MEPS 183:49-57 (1999)  -  doi:10.3354/meps183049

ABSTRACT: Twenty-seven samples of undecompressed seawater were collected between 800 and 2000 m depth in the NW Mediterranean. Most of the samples (24) were collected at the same sampling station. Potential ¹⁴C-glucose uptake rates and potential bacterial production rates were measured concomitantly under natural pressure conditions and under atmospheric pressure conditions. Compression of 5 surficial water samples suggested that metabolic activity of surface-borne bacteria would decrease when carried down by particle sedimentation or by a cascading seawater mass. Conversely, during the stratified water period, intermediate and deep-sea bacteria appeared adapted to the natural pressure conditions; for 90% of the samples metabolic rates were higher in samples kept in their natural pressure conditions than in their decompressed counterparts. Even if variable with time and depth, a global estimation of this adaptation to the natural pressure conditions resulted in a 3.5-fold increase relative to the measures done on decompressed samples. The pressure effect appeared at 800 m depth, a relatively shallow depth comparative to the average depth of the global ocean. In deep waters, potential glucose uptake rate and potential bacterial production rate varied greatly as a function of time and depth, ranging from 2 to 80 pmol C metabolised l^-1 h^-1, and from 4 to over 400 pmol bacterial C produced l^-1 h^-1, respectively.

KEY WORDS: Deep-sea bacteria · Bacterial activity · Pressure-retaining sampler · Pressure effect