Bacteria abundance measured on water bottle samples during HUDSON cruise HUD95/16@en

Samples for enumeration of bacterial abundance were drawn from Niskin bottles and immediately preserved in 20 mL glass scintillation vials with 0.2 µm filtered (Millex-GS, Millipore Corp.) formalin (2% final concentration). Preserved samples were stored dark in a refrigerator. Each sample was poured over triplicate polycarbonate membranes (0.2 µm black Nuclepore) underlain by pre-wetted backing filters (Sartorius #11306) which aid in distributing cells in an even manner on the Nuclepore surface. Cells were stained for 10 minutes with DAPI at a final concentration of 2 µg/ml and then drawn through the filters at a vacuum pressure of about 150 mm Hg. The filters were examined by epifluorescence microscopy (Leitz Orthoplan) under ultraviolet excitation from a mercury arc lamp (HBO 100W).