Chemical signal-response in diatoms of the genus Amphora

The marine diatom Ampbora coffeaeformis is positively chemotactic to glucose and sugars of similar structure. A free hydroxyl group is required at the 2-position of the ring; however, D-mannose (a toxic sugar) and L-glucose promote negative chemotaxis. Sugar metabolism is not necessary for a chemosen-sory response. When the direction of higher concentration in a D-glucose gradient is reversed, cells already orientated reverse their direction of movement. Pretreatment of diatoms with a uniform concentration of D-glucose before exposure to a concentration gradient eliminates the chemotactic response. When diatom cells attached to surfaces in the presence of 5 mM-Ca2+ are exposed to 0·25–0,625mM Ca2+, their motility drops rapidly. However, when this experiment is carried out in the presence of a uniform concentration of D-glucose or 3-O-methyl-D-glucose, motility is sustained for 25–30 min before falling. Speed of motility is also increased during this period. These results can be interpreted in terms of an agonist bindingsecretion coupling model for diatom chemotaxis.