CALCIUM CONTROL OF PHOTOTACTIC ORIENTATION IN Chlamydomonas reinhardtii: SIGN AND STRENGTH OF RESPONSE

Abstract— Chlamydomonas reinhardtii responds to a blue light stimulus by an oriented swimming (phototaxis) toward or away from the stimulus source. In this study it is established that the sign and strength of the phototactic response are a complex function of extracellular Ca²⁺], stimulus fluence rate, time of analysis after onset of stimulation and light pretreatment. At very low extracellular Ca²⁺] the response is weak and usually negative. At Ca²⁺] close to the preconditioning level, phototactic response becomes stronger and positive. As Ca²⁺] is raised further, the initial (2 s) response remains positive but the long term (20 s) becomes negative and very strong. At extremely high Ca²⁺] the cells become immobile. This bimodal behavior suggests that two different mechanisms determine the direction of the turn. Data cannot be explained in terms of a simple model. The model which accounts for most of the details of the behavior is that of Kamiya and Witman (1984), which proposes that positive response is triggered by a transient increase in intracellular Ca²⁺] and negative response by a decrease below unstimulated level of Ca²⁺, at least in the range of 10^-9-10^-6 M Ca²⁺]. The strong negative orientation which follows an initial positive response above this level of Ca²⁺], in these experiments, is best explained by an adaptation of the cells due to an increased (on average) intracellular Ca²⁺].