Mechanisms of signal transduction in photo‐stimulated secretion in Phaeocystis globosa

Phaeocystis globosa , a leading agent in marine carbon cycling, releases its photosynthesized biopolymers via regulated exocytosis. Release is elicited by blue light and relayed by a characteristic cytosolic Ca 2+ signal. However, the source of Ca 2+ in these cells has not been established. The present studies indicate that Phaeocystis ’ secretory granules work as an intracellular Ca 2+ oscillator. Optical tomography reveals that photo‐stimulation induces InsP 3 ‐triggered periodic lumenal [Ca 2+ ] oscillations in the granule and corresponding out‐of‐phase cytosolic oscillations of [Ca 2+ ] that trigger exocytosis. This Ca 2+ dynamics results from an interplay between the intragranular polyanionic matrix, and two Ca 2+ ‐sensitive ion channels located on the granule membrane: an InsP 3 ‐receptor‐Ca 2+ channel, and an apamin‐sensitive K + channel.