at in situ temperatures (2.5-4.1 degrees C). The barophilic response of thy midine incorporation was enhanced when SCW samples from the N site wer e incubated at 11.5 degrees C. It is proposed that this result indicat ed an elevating effect of pressure on cardinal temperatures and that t he SCW community was obligately psychrophilic when unpressurised. Comp arison of cell-specific incorporation rates determined under in situ c onditions showed bacteria in the SCW to have levels of activity compar able with bacteria from a depth of 150 m. Thymidine incorporation rate s were highest in SCW samples taken at the N site in May 1988 and Sept ember 1989. Thymidine incorporation by SCW samples taken immediately b efore (10 April 1994) the main spring-bloom-associated deposition of p hytodetritus was significantly lower and comparable with that determin ed for the oligotrophic S site. The attributes exhibited by the SCW co mmunity appeared to be highly localised. We conclude that the bacteria l communities of the SCW are active and adapted to their environment. Activity is influenced by the trophic nature of the site and may show temporal changes linked with episodic food supply. We postulate that t he existence of such communities is linked to the role of the sediment -water interface as the initial site of deposition of sea-surface deri ved labile organic material. (C) 1997 Elsevier Science Ltd.