NO EVIDENCE FOR AN ALLELOPATHIC ROLE OF OKADAIC ACID AMONG CIGUATERA-ASSOCIATED DINOFLAGELLATES

Serine/threonine protein phosphatases are regulatory enzymes critical to growth and replication in eukaryotes. Okadaic acid (OA), a potent serine/ threonine protein phosphatase inhibitor, has been well studied in vertebrates; however, little is known about the role this phycotoxin plays in the ecology of its producer, the benthic dinoflagellate Prorocentrum lima Ehrenberg. The frequency and diversity of toxins produced by benthic dinoflagellates suggests that toxin production might play a role in competition among algal species. To assess the role of OA in growth competition, dinoflagellate species that co-occur with P. lima were grown in medium preconditioned by P. lima. Exudates from P. lima inhibited the growth of three of the four co-occurring species. All four dinoflagellate species were found to possess protein phosphatases that are inhibited by low concentrations of OA (IC₅₀∼5 nM). Furthermore, when treated in culture for 24 h with either purified OA or medium preconditioned by P. lima, their protein phosphatase activity was substantially inhibited. In contrast, P. lima 's protein phosphatase activity was refractory to OA both in vitro and in culture. To determine whether its growth inhibitory action was dependent on its phosphatase inhibitory activity, the P. lima -preconditioned medium was fractionated by HPLC and fractions tested for growth-inhibitory and protein phosphatase–inhibitory activity. We found that the two activities reside in different fractions. These results suggest that although OA has growth-inhibitory potential against other microalgae, it does not represent the major growth-inhibitory activity found in P. lima 's medium.