Depth-related cell-specific bacterial leucine incorporation rates on particles and its biogeochemical significance in the Northwest Mediterranean

A study of total and particle-associated bacteria in the Northwest Mediterranean revealed notable relationships with depth. Cell-specific leucine incorporation rates by total bacteria decreased significantly with depth (to 380 m), best fitting the power function log (rate) = 2.1-0.54 log (depth), r(2) = 0.57, P = <0.001. In contrast, cell-specific leucine incorporation rates by particle-associated bacteria increased with depth as described by the power function log (rate) = 0.4+0.84 log (depth), r(2) = 0.74, P = <0.01. We suggest that this relationship may reflect availability of usable dissolved organic matter (UDOM) and/or inorganic nutrients within the particle with increasing water depth. The latter relationship could also be interpreted in relation with time, i.e., be a consequence of the time taken for enzyme induction and solubilization of particulate organic matter (POM) to UDOM and the subsequent incorporation into bacterial protein during the sinking of the particle. These results may help to explain observations that sinking particles are rapidly decomposed between 100 and 500 m. Bacteria associated with particles in the Mediterranean contributed only 0.8-2.8% of total bacterial numbers but contributed 3-12% of total bacterial production in the upper 70 m. However, their contribution to total bacterial production in deeper waters was substantially higher (48% at 380 m). The activity of attached bacteria is, therefore, important and varies with depth, and this may help to explain the particularly low particle flux and hence the impoverished benthic environment found in the deeper waters of the Mediterranean.