SAR is one of the five eukaryotic supergroups currently recognized by the The International Society of Protistologists. This group includes the Stramenopiles, Alveolates, and Rhizarians. (Please note that some textbooks are slightly out of date, and still refer to the Stramenopiles and Alveolates as supergroup Chromalveolata).
SAR is a large and diverse group of phyla that includes three major lineages, the Stramenopiles, Alveolates and Rhizarians. The range of variation within these organisms is stunning. Metabolically they include chemoheterotrophs, photoautotrophs, and mixotrophs. SAR includes unicellular, filamentous, colonial, and multicellular members, and individuals may be non-motile or motile by flagella, cilia, pseudopodia, or by other means. Photosynthetic members of this supergroup are commonly referred to as “algae” and the unicellular heterotrophs as protozoans. Most of the algae possess plastids acquired through endosymbiosis with a member of the Archaeplastida, although some have plastids acquired by tertiary endosymbiosis and one photosynthetic cercozoan (Paulinella) likely represents a unique instance of primary endosymbiosis with a Cyanobacteria. A summary of the major SAR phyla is provided below:
- Oomycota – oomycetes, water molds
- Bacillariophyta – diatoms
- Chrysophyta – golden algae
- Phaeophyta – brown algae
- Ciliophora – ciliates
- Dinoflagellata – dinoflagellates
- Apicomplexa – apicomplexans
- Foraminifera – forams, foraminiferans
- Radiolaria – radiolarians
- Cercozoa – cercozoans
This is a large and diverse group of organisms that is also known as the Heterokonta or as heterokonts. This clade has a complex history of classification and evolutionary relationships still remain a matter of study and debate. There are estimated to be at least 25,000 stramenopile species. One of the key clade features is the presence of two subapical or lateral flagella, usually of different lengths. The anterior flagellum (usually) has one or two rows of glycoprotein appendages called mastigonemes. This gives the flagellum a “feathery” appearance. These are sometimes referred to as “tinsel-type” flagella. These flagella pull motile cells through the water. The posterior flagellum is “smooth” (“whiplash” flagella), trailing, and shorter, sometimes much reduced.
Stramenopiles may be unicellular, filamentous, or multicellular. Most have cellulose cell walls. Some of the multicellular brown algae may be massive, forming marine kelp forests. Stramenopiles may be heterotrophic or photoautotrophs.
Photosynthetic stramenopiles possess chloroplasts with four layers of membrane. Evidence indicates that they were acquired by secondary endosymbiosis from a unicellular red alga. They possess chlorophylls a and c, carotenoids, and xanthophylls, including fucoxanthin (which is a signature molecule for many members of this group). This collection of pigments typically gives these algae a brown, golden, or yellow-green color.
It is not clear whether non-photosynthetic stramenopiles (such as oomycetes) represent an evolutionary loss of plastids or descent from an ancestor that pre-dates the acquisition of plastids by the algal members of the group.
Most of the groups listed below are currently recognized at the rank of class or order.
Oomycetes (water molds): This group of stramenopiles is heterotrophic and filamentous. Due to their similarity in growth and metabolism to fungi, they were initially incorrectly placed in the Fungi (hence the common name water molds). These similarities to “true” fungi are now considered to be homoplasies. Water molds are common in fresh water and moist soil ecosystems. Many are decomposers, but a large number of species are plant parasites. The most famous example is Phytophthora infestans, the cause of potato blight. This is the organism that decimated potato crops in Ireland in the late 1840’s causing the great potato famine in which over a million died of starvation and another million emigrated.
Bacillariophyta (diatoms): This is a large and ubiquitous group of photoautotrophic stramenopiles that is found in marine, freshwater, and terrestrial ecosystems. Most are unicellular and have a zygotic cycle. Their photosynthetic pigments appear brown, due to the presence of fucoxanthin. Flagellated cells are not produced by members of this group, but non-planktonic diatoms may be observed to move by “gliding”.
Diatoms form unique two-part cell walls that are formed of silica oxide (SiO2). Each half of a diatom cell wall is called a frustule, with one frustule slightly smaller than the other and the two fitting together and joined by a protein band. Asexual reproduction (by mitosis) results in two daughter cells that each inherit one frustule from the parent, with a second new frustule formed. Both frustules are replaced by individuals that are formed sexually. Frustules appear either radially or bilaterally symmetrical. Radially symmetrical diatoms are described as “centric” while bilateral diatoms are “pennate.”
