Growth of bacteria in seawater filtered through 0.2 µm Nuclepore membranes. Implications for dilution experiments

Dilution experlments were conducted in the Caribbean and Sargasso Seas in a n attempt to measure bacterial growth under reduced grazing pressure. In this type of experiment, a small volume of the intact plankton sample is diluted into the same seawater from which all organisms have presumably been removed by filtration We noted that d ~l u e n t prepared by vacuum filtration (< 100 mm Hg pressure) of natural seawater through 0.2 pm Nuclepore membranes contained bacterla which could be retained by the same membranes upon refiltration Upon ~ncubation, these bacteria rapidly accumulated 3H-amino acids and grew at high rates (0.18 h-') The literature supports the evidence of ultramicrobacteria (those which might be expected to pass through 0.2 pm membranes): these may have passed into the diluent but were subsequently retained on 0 2 pm membranes poss~bly because they increased in cell size, formed ultramicrocolonies, or were aspherical In shape so that the orientation in relation to the membrane pore determined whether the cell was filtered or retained. Although the apparent specific growth rate of bacteria was in inverse relation to the degree of sample dilution, interpretable as a consequence of grazing on bacteria, the growth of bacteria in the diluent made it problematic to consider the dilution experlments in the conventional manner. It IS likely that the 0.22 pm cellulose ester membranes used by many others who have performed such experiments would b e much more effective in retaining ultramicrobacteria.