Title: "Chemotaxis in marine bacterium Vibrio alginolyticus" 

Abstract: We investigated the motility pattern and chemotaxis system of the polarly flagellated marine bacterium Vibrio alginolyticus. V. alginolyticus executes 3-step (run-reverse-flick) cycles which are distinctively different from the 2-step (run-tumble) pattern of Escherichia coli. This marine bacterium backtracks its forward swimming path and randomizes its moving direction by flicking its flagellum at the end of the backward swimming interval. Vibrio alginolyticus has a similar chemotaxis system as Escherichia coli, and our study showed that their chemotaxis networks respond to chemical cues in the same manner. However, at contrast to Escherichia coli, in which the motor bias is regulate by the chemotaxis network, in Vibrio alginolyticus, the switching rates of the flagellar motor is modulated so that swimming intervals in a favorable direction can be lengthened regardless of the motor rotation direction. As a result, despite their different motility patterns, both Escherichia coli and Vibrio alginolyticus use a biased random walk to migrate toward a nutrient source. To understand the effect of motility patterns on chemotaxis capacity, master equations similar to convection-diffusion equations were developed to describe the motion of these two bacteria in a chemical profile. It was found that by adopting the run-reverse-flick motility pattern, a 3-step swimmer has the same drift velocity but its diffusivity is reduced by half compared to a 2-step swimmer. As a result of the smaller diffusivity, the former localizes better around a nutrient source but does not explore as efficiently as the latter. We thus speculate that the 3-step motility pattern suits better for the marine environment where searching is unproductive and it is more important to exploit an existing, though transient, resource.