Ecophysiological responses of Ostreopsis towards temperature: A case study of benthic HAB facing ocean warming.

Reports of the benthic dinoflagellate Ostreopsis spp. have been increasing in the last decades, especially in temperate areas. In a context of global warming, evidences of the effects of increasing sea temperatures on its physiology and its distribution are still lacking and need to be investigated. In this study, the influence of temperature on growth, ecophysiology and toxicity was assessed for several strains of O. cf. siamensis from the Bay of Biscay (NE Atlantic) and O. cf. ovata from NW Mediterranean Sea. Cultures were acclimated to temperatures ranging from 14.5 °C to 32 °C in order to study the whole range of each strain-specific thermal niche. Acclimation was successful for temperatures ranging from 14.5 °C to 25 °C for O. cf. siamensis and from 19 °C to 32 °C for O. cf. ovata, with the highest growth rates measured at 22 °C (0.54-1.06 d-1) and 28 °C (0.52-0.75 d-1), respectively. The analysis of cellular content of pigments and lipids revealed some aspects of thermal acclimation processes in Ostreopsis cells. Specific capacities of O. cf. siamensis to cope with stress of cold temperatures were linked with the activation of a xanthophyll cycle based on diadinoxanthin. Lipids (neutral reserve lipids and polar ones) also revealed species-specific variations, with increases in cellular content noted under extreme temperature conditions. Variations in toxicity were assessed through the Artemia franciscana bioassay. For both species, a decrease in toxicity was observed when temperature dropped under the optimal temperature for growth. No PLTX-like compounds were detected in O. cf. siamensis strains. Thus, the main part of the lethal effect observed on A. franciscana was dependent on currently unknown compounds. From a multiclonal approach, this work allowed for defining specificities in the thermal niche and acclimation strategies of O. cf. siamensis and O. cf. ovata towards temperature. Potential impacts of climate change on the toxic risk associated with Ostreopsis blooms in both NW Mediterranean Sea and NE Atlantic coast is further discussed, taking into account variations in the geographic distribution, growth abilities and toxicity of each species.

The benthic toxic dinoflagellate Ostreopsis cf. ovata in the NW Mediterranean Sea: Relationship between sea surface temperature and bloom phenology.

Blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata can induce ecological and human health issues in certain temperate areas. In order to prevent these negative effects, long-term monitoring studies of O. cf. ovata blooms have been conducted in several impacted areas to have a comprehensive understanding of bloom dynamics and efficient tools for risk management. O. cf. ovata blooms were monitored every summer (from mid-June to the end of August) on five identified sites in Larvotto beach (Monaco, NW Mediterranean Sea), between 2007 and 2019. This time-series represents one of the largest time-series in the world describing blooms of this species. Bloom phenological features (timing, duration, maximum cell abundance and growth rate), were found to be highly variable throughout the studied period, and were analyzed as a function of different hydroclimatic parameters, including sea surface temperature (SST). The highest net growth rates were related to temperatures ranging between 21°C and 25°C, and did not coincide with maximal temperature records (27.5°C). Such results suggest that, although global warming possibly influences the expansion of O. cf. ovata from tropical to temperate waters, the definite impact of temperature on bloom dynamics might be more complex than a simple facilitation factor for algal growth, at least in NW Mediterranean waters. Furthermore, monthly SST anomalies calculated over this 13-year survey showed a strong positive correlation between spring SST positive anomalies and the bloom starting date, indicating that blooms occurred earlier in the season when spring SSTs were warmer than usual. Overall results provide tools to modelers and managers who are facing crucial challenges to predict the distribution and phenology of O. cf. ovata blooms in European coastal waters, moreover in a context of global warming.