In the Chilean fjords, the frequency of Harmful algae blooms (HAB) has increased over the last four decades. Important HAB-forming species remain understudied, such as the dinoflagellate Protoceratium reticulatum. Understanding its ecology is crucial to anticipate and manage future blooms. Therefore, this study focused on the production of cysts and allelopathic compounds. In places like the Bay of Ilque (41.6236° S, 73.0750° W), HABs entail a threat for shellfish and salmon activity. Water and sediment samples were collected during the austral summer of 2023, and no P. reticulatum blooms were detected. P. reticulatum cysts were quantified in the sediment throughout the bay, suggesting that this area acts as a reservoir of cysts with potential to trigger future blooms. Cyst germination and allelopathy production were studied under laboratory conditions. Cyst from P. reticulatum were incubated at 9, 15 and 21ºC. The germination success was extremely low, suggesting either non-viability of the cysts or the involvement of other environmental factors in the germination process. Co-cultures of P. reticulatum and Heterosigma akashiwo, and the cell-free filtrates of the co-cultures, reduced the growth rate of H. akashiwo, revealing that allelopathy is likely to be one of the mechanisms that favor P. reticulatum over other species to form blooms.
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