What Is a Radiolarian? Single Cells That Build Glass Skeletons

Radiolarians are single-celled organisms of the group formerly lumped together as protozoa, living suspended in seawater from the surface down to considerable depth. Each builds an internal skeleton of opaline silica, arranged with radial symmetry around a central capsule that divides the cell into an inner and an outer region. Fine projections extend outward through and beyond the skeleton, supported by stiff rods, and these capture prey, which consists of bacteria, algae and small animals. Many species host symbiotic algae within the outer region, which supply energy from photosynthesis and restrict those species to the sunlit upper layer. The skeletons range from simple spheres to nested concentric shells, spiny forms, bell shapes and elaborate lattices, and the geometric regularity of the structures made them a favourite subject of nineteenth-century scientific illustration.

Their silica skeletons make them useful in ways that carbonate-shelled organisms are not:

• •Silica survives in deep water where carbonate dissolves, so these are the fossils available below the depth at which shells disappear
• •Accumulated skeletons form radiolarian ooze on the deep sea floor, which hardens into chert
• •Rapid evolution in many lineages makes them precise for dating, particularly in deep marine sequences with no other fossils
• •Their record extends back over five hundred million years, which is longer than most microfossil groups
• •Assemblages indicate water temperature and productivity, since species have distinct preferences
• •They occur in rocks scraped off the ocean floor onto continents, which is how deep ocean history is read from land

Carbonate and silica behave in opposite ways with depth, and understanding that is essential to reading the deep sea record. Carbonate dissolves more readily in cold, high-pressure, carbon dioxide-rich deep water, so below a certain depth, which varies between oceans and over time, carbonate shells simply do not accumulate, and sediment below it contains only the non-carbonate fraction. Silica behaves differently, since seawater is everywhere undersaturated in it and dissolution proceeds throughout the water column rather than switching on at a depth, which means silica preservation depends on how fast a skeleton sinks and how much organic coating protects it. The consequence is that deep ocean sediments are dominated by silica-shelled organisms and by clay, which is why these organisms carry the record where nothing else does.

These organisms are known to a wider public almost entirely through the work of one nineteenth-century biologist and illustrator, whose plates of their skeletons are among the most reproduced scientific images ever made. He described thousands of species from material collected on a major oceanographic expedition and published them in volumes of drawings emphasising symmetry and geometric regularity, and those images influenced architecture and decorative art directly, with buildings and objects designed after them. The scientific standing of the work is mixed, since the illustrations regularised the specimens, imposing a symmetry cleaner than the organisms possess, and the species descriptions have been extensively revised. The plates remain valuable as a record of what was collected and as an episode where scientific illustration escaped into design, and they are a caution about how much a drawing can quietly improve its subject.

Because the group is old, abundant and rapidly evolving, it supports several kinds of investigation. Extinction and recovery patterns across major boundaries are traceable in detail, and the group suffered and recovered repeatedly, which supplies a record of how plankton communities respond to disturbance. Latitudinal patterns in assemblages track ocean circulation, and shifts in those patterns record changes in current systems over geological time. Silica availability in the oceans has varied, partly because the evolution of diatoms, which also build silica skeletons, drew down dissolved silica substantially and appears to have pushed radiolarians towards thinner skeletons, which is a documented case of competition between unrelated groups leaving a physical signature. Their chert deposits also record where ocean floor once lay, which is central evidence in reconstructing the arrangement of ancient oceans and continents.

Single cells building silica skeletons of radial symmetry, capturing prey with fine projections and in many species hosting algae. Silica survives in deep water where carbonate dissolves, so these fossils carry the record where nothing else does, forming ooze that hardens into chert. Competition from diatoms drew down ocean silica and appears to have thinned their skeletons.