Marine bacterial organisation around point-like sources of amino acids

Abstract To better understand the trigger for and use of motility in marine bacterial chemotaxis, specific (amino acids) chemical stimuli were used to test the effect on bacterial speed and reorientation. An assay system was developed to analyse bacterial behavioural responses to point-like nutrient sources (beads 10-40 mum). The marine bacteria Pseudoalteromonas haloplanktis, Shewanella putrefaciens, Deleya marina, the enteric bacterium Escherichia coli and an enriched assemblage of marine bacteria were used in the assay. E. coli responded to the amino acids (e.g. serine, leucine) in this assay in a manner similar to the classical capillary plating method, and accumulated near the attractants using biased random walks and maximum speeds of less than 30 mum s(-1). The marine bacteria travelled more than 100 mum s(-1) and reversed direction instead of randomly tumbling to reorientate. Marine bacteria also showed different chemotactic responses. The enriched marine community and P. haloplanktis were repelled by serine while leucine was an attractant for all three marine isolates. More importantly marine bacteria formed band specific distances (>20 mum) away from individual beads. The bands were composed of free-swimming bacteria rather than bacteria attached to the beads or chamber surfaces. The bands' formation time (25-180 s) and width (5-20 mum) varied depending on the bacterial strains and attractant. This variation in chemotactic responses by the marine isolates may reflect adaptations to specific ecological niches of bacterioplankton in the ocean. However, band forming ability by marine bacteria has still only been reported in experimental studies, nutrient coupling between nutrient sources and bands of chemotactic marine bacteria have yet to be detected in the natural environment. The fragility of the bands, a clear association between free-swimming bacteria and nutrient, indicates the difficulty and potentially sets limits for finding microzones of bacteria around particles in the open ocean.