Light-dependent signal transduction and transient changes in cytosolic Ca2+ in a unicellular green alga

The physiological function and the molecular mechanisms of Ca2+-mediated signal transduction processes were studied in the unicellular green alga Eremosphaera viridis by different electrophysiological and microfluorimetric techniques. A sudden blockage of photosynthetic electron transport by darkening or inhibitors causes a transient hyperpolarization of the plasma membrane, For the alga this transient hyperpolarization seems to be an important mechanism to release monovalent ions and to drive the uptake of divalent cations. The transient hyperpolarization is due to the opening of K+ channels and is caused by a rapid transient elevation of the cytosolic free Ca2+ concentration ([Ca2+](cy) spike). Different agonists like caffeine or InsP(3) which are known to release Ca2+ from internal stores in animal cells, also cause a transient hyperpolarization and a [Ca2+](cy) spike, similar to darkening, In Eremosphaera the transient hyperpolarization can be used as an indicator for [Ca2+](cy) spikes. The InsP(3) gated and the ryanodine/cADPR gated Ca2+ channels which obviously both mediate Ca2+ release from internal stores in Eremosphaera do not seem to be involved in the dark-induced [Ca2+](cy) spikes. Besides single [Ca2+](cy) spikes, the addition of Sr2+ (or caffeine in the absence of divalent cations) causes repetitive [Ca2+](cy) spikes which may last hours and resemble [Ca2+](cy) oscillations observed in excitable animal cells. These observations suggest that some principal molecular mechanisms causing single or repetitive [Ca2+](cy) spikes are conserved from animal to plant cells.