Intraspecific variability within Karlodinium armiger (Dinophyceae) on a toxicological and metabolomic level

The species Karlodinium armiger occasionally co-occurs with Karlodinium veneficum during harmful algal blooms. The only toxin of this species described so far is karmitoxin, a highly ichthyotoxic compound very similar to the karlotoxins produced by K. veneficum. However, information on K. armiger is limited and based on a single Mediterranean strain (K-0668), with few studies investigating its toxicity. Given the high intraspecific variability known in K. veneficum, it was a significant achievement when two additional strains of K. armiger (MD-D6 and MD-D7) were isolated from the Labrador Sea in 2017, enabling comparative studies within this species. The toxicity of these three strains was assessed using the fish gill cell line RTgill-W1 and the cryptophyte Rhodomonas salina. An untargeted metabolomics approach using high-resolution tandem mass spectrometry, along with a computational workflow, provided insights into the metabolomic differences between the strains. Despite being cultivated under identical conditions, the metabolomic profiles and toxicological properties were distinct, even between MD-D6 and MD-D7, which were isolated from the same water sample. While MD-D7 did not exhibit significant toxicity, MD-D6 showed high toxicity and lytic potential, similar to K-0668. Interestingly, karmitoxin was only detected in K-0668, and neither karlotoxins nor any known analogs were detected in any strain. Within this comprehensive workflow, some molecules were found in MD-D6 that share the same chemical space as karmitoxin, making them interesting targets for further research. In conclusion, this study evaluated the toxicological and metabolic variability in three different strains of K. armiger and identified some putative toxin candidates in MD-D6.