Rhizaria: a group recognised by genes, not by looks
No single visible feature ties the Rhizaria together. These mostly single-celled organisms were grouped largely on DNA evidence, and they range from soil amoebae to ocean drifters that build shells of glassy opal, the mineral celestite or chalky calcite. A few form colonies more than a metre long.
Most rhizarians are amoebae that reach out with thin, netted or stiffened extensions of the cell to trap and swallow prey. Nearly all are non-photosynthetic; the exceptions are the chlorarachniophytes and three species of Paulinella. Many foraminifera and radiolarians, though, host single-celled algae, and those partnerships add significantly to the ocean's primary production. There is even a slime mould, Guttulinopsis vulgaris, that manages a multicellular stage.
The group has three main branches: Cercozoa, varied amoebae and flagellates common in soil; Foraminifera, shelled amoebae living on the seabed; and Radiolaria, floating plankton with needle-like projections. Some members are parasites of plants or animals. One, Mikrocytos mackini, causes Denman Island disease in oysters; it is just 2 to 3 micrometres across and lacks mitochondria.
Naturalists sensed kinship early. In 1861 William Carpenter grouped foraminifera with Gromia because both had fine, granular, net-like pseudopodia. The wider idea that these and the radiolarians share an ancestor came only in the late twentieth century, through molecular trees and better microscopes, and Thomas Cavalier-Smith applied the name Rhizaria to the expanded group in 2002. For a long time many of them had been classed as animals, since they move and eat; later they were parked in the catch-all kingdom Protista.
Today Rhizaria sit within the SAR supergroup alongside stramenopiles and alveolates, a link first hinted at in 1993 from the shapes of mitochondria. Chemical traces of compounds they make suggest they already played an ecological role in the Ediacaran period. Full sexual life cycles have been demonstrated in foraminifera and Gromia, overturning any assumption that such simple cells reproduce only by splitting.
Source: Rhizaria
