Rhizaria are a diverse group of amoeboid protists (i.e., unicellular eukaryotes) characterised by the presence of pseudopodia, cytoplasmic extensions used for locomotion and/or predation. They include organisms spanning a wide size range, from a few micrometres up to several centimetres, or even meters for the largest. Rhizaria are distributed worldwide from surface to deep waters, but also in freshwater and terrestrial ecosystems. In pelagic ecosystems, some Rhizaria build complex tests using inorganic/organic material or a variety of minerals: calcium carbonate for Foraminifera, opaline for Radiolaria and Phaeodaria. Radiolaria are well known for their paleontological signatures, but little is known about the ecology of modern assemblages.
They are closely involved in the biogeochemical cycles of silica, carbon and strontium sulfate, carrying important amounts of such elements to the deep ocean. The last few decades have seen a growing interest in modern Rhizaria, likely thanks to large-scale studies that highlighted their apparently substantial contribution to marine biodiversity. In this seminar, I will highlight and discuss some of the major and latest advances that led to some major reconsideration in the role of Rhizaria in pelagic ecosystems. Such studies include global-scale quantification of rhizarian carbon and silica pool, revealing unsuspected high biomasses of large Rhizaria. Most of this work was enabled thanks to fine elemental quantification at single-cell levels and recent developments in in situ imaging technics combined with the improvement in supervised classification (i.e., machine learning). Despite major breakthroughs in the understanding of Rhizaria, several elements need to be dealt with in order to fully include Rhizaria in ecosystemic and/or biogeochemical models.