Summary: Trichomes of an sp. suspended in soft agar in Petri dishes showed active movement away from areas where gaseous exchange had been prevented by placing glass coverslips on the surface. This clearance response occurred only in the light and it was related to gradients of inorganic carbon that formed in the agar layer. In plates incubated in the dark trichomes accumulated in the central areas below glass coverslips but the substance eliciting this response could not be identified. Gradients of diffusible substances were established within lawns of cyanobacteria suspended in soft agar and it was demonstrated that trichomes of sp. moved towards CO. HCO and O in light-dependent chemotactic reactions. No chemotaxis occurred in response to these substances in the dark. The chemotactic responses were detected after 2 h but became increasingly distinct up to 8 h. The chemotactically active concentration of CO was greater than the atmospheric concentration since trichomes moved from air towards a source of CO. Using diffusivity coefficients it was calculated that trichomes of sp. accumulated at a CO partial pressure of 0.02 bar (equivalent to 0.83 mM). With O as the chemoattractant the value was 0.35 bar (14.56 mM). These results are discussed with reference to the roles of inorganic carbon and O in cyanobacterial metabolism and it is concluded that chemotactic behaviour may be important in movements within the photic zone of sediment environments.


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