Redescription and neotypification of <i>Alexandrium minutum</i> Halim—the type species of <i>Alexandrium</i> —from its type locality in Alexandria, Egypt, with insights into ribotype biogeography and toxicity

Abstract Alexandrium minutum Halim, the generitype species of the toxigenic Alexandrium, was described from a 1958 bloom in the Eastern Harbour, Alexandria (Egypt), but has never been re‐examined with modern methods from the type locality. Here, we establish a monoclonal strain (TIO1796) by cyst germination from Alexandria's port sediment and a second strain (TIO1816) from plankton samples from the Gulf of Suez. Cell morphology of both cultured strains and plankton samples was examined using scanning electron and light microscopy. The plate formula of A. minutum was identified as Po, *4′, 6′′, 6C, 9S, 5′′′, 2p, 1′′′′. Our results confirmed the consistent presence of a ventral pore and a variable connection between Po and 1′. Molecular phylogenetic analyses based on LSU rRNA gene and ITS rRNA region sequences revealed four ribotypes (A–D) of A. minutum . The LSU rRNA gene and ITS rRNA region sequences of both strains were identical and belong to the widespread ribotype A (“Global/Mediterranean clade”). The other ribotypes are restricted to the Asia‐Pacific region. Strain TIO1796 (type locality) was non‐toxic regarding paralytic shellfish toxin (PST &lt; 0.024 pg · cell −1 ), whereas TIO1816 produced GTX1/4‐dominated toxins (0.48 pg PST · cell −1 ). These results (i) anchor the name A. minutum to ribotype A via neotypification of strain TIO1796, (ii) document a PST‐producing A. minutum in the Red Sea, and (iii) provide evidence supporting that A. minutum constitutes a cryptic species complex analogous to the A. tamarense species complex. The historical decline of A. minutum blooms in Alexandria since the 1990s correlates with near‐absent cyst banks in recent sediments.