Ancient DNA and microfossils reveal dynamics of three harmful dinoflagellate species 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 (sedaDNA) 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 sedaDNA 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 sedaDNA 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 sedaDNA 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. HighlightsO_LIMetagenomic sedaDNA and hybridisation capture enabled analyses of harmful dinoflagellates off Tasmania C_LIO_LISequence validation was used to confirm the presence of Alexandrium spp., Gymnodinium spp. and Noctiluca scintillans C_LIO_LIAlexandrium and Gymnodinium have been present in Tasmanian waters during the past [~]9 000 years C_LIO_LINoctiluca scintillans sedaDNA derived relative abundance correlates with its recorded increase since 2010 C_LI