Aliquots (100 ml) of the primary deep-sea water sample were placed in sterile 150 ml polyethylene sample bags. Some incubations were supplemented with 5 parts peptone (Oxoid, UK) to 1 part yeast extract (Oxoid, UK) [w/w]: PYE togivefinalconcentrationsof6mgl_1 or 30mgl_1 PYE Bags were heat sealed, taking care to exclude all air. Enrichmen experiments (of one-month duration) were carried out at both surface pressure(0.1MPa)and in-situ pressure (31MPa).For enrichments under in-situ pressure, bags(with and without PYE) were placed in titanium pressure chambers filled with tap water and incubated at 31 MPa and at 4 1C. Enrichments (with and without PYE)that were incubated at atmospheric pressure (0.1 MPa) were immersed in buckets filled with tap water and maintained at4 1C in the dark. Control samples were fixed immediately upon recovery by the addition of 1.5 ml of 40% (w/v) particle-free formalin (BDH, UK) to 100 ml aliquots of the primary deep-sea water sample. Time-zero samples were also incubated in the dark at 4 1C. After one month of incubation, bacterial activity with in samples was halted by the addition of particle-free formalin to bring the final concentration to 1.5% (w/v) formaldehyde in each bag.