Chloroplast and oxygen evolution changes in Symbiodinium sp. as a response to latrunculin and butanedione monoxime treatments under various light conditions.

The actin cytoskeleton is a dynamic structure that provides an interactive platform for organelles and cellular components. It also serves as track for membranes and vesicles that move via myosin. The actin cytoskeleton of Symbiodinium is a well-organized reticular structure suggestive of multiple membrane interactions, very likely including those of the chloroplast. The Symbiodinium chloroplast membrane network is, in turn, a highly organized structure, suggestive of being under the control of an organizing network. We visualized the chloroplast membranes of cultured Symbiodinium sp. under various light conditions and observed changes dependent on illumination intensity. Since we suspected interaction between these two organelles, and we knew that the Symbiodinium actin cytoskeleton collapses upon treatment with either latrunculin B, an actin microfilament-disrupting agent, or butanedione monoxime, a myosin function inhibitor, we tested the Symbiodinium sp. oxygen evolution in their presence. Upon latrunculin B addition, the oxygen production decreased compared to non-treated cells; however, this was not observed after a 24 h latrunculin treatment. On the contrary, butanedione monoxime treatment caused a non-recoverable dysfunction of the chloroplast causing a severe loss in oxygen production even after long-term exposure. Using electron microscopy, we observed an alteration of the Symbiodinium sp. chloroplast distribution after latrunculin B treatment, with respect to untreated cells. Furthermore, a thorough disorganization of the chloroplast grana was observed after butanedione monoxime treatment. These data suggest that an actomyosin system would be important for chloroplast organization and distribution, and critical for normal photosynthetic function of Symbiodinium sp.