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Dinoflagellates are haploid eukaryotic microalgae in which rapid proliferation causes dense blooms, with harmful health and economic effects to humans. The proliferation mode is mainly asexual, as the sexual cycle is believed to be rare and restricted to stressful environmental conditions. However, sexuality is key to explaining the recurrence of many dinoflagellate blooms because in many species the fate of the planktonic zygotes (planozygotes) is the formation of resistant cysts in the seabed (encystment). Nevertheless, recent research has shown that individually isolated planozygotes in the lab can enter other routes besides encystment, a behavior of which the relevance has not been explored at the population level. In this study, using imaging flow cytometry, cell sorting, and Fluorescence In Situ Hybridization (FISH), we followed DNA content and nuclear changes in a population of the toxic dinoflagellate Alexandrium minutum that was induced to encystment. Our results first show that planozygotes behave like a population with an “encystment-independent” division cycle, which is light-controlled and follows the same Light:Dark (L:D) pattern as the cycle governing the haploid mitosis. Resting cyst formation was the fate of just a small fraction of the planozygotes formed and was restricted to a period of strongly limited nutrient conditions. The diploid-haploid turnover between L:D cycles was consistent with two-step meiosis. However, the diel and morphological division pattern of the planozygote division also suggests mitosis, which would imply that this species is not haplontic, as previously considered, but biphasic, because individuals could undergo mitotic divisions in both the sexual (diploid) and the asexual (haploid) phases. We also report incomplete genome duplication processes. Our work calls for a reconsideration of the dogma of rare sex in dinoflagellates. SI

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The research leading to the last developments of Diva has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 283607, SeaDataNet 2, and from project EMODNET (MARE/2008/03 - Lot 3 Chemistry - SI2.531432) from the Directorate-General for Maritime Affairs and Fisheries. Additional funding from the Local Government of the Balearic Islands (CAIB-51/2011 Grant) is also acknowledged. C. Troupin post-doctoral position is funded by MyOcean2 EU FP7 project Trabajo presentado en el 7th Coastal Altimetry Workshop, celebrado los días 7 y 8 de octubre de 2013 en Boulder, Co. (Estados Unidos) Peer Reviewed

Abstract. The last glacial period (LGP; ca. 110–10 kyr BP) was marked by the existence of two types of abrupt climatic changes, Dansgaard–Oeschger (DO) and Heinrich (H) events. Although the mechanisms behind these are not fully understood, it is generally accepted that the presence of ice sheets played an important role in their occurrence. While an important effort has been made to investigate the dynamics and evolution of the Laurentide ice sheet (LIS) during this period, the Eurasian ice sheet (EIS) has not received much attention, in particular from a modeling perspective. However, meltwater discharge from this and other ice sheets surrounding the Nordic seas is often implied as a potential cause of ocean instabilities that lead to glacial abrupt climate changes. Thus, a better comprehension of the evolution of the EIS during the LGP is important to understand its role in glacial abrupt climate changes. Here we investigate the response of the EIS to millennial-scale climate variability during the LGP. We use a hybrid, three-dimensional, thermomechanical ice-sheet model that includes ice shelves and ice streams. The model is forced off-line via a novel perturbative approach that, as opposed to conventional methods, clearly differentiates between the spatial patterns of millennial-scale and orbital-scale climate variability. Thus, it provides a more realistic treatment of the forcing at millennial timescales. The effect of both atmospheric and oceanic variations are included. Our results show that the EIS responds with enhanced ice discharge in phase with interstadial warming in the North Atlantic when forced with surface ocean temperatures. Conversely, when subsurface ocean temperatures are used, enhanced ice discharge occurs both during stadials and at the beginning of the interstadials. Separating the atmospheric and oceanic effects demonstrates the major role of the ocean in controlling the dynamics of the EIS on millennial timescales. While the atmospheric forcing alone is only able to produce modest iceberg discharges, warming of the ocean leads to higher rates of iceberg discharges as a result of relatively strong basal melting at the margins of the ice sheet. Our results clearly show the capability of the EIS to react to glacial abrupt climate changes, and highlight the need for stronger constraints on the ice sheet's glacial dynamics and climate–ocean interactions.