Eppur si muove: many water column bacteria are motile

Models of chemosensory behaviour and microscale patchiness in the water column often assume that most or all suspended bacteria are motile. Yet direct observations on fresh seawater samples seem to indicate that only a few percent of suspended bacteria swim. Consequently, most experimental studies on the role of bacterial chemotaxis in the water column depend on pure strains of bacteria or on incubations enriched with organic substrates. Here it is shown that a large fraction (typically >20%) of the bacteria tin terms of direct counts) are, in fact, motile and show chemosensory behaviour. The discrepancy is due to the tendency of motile bacteria to attach rapidly to solid surfaces; in microscope preparations that are suited for observation of mu-m-sized particles most motile cells will become attached within a few seconds.