Recovering sedimentary ancient DNA of harmful dinoflagellates accumulated over the last 9000 years off Eastern Tasmania, Australia.

Harmful algal blooms (HABs) have had significant adverse impacts on the seafood industry along the Tasmanian east coast over the past 4 decades. To investigate the history of regional HABs, we performed analyses of sedimentary ancient DNA (sedaDNA) in coastal sediments up to ~9000 years old collected inshore and offshore of Maria Island, Tasmania. We used metagenomic shotgun sequencing and a hybridisation capture array ("HABbaits1") to target three harmful dinoflagellate genera, Alexandrium, Gymnodinium, and Noctiluca. Bioinformatic and DNA damage analyses verified the authenticity of the sedaDNA sequences. Our results show that dinoflagellates of Alexandrium genera have been present off eastern Tasmania during the last ~8300 years, and we sporadically detected and unambiguously verified sequences of Gymnodinium catenatum that were present offshore up to ~7600 years ago. We also recovered sedaDNA of the fragile, soft-bodied Noctiluca scintillans with increased relative abundance since 2010, consistent with plankton surveys. This study enabled us to identify challenges of sedaDNA sequence validation (in particular for G. catenatum, a microreticulate gymnodinoid species) and provided guidance for the development of tools to monitor past and present HAB species and improvement of future HAB event predictions.