Marine algae from the genus Karlodinium are known to be involved in fish-killing events worldwide. Here we report for the first time the chemistry and bioactivity of a natural product from the newly described mixotrophic
dinoflagellate Karlodinium armiger. Our work describes the isolation and structural characterization of a new polyhydroxypolyene named karmitoxin. The structure elucidation work was facilitated by use of 13C enrichment and high-field 2D NMR spectroscopy, where 1H−13C long-range correlations turned out to be very informative. Karmitoxin is structurally related to amphidinols and karlotoxins; however it differs by containing the longest carbon−carbon backbone discovered for this class of compounds, as well as a primary amino group.
Karmitoxin showed potent nanomolar cytotoxic activity in an RTgill-W1 cell assay as well as rapid immobilization and eventual
mortality of the copepod Acartia tonsa, a natural grazer of K. armiger.