Diatoms are abundant and ecologically important as a major form of phytoplankton. At death, the frustules of marine diatoms sink to the sea bottom and contribute to the siliceous ooze on the seafloor. Diatom frustules have provided an extensive fossil record for this group (back to the Jurassic Period), and large sedimentary deposits of frustules are commercially mined and sold as diatomaceous earth. Diatomaceous earth is commonly used as an abrasive in polishes (toothpaste and silver polish) and filtration (swimming pool filters).
Chrysophytes (golden algae and yellow-green algae): The golden algae and yellow-green algae are often placed in separate classes. Golden (also called golden-brown) algae possess darker fucoxanthin pigments, while the yellow-green algae possess other, more yellowish xanthophyll pigments. Chrysophytes include groups of unicellular, filamentous, or colonial photoautotrophic algae that are often found in freshwater ecosystems, but may also be found in marine and terrestrial ecosystems. The algae are quite variable in many of their features. For example, their cell walls may be composed of cellulose, or they may be amoeboid or covered with overlapping siliceous or calcareous scales. Most form a biflagellate cell at some stage in their life cycle. Their evolutionary relationships remain an area of active research.
Phaeophyta (brown algae): This is a large (1,500-2,000 spp.) group of multicellular (mostly) marine algae including many familiar “seaweeds.” Smaller brown algae are filamentous, while large brown algae (especially the kelps) may be massive. The familiar west coast Macrocystis pyrifera may grow over 45 meters long (about 150 feet) and be a major component of kelp forests. These large marine algae show a large degree of differentiation with a “holdfast” that anchors the kelp to the rocky ocean floor, elongated stem-like portions called stipes, and flattened leaf-like portions called blades. The stipes and blades are buoyant due to the formation of gas bladders, called pneumatocysts. The stipes of some kelps have a central zone of elongated conducting cells.
Brown algae appear deep brown to brownish green due to the presence of the xanthophyll pigment fucoxanthin. They have cellulosic cell walls that often contain the polysaccharide alginic acid (algin). Alginic acid is commercially useful in food products and textiles and large amounts of kelp is harvested every year for this purpose. All have sporic life cycles with isomorphic or heteromorphic generations. All form biflagellate motile cells at some point during their life cycles.
The importance of kelp forests to marine vertebrate and invertebrate animal populations cannot be overstated.
The Alveolates are a large group of over 10,000 described species. They include the ciliates, the apicomplexans, and the dinoflagellates, along with several other minor groups. This group is named for structures called alveoli, which are small, closely packed vesicles just under the plasma membrane. These often form a pellicle, which is used in various species for motility, defense, or host cell invasion. Many alveolates, but not all, have plastids. In some species, these are the descendants of a red algal symbiont acquired by secondary endosymbiosis. Other species have lost the original endosymbiont, and some have replaced it with a plastid of a different origin.
Ciliates are extremely common in marine, freshwater, and terrestrial environments. There are about 4,000 described species. They are named for their cilia, which function to move the organism through the water, and/or to direct food into an oral groove. Ciliates have two nuclei that differ in morphology. The micronucleus (there may be one or more) contains the diploid genome, which is not actively transcribed. The macronucleus is polyploid and actively transcribes the genes necessary for metabolism.
Normal reproduction in ciliates is asexual and takes place via transverse binary fission. Ciliates also have a unique method of sexual reproduction, called conjugation. Two ciliates in close proximity form a cytoplasmic bridge between them. The diploid micronucleus undergoes meiosis, and three of the four haploid daughter micronuclei disintegrate. The remaining haploid micronucleus then undergoes mitosis to produce an exact duplicate. The two ciliates exchange one micronucleus, then the two fuse to form a new diploid micronucleus.
Dinoflagellates: There are about 2,000 described species of dinoflagellates, which are unicellular organisms with two flagella of different morphology. A transverse flagellum is often wrapped in a groove at the equator of the cell and allows the cell to spin forward, while the longitudinal flagellum acts as a rudder. Dinoflagellates can be autotrophs, heterotrophs, or mixotrophs. Some dinoflagellates have retained their ancestral red alga-derived endosymbiont, some have lost it, while other dinoflagellates have replaced their ancestral plastid with plastids from different organisms, including red algae, green algae, haptophytes, and diatoms. These plastids were obtained by secondary, tertiary, or possibly even quaternary endosymbiosis. Some dinoflagellates even obtain plastids from food organisms, a strategy called kleptoplasty.
