Rhizaria represent a widespread supergroup of single-celled eukaryotic organisms. These microscopic life forms inhabit diverse environments, from oceans and freshwater bodies to soils across the globe. Despite their small size, they exhibit remarkable variety in their shapes and structures. Rhizaria play significant roles in various ecosystems.
Defining Features of Rhizaria
Rhizaria are characterized by their pseudopods, which are temporary extensions of their cytoplasm used for movement and feeding. These pseudopods appear as slender, thread-like structures, and can be categorized into filopodia, reticulopodia, or axopodia depending on their morphology and internal support. Filopodia are typically thin and branching, while reticulopodia form intricate, net-like structures that can fuse and re-fuse. Axopodia are straight, needle-like pseudopods supported by internal microtubules. These extensions allow Rhizaria to capture food and navigate their surroundings.
Rhizaria are eukaryotic, meaning their cells contain a true nucleus and other membrane-bound organelles. Many members of this supergroup also possess shells or skeletons, known as tests, which provide protection and support. These tests can be composed of various materials, including calcium carbonate, silica, or strontium salts. Most Rhizaria are unicellular, though some multicellular forms exist.
Diverse Forms of Rhizaria
The supergroup Rhizaria encompasses three major subgroups, each with distinct morphological characteristics and ecological niches.
Foraminifera
Foraminifera, or “forams,” are amoeboid protists found in marine environments, both on the seafloor (benthic) and in the water column (planktonic). Their most recognizable feature is their multi-chambered calcareous shells, or tests, made of calcium carbonate. These tests can range in size from 100 micrometers to several centimeters. Their intricate designs contribute to their beauty. Foraminifera extend a network of thin, granular pseudopods called reticulopodia through pores in their shells to capture food, which includes bacteria, algae, and small animals. Their fossilized remains contribute to geological formations like chalk cliffs.
Radiolaria
Radiolaria are marine planktonic protists known for their intricate, glassy skeletons composed of opaline silica. These skeletons display radial or bilateral symmetry, with needle-like axopodia radiating from the cell body. These axopodia are supported by microtubules and are used for buoyancy and capturing food particles. Radiolarians are 0.1 to 0.2 millimeters in diameter. When radiolarians die, their siliceous skeletons sink to the ocean floor, forming siliceous ooze that contributes to the fossil record.
Cercozoa
Cercozoa are a diverse subgroup within Rhizaria, including amoeboid and flagellated species. Unlike foraminifera and radiolaria, many cercozoans lack mineralized shells, though some produce shells of siliceous scales or organic materials. They typically possess filose pseudopods, which are thin and thread-like, used for feeding and movement. Cercozoa are common in various habitats, including soil and aquatic environments, where they act as predators of bacteria and other microorganisms. Notable examples include chlorarachniophytes, which possess chloroplasts acquired through secondary endosymbiosis, retaining a vestigial nucleus called a nucleomorph.
Ecological Roles and Impact
Rhizaria play roles in global ecosystems, influencing nutrient cycles and contributing to food webs across marine, freshwater, and terrestrial environments. Their feeding strategies position them as primary consumers. They graze on bacteria, algae, and detritus, transferring energy through food chains.
Beyond grazing, Rhizaria contribute to biogeochemical cycles, particularly the carbon and silicon cycles. The calcareous shells of Foraminifera and the siliceous skeletons of Radiolaria sink to the ocean floor upon death, locking away carbon and silicon from the atmosphere and contributing to sedimentary rock formation like limestone and chert. Foraminifera also participate in the nitrogen cycle through denitrification, a process usually associated with prokaryotes. Some Rhizaria are utilized as bioindicators, providing insights into past climates and environmental health due to their widespread presence and sensitivity to environmental changes.