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 Ca-cyt(2+) 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. Br(2)BAPTA (K-d = 1.6 mu M) was a more effective inhibitor of egg activation than BAPTA (K-d = 0.17 mu M). Localized microinjection of Ca2+ produced only localized cell wall exocytosis at the injection site. Eggs injected with Br(2)BAPTA at intracellular concentrations, which blocked egg activation, exhibited prolonged fertilization potentials. Ca-45(2+) influx across the plasma membrane increased during fertilization. Our results show that a large transient global elevation of Ca-cyt(2+) 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.