Protoctista is a paraphyletic grouping of lower eukaryotes that includes all eukaryotes except metazoans and green plants. Protoctistans are predominantly single-celled forms, although several groups have developed colonial and multicellular morphologies. Protoctistans may be either photosynthetic, or heterotrophic and nonphotosynthetic.

Algae is the term traditionally used to describe the polyphyletic array of photosyn-thetic single-celled and multicellular eukaryotes, excluding land plants. Different algal groups utilize different combinations of pigments for photosynthesis, their chloroplasts having been derived from at least three independent endosymbiotic events with photo-synthetic bacteria or from secondary endosym-bioses with other chloroplast-containing eukaryotes.

Nonphotosynthetic protoctistans are represented by a polyphyletic array of predominantly single-celled eukaryotes. Formerly called protozoans, reflecting earlier ideas of their close evolutionary ties to animals or meta-zoans, the nonphotosynthetic groups encompass a diversity of unrelated forms with flagellated and amoeboid morphologies

Foraminiferans. Foraminiferans are a pro-toctistan group that appears to have diverged early in the history of eukaryotes. Distinctive characteristics of foraminiferans include the possession of anastomosing pseudopodia (retic-ulopodia) and an extracellular test, or shell, that may be organic in composition, composed of agglutinated material, or of biomin-eralized calcite, aragonite, or silica. Although most foraminiferans live in benthic marine habitats, a few lineages have diversified into the plankton. Meroplanktonic forms include Tretomphalus and other closely related benthic taxa. Members of this group live attached to sea grasses and marine macroalgae during most of their life cycle. Just prior to reproduction, gametogenic individuals develop an enlarged, spherical gas-filled chamber, detach from their phytal substrate, and ascend into the plankton, where they release flagellated gametes into the surface waters.

Holoplanktonic foraminiferal species are widely distributed in the open ocean from subpolar to tropical regions. Spinose species are found living in warm, subtropical to tropical waters, where they dwell in surface waters of the photic zone. Most spinose planktonic species are hosts to intracellular photosyn-thetic eukaryotic symbionts, such as dinofla-gellates, chlorophyceans, and haptophytes. Although the photosynthetic products of the endosymbionts provide the host cells with a source of endogenous nutrition, spinose species (such as Hastigerina pelagica and Orbulina universa) are known to feed on a wide range of prey items, including copepods, crab zoea, and fish larvae. Nonspinose species, such as Globorotalia truncatulinoides, are generally herbivorous, feeding on diatoms and nonphoto-synthetic flagellated eukaryotes. Many non-spinose species live in the photic zone as juveniles but descend into deeper waters as adults. Other nonspinose taxa, such as Globorotalia species, spend their entire life cycle at depth.

Although living planktonic foraminifer-ans are relatively sparsely distributed in the plankton, occurring in densities of from 1 to 10 per cubic meter, their empty shells form significant deposits in oceanic sediments, and also did so in the past. The earliest fossil planktonic foraminiferans are found in Middle Jurassic sedimentary rocks of Europe. Fossil planktonic foraminiferans appear to have undergone three episodes of evolutionary diversification in the past—during the mid-Cretaceous, the Paleocene to Middle Eocene, and the Early Miocene. Recent reconstructions of the evo lutionary relationships of planktonic foraminiferans based on DNA sequence data indicate that modern planktonic foraminifer-ans are derived from at least two different benthic groups.

Euglenids. Euglenids are a group of primitive, single-celled eukaryotes that are most closely related to the parasitic trypanosomes. Euglenids possess a pellicle composed of helically arranged, interlocking proteinaceous strips. Primitive euglenids are nonphotosyn-thetic and phagotrophic. Most derived euglenids are photosynthetic, possessing chlorophylls a and b as their primary photo-synthetic pigments, and beta carotene and other carotenoid derivatives as accessory pigments. The flagella of euglenids are typically covered with a single row of fine hairs. A pigmented eyespot located at the base of one fla-gellum is believed to function as a light-sensing organ.

Euglenids live primarily in shallow, freshwater habitats with high nutrient concentrations and, thus, are considered to be environmental indicators of eutrophication and pollution.

Radiolarians and acantharians. Radiolar-ians and acantharians are nonphotosynthetic, single-celled eukaryotes that possess axopodia (pseudopodia underlain by an axial bundle of cross-linked microtubules). Both groups are wholly marine, dwelling exclusively in the waters of the open ocean, but like planktonic foraminiferans, radiolarians and acantharians are found in relatively low densities, from 1 to 10 per cubic meter of seawater. Radiolarians and acantharians have traditionally been classified together in the same group, but recent studies based on DNA sequence data indicate that these groups may have separate and distinct evolutionary histories.

Radiolarians possess a perforate organic wall, or capsule, that surrounds the cell body, and intricate siliceous skeletons. The cytoplasm of radiolarians is highly compartmentalized and organized into specialized zones with specific functions. Radiolarian species with nonspherical, bilaterally symmetrical skeletons feed primarily on bacterioplank-ton, while species with small spherical skeletons feed predominantly on photosynthetic bacteria and eukaryotes. The large, gelatinous colonial species are omnivorous gener-alists, feeding on heterotrophic flagellates, photosynthetic eukaryotes, bacteria, and cyanobacteria. Several radiolarian taxa host dinoflagellate and prasinophycean endosym-bionts that live in the cytoplasmic zone exterior to the capsule. The fossil record of radi-olarians extends from the Recent all the way back to Cambrian times.