Many dinoflagellates are armored, with cellulose plates around their plasma membrane for protection. Other dinoflagellates lack this armor, and are referred to as naked. Dinoflagellates exhibit both sexual and asexual reproduction. Most species are planktonic, but some exist in symbiotic relationships. For instance, dinoflagellates can be mutualists with coral and sea anemones. Some dinoflagellates are bioluminescent.
Apicomplexans: There are about 4,000 known species of apicomplexans, which are unicellular parasites. Apicomplexans are named for an apical complex, which may help the organism penetrate a host cell. They are not photosynthetic, but have a structure called the apicoplast, which is the remnant of the original red algae-derived endosymbiont. Apicomplexans have complex life cycles that can include asexual and sexual stages and host switching. Some apicomplexans are major human health threats, mostly notably the four species of genus Plasmodium that cause malaria, resulting in an estimated 300-900 million deaths per year.
Rhizaria is a lineage that includes organisms that are (mostly) unicellular and amoeboid. They may form a siliceous skeleton or outer calcareous covering called a “test.” Slender (filose) pseudopodia extend through pores in the test. These function in feeding and/or movement. Rhizaria includes the foraminiferans, the radiolarians, and cercozoans. The majority of these organisms are marine. Many are planktonic.
Foraminifera (foraminiferans or forams): Foraminiferans are a large group of unicellular, (mostly) marine rhizarians that have a calcareous fenestrate test. Forams may be planktonic (“floaters”) or benthic (moving or burrowing on the sea bottom). They have thread-like (filose) pseudopodia or a cytoplasmic web that they use for trapping organic matter or prey. They are heterotrophic, but may contain unicellular algal endosymbionts (green algae, red algae, chrysophytes, diatoms, or dinoflagellates). Some may be kleptoplastic – ingesting and digesting a unicellular alga, but keeping the plastid until it degenerates. For this reason, planktonic forams are considered to be phytoplankton. Forams have a sporic life cycle. Gametes are biflagellate.
Thanks to their calcareous tests, foraminiferans are readily fossilized. Planktonic and benthic forams have a long fossil record, extending back to the Cambrian Period. They are geologically useful as index fossils, with about 50,000 named species (about 10,000 species still extant). They are widely used in the study of paleoclimatology and paleoceanography. Foraminiferans are major contributors to sedimentary chalk deposits.
Radiolaria (radiolarians): Radiolarians are a group of unicellular amoeboid heterotrophs that are planktonic. They have a siliceous internal skeleton. Radiolarians trap food using spikey filose pseudopodia that are reinforced by microtubules. Although heterotrophic they may contain zooxanthellae (dinoflagellate endosymbionts).
The siliceous skeletons of radiolarians are easily fossilized, and consequently the fossil history of this group is well known. Their fossil history begins in the Cambrian Period. Most (about 90%) of the known species of radiolarians are extinct. Radiolarians are major contributors to seafloor siliceous ooze.
Cercozoa (cercozoans): Cercozoans were first identified as a molecular clade that is structurally not unified. This group includes marine, freshwater, and terrestrial organisms that may be flagellated or amoeboid. Most feed by filose pseudopodia. Tests, if present, are of overlapping siliceous or organic scales or plates. Most cercozoans are heterotrophic, but this group also includes the mixotrophic chlorarachniophytes. Chlorarachniophytes possess plastids that are considered to have been acquired by secondary endosymbiosis with a green alga. Chlorarachniophyte plastids are bounded by four layers of membrane and have “nucleomorphs” which are thought to be the remnants of the algal nucleus. Cercozoa also includes Paulinella chromatophora, a unicellular photoautotroph whose plastid represents a possible instance of primary endosymbiosis. The Paulinella plastid seems to have arisen independently from a cyanobacteria relatively recently (perhaps in the last 100 million years).
 There are two reasons that this group is no longer used. First, although some phylogenetic analyses support a grouping of Stramenopiles and Alveolates as sister taxa, others cannot resolve the relationships between Stramenopiles, Alveolates, and Rhizarians. Second, when Chromalveolata was originally proposed, it was united based on the hypothesis that the members of the group were descended from a single biflagellate organism that engulfed a red alga via secondary endosymbiosis. It therefore included other groups, such as cryptophytes and haptophytes, which are not closely related, as indicated by molecular analysis. Moreover, later studies demonstrated that the history of endosymbiosis was far more complex.