Summary: The distribution and abundance of mat-formingwere investigated at a siltdepositing freshwater site in England over a period of 4 years. Maximum growth occurred during April and May and experiments showed that mat buried by fine sediment could regain the surface by gliding movements. The number of trichomes reaching the surface was light-dependent but chemotactic responses were also apparent. Gliding rates of 1-2 μm s were recorded and the trichomes moved through the sediment as a left-handed screw with a pitch of 60°. A surface illumination of 1 klx (4 W m) was optimal for mat formation. Light intensities exceeding 2·5 klx produced a photophobic response resulting in lateral trichome migration to regions of lower surface illumination when these were provided. Migration into the sediment was not observed in the surface intensity range 0-10 klx. The onset of light saturation measured by the CO method was 3 klx for , close to the optimum for mat development. Experiments employing CO in the presence of sulphide and absence of oxygen suggested that the organisms were adapted to an aerobic environment.


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