Species from several different phytoplankton groups have been shown to differ genetically between biogeographically different areas. These variations are important when dealing with toxic species, and may sometimes explain differences in toxicity. The variations are also important when considering the potential use of molecular probes for identification of phytoplankton species within groups of organisms that are difficult to identify.

Isolates of Pseudo-nitzschia delicatissima from biogeographically different areas of the world were examined using TEM and fluorescent rRNA-probes. Differences in both morphology and probing results will be discussed. The results indicate that probing with fluorescent rRNA-probes might be an easy way of screening for genetic variability. It may also be a means of understanding diversity of toxicity in toxic species.