Diversity and ecology of Radiolaria in modern oceans

Abstract

Among the many inhabitants of planktonic communities, several lineages have biomineralized intricate skeletons. These have existed for millions of years and include the Radiolaria, a group of marine protists, many of which bear delicate mineral skeletons of different natures. Radiolaria are well known for their paleontological signatures, but little is known about the ecology of modern assemblages. They are found from polar to tropical regions, in the sunlit layers of the ocean down to the deep and cold bathypelagic. They are closely involved in the biogeochemical cycles of silica, carbon and strontium sulfate, carrying important amounts of such elements to the deep ocean. However, relatively little is known on the actual extent of genetic diversity or biogeographic patterns. The rapid emergence and acceptance of molecular approaches have nevertheless led to major advances in our understanding of diversity within and evolutionary relationships between major radiolarian groups. Here, we review the state of knowledge relating to the classification, diversity and ecology of extant radiolarian orders, highlighting the substantial gaps in our understanding of the extent of their contribution to marine biodiversity and their role in marine food webs.

From the Surface Ocean to the Seafloor: Linking Modern and Paleo‐Genetics at the Sabrina Coast, East Antarctica (IN2017_V01)

“…In a contrary pattern, Retaria were found mostly below the chl max and in deep waters. Retaria include the unicellular predatory Foraminifera and Radiolaria, they are globally distributed and are highly abundant in the Southern Ocean south of 45°S, with several classes adapted to thrive from the surface to the deep Ocean (Abelmann & Gowing, 1997; Biard, 2022; Boltovskoy, 2017). Retaria play a key role in the carbon export in the global Ocean (Guidi et al., 2016) with classes belonging to this phylum often found in sediment traps (Fontanez et al., 2015).…”

“…(2016) determined Collodaria as a dominant group in the deep ocean (using water samples from the Atlantic, Pacific, and Indian Ocean collected between 3,000 and 4,000 m depth), again, reinforcing that proximity of water column organisms to the seafloor plays a role in their representation in the sed aDNA record. Collodaria are colonial, and radiolarian colonies have been shown to have exceptionally high SSU gene copy numbers (37,474 ± 17,799 per colonial cell; Biard, 2022) compared to a few hundred in diatoms (Godhe et al., 2008), which may bias SSU‐based abundance data toward colonial radiolaria. Collodaria nourish through photosynthetic endosymbionts, which provides a conundrum as these would not be able to survive in the ice‐covered environment experienced during the last glacial maximum at our sites on the Sabrina Coast (when Collodaria reads were found).…”

Elemental content allometries and silicon uptake rates of planktonic Rhizaria: Insights into their ecology and role in biogeochemical cycles

“…In addition, it has been suggested that cells with little organic matter are less attractive and more difficult for predators to grasp (Kiørboe 2013), in addition to their large size making them less accessible to small grazers. Only a few predators have been reported for Radiolaria (summarized in Biard 2022 b ) and even less for Phaeodaria, which are usually larger organisms than Radiolaria. As proposed by Stukel et al (2018), another advantage of this low C strategy is the maintenance of neutral buoyancy.…”

“…Among them, Retaria includes Foraminifera and Radiolaria (Sierra et al 2013). Foraminifera build calcium carbonate skeletons, whereas among Radiolaria, the skeleton is made of strontium sulfate for the order Acantharia and biogenic Si for the orders Nassellaria, Spumellaria, Collodaria, and Orodaria (Nakamura and Suzuki 2015; Suzuki and Not 2015; Biard 2022 b ). Phaeodaria, whose skeletons are made of biogenic Si, have long been considered radiolarians, but are now classified as Cercozoa, a sister group to Retaria (Sierra et al 2013).…”

“…Siliceous Rhizaria inhabit the open ocean worldwide from the surface to the bathypelagic realm (Nakamura and Suzuki 2015; Suzuki and Not 2015; Biard 2022 b ). Their distribution is taxon dependent and influenced mostly by temperature, dissolved Si concentration and the amount of food available (Boltovskoy and Correa 2016; Biard and Ohman 2020).…”