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 Ca2+ signal. However, the source of Ca2+ in these cells has not been established. The present studies indicate that Phaeocystis’ secretory granules work as an intracellular Ca2+ oscillator. Optical tomography reveals that photo-stimulation induces InsP3-triggered periodic lumenal [Ca2+] oscillations in the granule and corresponding out-of-phase cytosolic oscillations of [Ca2+] that trigger exocytosis. This Ca2+ dynamics results from an interplay between the intragranular polyanionic matrix, and two Ca2+-sensitive ion channels located on the granule membrane: an InsP3-receptor-Ca2+ channel, and an apamin-sensitive K+ channel.