Program     Harmful algal blooms: mechanisms, monitoring, and prevention in a rapidly changing world       Poster Proofs for production of sexual resting cyst by the toxic dinoflagellate Karenia mikimotoi from clonal cultures and marine sediments P-B1-22-S Yuyang Liu* , CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences;University of Chinese Academy of Sciences Zhangxi Hu, CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences;Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology Yunyan Deng, CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences;Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology Yingzhong Tang, CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences;Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology Presenter Email: lyy9303130@163.com The toxic dinoflagellate Karenia mikimotoi is known to form large-scale and dense harmful algal blooms (HABs) in coastal waters worldwide and cause serious economic loss in aquaculture and fisheries and other adverse effects on marine ecosystems. Therefore, understanding the ecology of this species is extremely urgent, especially in the mechanisms of overwintering, bloom initiation, and geographic expansion. Resting cyst, as an important survival strategy in life cycle of dinoflagellates, has been proven to play vital roles in the initiation and termination of blooms and geographical expansion in many HABs-forming dinoflagellates. Whether or not K. mikimotoi forms resting cyst, however, has been a puzzling issue to the research community of HABs. Here, we provide visual and molecular confirmations of the production of sexual, thin-walled resting cysts by K. mikimotoi based on observations of laboratory cultures and detections in marine sediments. Evidences from the light and scanning electron microscopic observations in laboratory cultures include cell pairs in sexual mating, cells in fusion, planozygotes (two longitudinal flagella), thin-walled resting cysts and their germination processes (e.g. new germling with two longitudinal flagella), which together confirmed that K. mikimotoi produces sexual resting cysts homothallically. Evidences from marine sediments collected from locations where K. mikimotoi bloomed frequently include positive PCR detection using species-specific primers, positive detection with fluorescence in situ hybridization (FISH) using species-specific oligonucleotide probe which targets the LSU rDNA D2 domain of K. mikimotoi and was labeled with either FITC or Cys3, light microscopic observation of cysts labeled with FISH probe, and identity confirmation with single-cell PCR sequencing for cysts labeled with FISH probe. The confirmation of sexual resting cyst production by K. mikimotoi in laboratory cultures and field sediment samples provides a possible mechanism accounting for the recurrences of annual blooms at certain regions and the global expansion of K. mikimotoi blooms during the past decades. Although a postulated population initiation based on the cyst abundance in sediments reflected in our FISH detection may lead to a bloom (e.g. 107 cells L-1) in about a month for a shallow water, the extremely low abundance of cysts, however, necessitates more extensive surveys on temporal and spatial distribution of cysts in the field in order to reveal the exact roles played by resting cysts in the population dynamics of K. mikimotoi, which is ongoing now.

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