Many marine dinoflagellates emit bright discrete flashes of light nearly instantaneously in response to either laminar or turbulent flows as well as to direct mechanical stimulation. The flash involves a unique pH-dependent luciferase and a proton-mediated action potential across the vacuole membrane. The mechanotransduction process initiating this action potential is unknown. The present study investigated the role of Ca(2+) in the mechanotransduction process regulating bioluminescence in the dinoflagellate Lingulodinium polyedrum. Calcium ionophores and digitonin stimulated luminescence in a Ca(2+)-dependent manner in the absence of mechanical stimulation. Mechanically sensitive luminescence was strongly inhibited by the intracellular Ca(2+) chelator BAPTA-AM [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethyl ester]; there was only a partial and irreversible dependence on extracellular Ca(2+). Ruthenium Red, a blocker of intracellular Ca(2+) release channels, inhibited mechanically sensitive luminescence. Luminescence was also stimulated by increasing K(+), even in the absence of extracellular Ca(2+); K(+) stimulation was inhibited both by BAPTA-AM and Ruthenium Red. These results support the hypothesis that Ca(2+) mediates stimulated bioluminescence and also indicate the involvement of intracellular Ca(2+) stores. Rapid coupling between mechanical stimulation and mobilization of intracellular Ca(2+) stores might occur through a mechanism similar to excitation-contraction coupling in skeletal muscle.