Cytoplasmic calcium and Fucus egg activation

Eggs of the marine brown alga, Fucus serratus, exhibit small transient elevations of cytosolic Ca2+ of variable magnitude, corresponding to the onset of the fertilization potential. Microinjection of Ca2+ buffers (BAPTA (1-2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid)) at concentrations sufficient to block any global fertilization-associated Ca2+cyt elevation did not inhibit egg activation (monitored as exocytosis of cell wall) or subsequent development. However, egg activation could be inhibited with higher buffer concentrations. Br2BAPTA (Kd = 1.6 μM) was a more effective inhibitor of egg activation than BAPTA (Kd = 0.17 μM). Localized microinjection of Ca2+ produced only localized cell wall exocytosis at the injection site. Eggs injected with Br2BAPTA at intracellular concentrations, which blocked egg activation, exhibited prolonged fertilization potentials. 45Ca2+ influx across the plasma membrane increased during fertilization. Our results show that a large transient global elevation of Ca2+cyt is not necessary for Fucus egg activation but rather a localized elevation to micromolar levels results, at least in part, from increased Ca2+ influx across the plasma membrane. This is needed for early fertilization events, including the generation of the fertilization potential and cell wall secretion.