Formation of 30- to 40-Micrometer-Thick Laminations by High-Speed Marine Bacteria in Microbial Mats

The precision with which motile heterotrophic bacteria could position themselves in microbial mats was determined. This required the development of a technique to view motile bacteria in situ. This was successfully achieved by replacing a 1-cm-diameter minicore from the mat sediment with 210- to 300- μm-diameter glass beads or sieved agar. After allowing 3 days for regrowth of the mat into the transparent medium, a cross section showed that bacteria formed a layer as thin as 30 to 40 μm at a depth of 500 μm below the surface. Bacterial concentrations in this microlamination were 20 times above background. Mean speeds were 200 μm s^-1 inside and 60 μm s^-1 outside the microlamination. The percentages of bacteria turning per 30 s were 93% inside and 10% outside the microlamination. Artificial chemical gradients were unsuccessful in stimulating microlamination formation or in eliciting the same extent of speed and turning responses. The significance of the results is that it is now possible to microscopically examine sedimentary bacteria in situ. Our first examination indicates that some bacteria form chemotactic microlaminations by increasing their turning frequency. This behavior is opposite that described in the enteric-based model of chemotactic movement, in which positive chemotaxis is achieved by decreasing the turning frequency.