Bioluminescence of the heterotrophic dinoflagellate <i>Polykrikos kofoidii</i> Chatton 1914 (Dinophyceae)

Heterotrophic dinoflagellates are important predators of phytoplankton, and many species produce bioluminescence, which is thought to serve as an antipredator function. The present study investigated the bioluminescence of the heterotrophic dinoflagellate Polykrikos kofoidii, an important predator of toxic and bloom-forming dinoflagellates. Its flashes were slow and dim compared to those of other dinoflagellates but with a similar spectral emission. Based on hyperspectral and laser confocal microscopy, autofluorescence consistent with that of luciferin was distributed throughout the cell, with only a few punctate sources typical of scintillons from other luminescent dinoflagellates. Polykrikos kofoidii consumed whole prey, which initially remained intact with measurable autofluorescence of chlorophyll, if plastid-containing, and luciferin, if luminescent. Polykrikos kofoidii encoded a luciferase gene with three conserved catalytic domains with an N-terminal region that appeared to contain a luciferin-binding protein-like motif. In three of the nine publicly available P. kofoidii transcriptomes, a luciferin-binding protein with homology to Noctiluca scintillans hybrid luciferase-luciferin binding domain was detected. The slow and dim flashes of P. kofoidii resulted in a bioluminescence signature that was distinct from other luminescent dinoflagellates, whereas the diffuse cellular distribution of luciferin fluorescence was unlike the punctate scintillon emission observed in previously studied luminescent dinoflagellates. This suggests that the cellular regulation of luciferin in P. kofoidii may differ from that of other dinoflagellates.