The objective of the DRASTIC project is to better understand marine environmental changes in high latitudes, and their consequences on the fate of planktonic siliceous organisms, such as diatoms and Rhizaria. The originality of the project was to develop a transdisciplinary approach (including ecology, physiology, biogeochemistry, and genomics) to quantify the role of Rhizaria and Diatoms in the silica cycle from the Arctic Ocean to the North Atlantic, in a spirit of low-carbon research mission using a sailboat as a scientific base.

Diatoms and Rhizarias play a key role in the trophic networks of the most productive coastal or open-ocean ecosystems, as well as in the biology-mediated transfer of CO2 from the surface to the ocean interior (the so-called biological pump of carbon).

Pelagic diatoms have been extensively studied, but these photosynthetic organisms are not the only ones that use dissolved mineral silica (dSi) to form a skeleton or a siliceous carapace. Recently, the combination of genomic and biogeochemical data has revealed that another group of silicifiers, the Rhizaria, has been largely underestimated and correlates with carbon export fluxes at 150 m depth, highlighting an unexpected role of this taxa in the carbon biological pump, and also in the silica cycle (Llopis Monferrer et al. 2020, 2022 ; Biard et al., Guidi et al., 2016).

To better understand the interplay of diatom and rhizarias in the ecosystems, their role in the food web, and determine the associated silicon and carbon flows, it is essential to better constrain the distribution, the biodiversity and the ecophysiology of these organisms.

In the literature, sediment samples are practically the only available data on Rhizaria in the World Ocean. Swanberg et al (1986) reported that Rhizaria are constant and usually abundant components of the plankton of western Norwegian fjords. Between Atlantic water and the Arctic Ocean, the Norwegian coast is a "sentinel site" where global biodiversity change has to be surveyed. In this regard, Bjorklund et al. (2012) have already reported the Northward migration of some Rhizaria species. However, live Rhizaria diversity in Norwegian Sea and fjord is poorly explored and no physiological data are available, particularly in comparison with diatoms. This site offers sharp environmental gradients with latitude and from the coast to the deep ocean on a limited sampling area. We will compare our results with data collected further south, in the North Atlantic (APERO project, June-July 2023), and in the Iroise Sea (RHIZET project) and thus get a more comprehensive picture of the distribution of Rhizaria and diatoms from the Artic to the Atlantic Ocean.

To reach our goal, we have sampled seawater along the Norwegian coast, in a north-south transect from Tromsø to Bergen. Ultimately, this project should help us to answer the following questions

1. What is the latitudinal richness and the ecologic structure of the siliceous planktonic assemblages? Do we find the same species as those reported about 30 years ago (Swanberg et al., 1986) and in the sediment ?
2. Can we link the biodiversity and biomass of rhizarians and of the microplankton community to environmental factors (nutrient concentrations, temperature, depth, light, etc.)? How do rhizarias and diatoms respectively contribute to the silica and carbon cycle?
3. What are the regulating factors at the cell level of major biogeochemical processes?