Ostreopsis blooms: an indicator of the need to act in the climate and environmental emergency

12th International Conference on Modern and Fossil Dinoflagellates (DINO12), 4-8 July 2022, Las Palmas de Gran Canaria, Spain

The life in our planet is a history of continuous extinction and survival of organisms adapting to progressive or abrupt changes in the ecosystems. There is no doubt that we are facing a period of rapid changes in our planet caused by human pressures since the industrial revolution, with global warming as a major driving force. The argument of this presentation is that, through the genus Ostreopsis (Figure 1), nature is talking about the ongoing planetary changes. The biogeographic distribution of the identified Ostreopsis species, first identified in tropical areas and present in warm temperate coasts, has expanded from 20°S - 28°N in 1995, to 35°S-45°N in 2020 (Tester et al. 2020). This increased trend can be explained by a better and targeted monitoring, as well as by improved detection and species characterization by molecular tools. But the interest on Ostreopsis since the beginning of the 21st century is mainly due to the occurrence of high biomass blooms of certain species in temperate beaches during the summer vacation period. These proliferations have been associated to bathing water quality deterioration, respiratory and cutaneous irritations on humans exposed to marine aerosols, massive mortalities of some benthic macrofauna and the potential risk of food borne poisonings, because some Ostreopsis species synthesize analogues of the potent palytoxin related to fatalities (although not completely proved yet) in the tropics. Which are the main drivers of Ostreopsis blooms at present and which could be the expected trends in the projected scenarios of global warming? Focusing on O. cf. ovata, that proliferates recurrently in some Mediterranean beaches, seawater temperature is a major factor modulating its bloom phenology (Drouet et al. 2022). In the 13-years data series in Larvotto beach (Monaco), the highest in situ net growth rates were estimated in the 21°C - 25°C temperature range, while maximal values reached 27.5°C. Furthermore, blooms occurred earlier in the season when spring Sea Surface Temperatures were warmer than usual. The potential expansion of another species, O. cf. siamensis, towards warming waters of the Bay of Biscay (NE Atlantic) was also documented using in situ cells sampling, eDNA and connectivity modeling in the area (Drouet et al. 2021). Ostreopsis blooms occur in shallow, illuminated and relatively calm coastal waters, where cells thrive mainly attached to biotic (Figure 2) and abiotic surfaces. Being photosynthetic, inorganic nutrients should be available to allow high biomass blooms, although the direct link has not been clearly established. Often, Ostreopsis cells thrive as epiphyte of dense macroalgal turf carpets indicating that the microalga tolerates eutrophic and deteriorated habitats. With the available knowledge we can design strategies to mitigate the impacts of Ostreopsis blooms, diminish their occurrence while also acting in the present climate and environmental emergency

Peer reviewed