Diversity of Protists III

Rhizaria comprises three major subgroups: Chlorarachniophyta, Foraminifera, and Radiolaria. Chlorarachniophyta are phototrophic protists found in freshwater and marine environments. Foraminiferans are strictly marine and build calcium carbonate tests. Radiolarians are planktonic marine protists with silica-based skeletons. Each group occupies distinct ecological niches and exhibits different metabolic strategies.

Chlorarachniophytes exemplify secondary endosymbiosis, where a non-photosynthetic eukaryotic host engulfed a photosynthetic algal eukaryote. Their chloroplasts are surrounded by four membranes and retain a nucleomorph—a vestigial nucleus from the engulfed alga. This nucleomorph represents a rare intermediate state in plastid integration, preserving genomic evidence of the endosymbiotic event.

Foraminiferans construct elaborate tests composed of calcium carbonate that provide structural protection and support pseudopodial feeding. These tests have high fossilization potential, making them indispensable in biostratigraphy and petroleum geology. Their distribution across geological strata helps infer the relative age of sedimentary layers and assess potential for oil deposits.

Radiolarians inhabit the upper 100 meters of the ocean and maintain buoyancy through lipid droplets, large cytoplasmic vacuoles, and stiff needle-like pseudopodia that counteract gravitational sinking. Their symmetrical silica-based skeletons are highly ornate and remain after death, contributing to siliceous ooze on the seafloor. They feed on bacteria and organic detritus.

Benthic foraminiferans reside on the ocean floor, feeding on particulate organic matter and engaging in nutrient-exchange symbioses with algae. Planktonic foraminiferans, by contrast, host phototrophic endosymbionts and occupy upper ocean layers where sunlight is accessible. Both groups use pseudopodia extending from their tests to capture food.

Chlorarachniophytes contain at least five genomes: the host nuclear genome, host mitochondrial genome, nucleomorph genome, algal mitochondrial genome, and plastid genome. Over evolutionary time, many genes from the endosymbiont have transferred to the host nucleus, resulting in genome reduction within the nucleomorph and chloroplast. This genomic integration reflects evolutionary streamlining.

Radiolarians form symbiotic, but not endosymbiotic, relationships with algae, feeding on bacteria and organic matter independently. Foraminiferans, particularly planktonic species, host phototrophic endosymbionts that provide nutrients. This distinction reflects different evolutionary strategies: radiolarians maintain autonomy while foraminiferans rely on integrated algal partners for energy.