PHOTOINHIBITION OF MECHANICALLY STIMULABLE BIOLUMINESCENCE IN THE HETEROTROPHIC DINOFLAGELLATE PROTOPERIDINIUM-DEPRESSUM (PYRROPHYTA)

Photoinhibition of mechanically stimulable bioluminescence (MSL) in th e heterotrophic dinoflagellate Protoperidinium depressum Bailey was in vestigated using samples collected from the Massachusetts and southern Texas coasts. The times for both photoinhibition of MSL (ca. 10 min) and dark recovery from photoinhibition of MSL (ca. 45 min) in this spe cies were similar to those reported for autotrophic dinoflagellates. T he degree of photoinhibition of MSL was a linear function of the logar ithm of photon flux density (PFD). The threshold PFDs for the photoinh ibition of MSL were 0.02, 0.6, and 21 mu mol photons . m(-2). s(-1) fo r broad-band blue, green, and red light, respectively. These PFDs are lower than those required for photoinhibition of MSL by the autotrophi c dinoflagellates Pyrocystis lunula and Ceratium fusus. We speculate t hat photosynthetic pigments in autotrophic dinoflagellates shield the photoreceptor that causes photoinhibition of MSL, thus lowering the se nsitivity of these dinoflagellates to light. When field-collected P. d epressum were kept in the laboratory without growth for a week, photoi nhibition of MSL's sensitivity to light increased progressively along with 1) a decrease in its bioluminescence capacity (BCAP), 2) a decrea se in the ratio of MSL to BCAP (MSL / BCAP), and 3) a decrease in the orange pigmentation (probably carotenoid) of the dinoflagellate. The a ction spectrum for photoinhibition of MSL in P. depressum was characte rized primarily with a broad peak in the blue extending into the green . We suggest that carotenoid was not a photoreceptor for the photoinhi bition of MSL in P. depressum because the peak of the action spectrum was too broad and extended too far into the green part of the spectrum , and because the orange pigment present decreased as photoinhibition of MSL became more sensitive to light.