Recovering sedimentary ancient DNA of harmful dinoflagellates off Eastern Tasmania, Australia, over the last 9 000 years

Harmful algal blooms (HABs) have significantly impacted the seafood industry along the Tasmanian east coast over the past four decades. To investigate the history of regional HABs, we applied sedimentary ancient DNA analyses ( sed aDNA) to coastal sediments up to ∼9 000 years old collected inshore and offshore Maria Island, Tasmania. We used metagenomic shotgun sequencing combined with a hybridisation capture array (‘HABbaits1’) to target harmful dinoflagellates of the genera Alexandrium, Gymnodinium, and Noctiluca scintillans . Bioinformatic analyses were used to verify sed aDNA sequences and their presence in older layers, especially for microreticulate cyst forming species including Gymnodinium catenatum due to its important role in shellfish toxicity. Our results show that the Alexandrium genus (up to 854 and 20 reads per sample inshore and offshore, respectively, based on capture-data) has been present off eastern Tasmania within the last ∼8 307 years. For G. catenatum we detected a total of only 9 unambiguously verified reads sporadically between ∼7 638 years ago and the present in the offshore core. We recovered verified sed aDNA of the fragile, non-fossilising species N. scintillans , along with evidence of increased relative abundance from 2010, consistent with plankton surveys. This study identifies challenges regarding sed aDNA sequence validation of some species (in particular, for G. catenatum ), and provides guidance for the development of tools to monitor past and present HAB species and events, and to improve future HAB event predictions. Highlights ### Competing Interest Statement The authors have declared no competing interest.