Clustering of Marine Bacteria in Seawater Enrichments

Seawater enrichments of marine bacteria clustered in 20- to 50-μm-wide bands near air-water interfaces. The cells within the band travelled at up to 212 μm s^-1 and at an average speed of 163 μm s^-1. Mean cell speeds peaked mid-run at 187 μm s^-1. At the end of the run, bacteria reversed direction rather than randomly reorienting. The duration of the stops during reversal was estimated at 18 ms, six to seven times shorter than that found in enteric bacteria. Cells hundreds of micrometers from the band travelled at half the speed of the bacteria in the band. The fastest isolate from the seawater enrichment was identified as Shewanella putrefaciens and had an average speed of 100 μm s^-1 in culture. Air-water interfaces produced no clustering or speed changes in isolates derived from enrichments. Salinity and pH, however, both influenced speed. The speed and reversal times of the seawater enrichments indicate that the bacteria in them are better adapted for clustering around small point sources of nutrients than are either enteric or cultured marine bacteria.