The history of the Arctic Ocean during the Cenozoic era (0-65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1-2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change. © 2006 Nature Publishing Group

Radiolaria are unicellular holoplanktonic protozoa with siliceous or strontium sulfate skeletons. Mainly studied by micropaleontologists because of their excellent fossil record, they are also key members of planktonic communities and play important roles in various oceanic ecosystems. This chapter presents an overview of the current knowledge on living Radiolaria (orders Acantharia, Collodaria, Nassellaria, Spumellaria and Taxopodia). Besides general considerations on Radiolaria as a whole, it focuses on the taxonomy, biology, and ecology of each radiolarian order. Finally this chapter provides insights on research perspectives to improve our knowledge of living radiolarians and their ecological role in marine ecosystems.

Symbiotic interactions between pelagic hosts and microalgae have received little attention, although they are widespread in the photic layer of the world ocean, where they play a fundamental role in the ecology of the planktonic ecosystem. Polycystine radiolarians (including the orders Spumellaria, Collodaria and Nassellaria) are planktonic heterotrophic protists that are widely distributed and often abundant in the ocean. Many polycystines host symbiotic microalgae within their cytoplasm, mostly thought to be the dinoflagellate Scrippsiella nutricula, a species originally described by Karl Brandt in the late nineteenth century as Zooxanthella nutricula. The free-living stage of this dinoflagellate has never been characterized in terms of morphology and thecal plate tabulation. We examined morphological characters and sequenced conservative ribosomal markers of clonal cultures of the free-living stage of symbiotic dinoflagellates isolated from radiolarian hosts from the three polycystine orders. In addition, we sequenced symbiont genes directly from several polycystine-symbiont holobiont specimens from different oceanic regions. Thecal plate arrangement of the free-living stage does not match that of Scrippsiella or related genera, and LSU and SSU rDNA-based molecular phylogenies place these symbionts in a distinct clade within the Peridiniales. Both phylogenetic analyses and the comparison of morphological features of culture strains with those reported for other closely related species support the erection of a new genus that we name Brandtodinium gen. nov. and the recombination of S. nutricula as B. nutricula comb. nov.

Phaeodarians are unicellular marine protists characterized by the "central capsule" containing the nucleus, the "phaeodium", or mass of brown particles, and a siliceous skeleton called the "scleracoma". Phaeodaria have long been classifi ed as a member of the Radiolaria; however, this protist group now belongs to the phylum Cercozoa. The ancestor of phaeodarians is thought to have appeared in the upper Triassic Epoch according to the fossil record. They reproduce by cell division and swarmer production. These plankton are heterotrophic, and they presumably feed on organic materials suspended in the water column or capture other plankton. Although this group is widely distributed in the world ocean from the surface to deep waters, they have attracted little attention from marine researchers partly because their abundance has long been underestimated. Recent study, however, revealed that phaeodarians are more numerous than expected, and their high abundance is sometimes reported. Considering their occasional high biomass and the fact that their scleracoma is made chiefl y from silica, this plankton group plays an important role in local ecosystems and has a large impact in the silica cycle of the ocean. Knowledge of phaeodarian is indispensable for future oceanography; therefore the hitherto-known information on this marine protist is comprehensively reviewed in this chapter.

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Collodaria are ubiquitous and abundant marine radiolarian (Rhizaria) protists. They occur as either large colonies or solitary specimens, and, unlike most radiolarians, some taxa lack silicified structures. Collodarians are known to play an important role in oceanic food webs as both active predators and hosts of symbiotic microalgae, yet very little is known about their diversity and evolution. Taxonomic delineation of collodarians is challenging and only a few species have been genetically characterized. Here we investigated collodarian diversity using phylogenetic analyses of both nuclear small (18S) and large (28S) subunits of the ribosomal DNA, including 124 new sequences from 75 collodarians sampled worldwide. The resulting molecular phylogeny was compared to morphology-based classification. Our analyses distinguished the monophyletic clade of skeleton-less and spicule-bearing Sphaerozoidae from the sister clades Collosphaeridae (skeleton-bearing) and Collophidiidae (skeleton-less), while the Thalassicollidae was not retrieved as a monophyletic clade. Detailed morphological examination with electron microscopy combined with molecular analyses revealed many discrepancies, such as a mix between solitary and colonial species, co-existence of skeleton-less and skeleton-bearing specimens within the Collosphaeridae, as well as complex intraspecific variability in silicified structures. Such observations challenge a morphology-based classification and highlight the pertinence of an integrative taxonomic approach to study collodarian diversity.

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