Role of Rhizaria in biogeochemical Cycles in the epi-and mesopelagic ocean – RhiCycle

-Imagery in situ (underwater vision profiler 5 and 6) -Elemental composition (carbon and silicon) of marine rhizarians -Vertical flux quantification -Geochemical quantification of total fluxes (carbon and biogenic silica)

Rhizaria

- Quantification of elemental contents in ~1000 Rhizaria individuals and development of a mathematical model enabling prediction of these contents from a simple measurement of specimen size.

- Rhizarians have abnormally low cellular carbon densities compared with other planktonic groups. We interpret this anomaly as an ecological adaptation of certain Rhizaria to life in the deep ocean. In particular, this allows them to increase their size, necessary to capture the particles they feed on, reduce their metabolic requirements and balance the weight of their siliceous skeletons.

- Thanks to global quantification using in situ imaging data, we have used machine learning approaches to model the global distribution of Rhizaria biomass in the two surface layers of the oceans (0-200 m: the surface layer; 200-1000 m the mesopelagic layer). This modeling reveals that carbon biomass is lower than previously quantified in 2016, but that Rhizaria have a preponderant biomass in the Southern Ocean, particularly in the mesopelagic. In this deep layer, Rhizaria are particularly abundant, regardless of latitude. In addition to their carbon biomass, we have modeled their biogenic silica biomass. These underline their central role in the silicon cycle in the deep ocean, where they could be the main contributors to this cycle.

Methodological development:

- Based on the Underwater Vision Profiler 6 camera, we have developed a new method for measuring the falling (or swimming) speeds of particles (or planktonic organisms) directly in situ: VisuTrap. This approach collects a sequence of in situ images at high frequency (1Hz) to 1) detect particles larger than 100 µm, 2) identify them in a sequence of successive images, and 3) measure their velocity.

- This approach has been used on three different platforms: a cylinder-conical sediment trap deployed on a quasi-Lagrangian drift line; a PPS5 sediment trap anchored at a fixed depth for 1 year; and a modified Neutrally Buoyant Sediment Trap float.

- To date, the above-mentioned platforms have been deployed on a campaign in California, one in the Indian Ocean, one in the Mediterranean Sea and one in the Atlantic Ocean. The fixed trap was deployed for two consecutive years between France and Corsica. Finally, the float was deployed during the APERO campaign, alongside a floating line, itself equipped with cylinder-conical traps.

- This technological development gives us a completely new insight into the mechanisms by which particles fall into the oceans, a central mechanism in the carbon cycle.

Rhizaria:

One of RhiCycle's initial objectives was to quantify their vertical downward fluxes. This has yet to be determined. In addition, our global models show a high biomass in the Southern Ocean. This question of vertical fluxes in this ocean should therefore be addressed.

VisuTrap:

Generalize the approach developed to other platforms.

NA

Planktonic organisms play crucial roles in biogeochemical cycles that regulate Earth’s climate. Within planktonic communities, Rhizaria are diverse protists that build complex mineralized skeletons, for some, of biogenic silica. In recent years, they have begun to emerge as important organisms structuring planktonic communities and modulating biogeochemical processes linked to the silica and carbon cycles. Yet, considering their broad size spectra and extended range of vertical habitats, they still lack a full characterization and quantification of their impacts (i.e., export and flux attenuation) on these processes. To this end, the RhiCycle project will evaluate the role and quantitative contribution of Rhizaria in marine biogeochemical cycles, in particular the biological carbon pump and marine silica cycle, extending from the epipelagic to the mesopelagic ocean. Built around a multidisciplinary team of internationally recognized experts, an original combination of tools and expertise encompassing marine biogeochemistry, quantitative in situ imaging, sediment trap fluxes and molecular characterizations of diversity will be used. Designed to complement regional field studies and oceanographic cruises in a variety of ecosystems, laboratory experiments will generate information needed to interpret large-scale studies in different major oceanic basins (Northeastern Pacific, North Atlantic). Ultimately, since the full characterization and quantification of the impact of rhizarians on these processes has not yet been done in any part of the ocean, and given their new-found importance in marine biota, this project will significantly improve upon our knowledge of rhizarian ecology as well as expand understanding of key biogeochemical processes affecting the mesopelagic ocean.