Acantharians possess a microfibrillar, meshlike capsule and a skeleton composed of strontium sulfate spicules. Most acantharians dwell primarily in the photic zone, as they possess photosymbiotic haptophytes. The fossil record of acantharians is much younger than that of radiolarians, the earliest fossil acantharians being Eocene in age.

Heliozoans. Heliozoans are nonphotosyn-thetic, single-celled eukaryotes that possess axopodia, an organic wall, and siliceous or organic surface scales or spines. Heliozoans are primarily a freshwater planktonic group, but a few species, such as Heterophrys marina, are found living in marine waters.

Like foraminiferans, radiolarians, and acan-tharians, heliozoans use their web of radiating pseudopods to capture prey items from the water column. Studies have shown that helio-zoans are ecologically important components of plankton communities in freshwater lakes; many species feed on photosynthetic and het-erotrophic nanoplankton. Several species, such as Heterophrys myriopoda, are hosts to photosynthetic endosymbionts.

Alveolates. Alveolates are another morphologically diverse group of heterotrophic and photosynthetic protoctistans. Dinofla-gellates, ciliates, and apicomplexans together make up the alveolate clade. Members of these three groups possess either cilia or flagella and a distinctive alveolar membrane system, which consists of flattened membrane-bound sacs located below the outer cell membrane.

Dinoflagellates are an important component of marine ecosystems. Nearly all dinoflagellates are planktonic marine forms, with only a few taxa found in freshwater ecosystems. Dinofla-gellates possess a distinctive pattern of spiral movement, being propelled by two flagella— one longitudinal flagellum that extends in a direction opposite to movement, and a transverse flagellum that encircles the cell, lying in a girdlelike depression. Dinoflagellates also possess a theca, or test, made up of rigid cellulose plates. As a group, dinoflagellates include both photosynthetic and heterotrophic forms; approximately half of all free-living species lack chloroplasts and are heterotrophic. Pho-tosynthetic dinoflagellates are yellowish-brown in color and possess chlorophylls a and c as their primary photosynthetic pigments, and beta and gamma carotenes, xanthins, and a unique carotenoid called peridinin, as accessory pigments. Heterotrophic dinoflagellates feed on a wide range of prey items, including bacterioplankton, photosynthetic and non-photosynthetic single-celled eukaryotes, and metazoans such as copepods. Some dinofla-gellate groups enter into endosymbiotic associations with other marine organisms, including planktonic cnidarians, foraminiferans, and radiolarians. Many dinoflagellates, such as Noctiluca, are bioluminescent and light up the waves in the ocean at night.

Several dinoflagellate groups produce toxins that can impact both human health and that of marine organisms. The dinoflagellate

Swarm of Noctiluca plankton at sea surface in summer. Many dinoflagellates, such as Noctiluca, are bioluminescent and light up the ocean waves at night. (Frank Lane Picture Agency/Corbis)

Pfiesteria is believed to have caused the massive fish kills that occurred in 1997 in numerous tributaries of coastal North Carolina. Planktonic dinoflagellates are responsible for "harmful algal blooms" (HAB), such as "red tides." Life-threatening paralytic shellfish poisoning may result from human consumption of filter-feeding marine mollusks and crustaceans that have been feeding on these dinoflagellates. Other dinoflagellates are responsible for neurotoxic shellfish poisoning and ciguatera fish poisoning.

Many species of dinoflagellates undergo an encystment stage during their life cycle, producing resistant cysts that are preserved in the fossil record. The earliest dinoflagellate cyst is the Silurian Arpylorus. No record of dinoflagellate cysts is found again until the Late Triassic.

Ciliates are another alveolate group that is widely distributed in the plankton of marine and freshwaters. Ciliates possess cilia, a special class of flagella, arranged in rows over the surface of the cell body. The cilia are anchored inside the cell via the infraciliature, a network of three types of fibers. In some ciliate groups, cilia cover the entire cell surface, while in other taxa the body ciliature is reduced or present only during certain stages of the life cycle. Some ciliates feed by phagocytosis, while others utilize a specialized oral apparatus called the cytostome. Most ciliates are omnivorous and feed on a diversity of bacte-rioplankton, nanoplankton, and microplankton. Several planktonic oligotrich ciliates enter into facultative symbioses, by sequestering and retaining functioning chloroplasts in their cytoplasm. This behavior has been observed in both marine and freshwater olig-otrichs. Blooms of the red-pigmented, chloro-plast-sequestering marine holotrich Meso-dinium rubrum are responsible for nontoxic red tides that may, nevertheless, negatively affect marine habitats through oxygen depletion of the overlying waters.

Tintinnids are a specialized group of spirotrich ciliates that are an important component of the marine plankton. The cell body of tintinnids is enclosed in a cup-shaped organic test, or lorica, that may be covered with agglutinated particles such as sand grains, diatoms, or coccolith plates. Tintinnids feed on other small plankton, such as diatoms, dinofla-gellates, coccolithophores, silicoflagellates, bacteria, radiolarians, and other ciliates. As a group, tintinnids are the most diverse in warm subtropical to tropical waters, but they occur in relatively low abundances. They have the lowest diversity but the highest abundances in colder water; for example, tintinnids are second only to diatoms in abundance in the Antarctic plankton. The fossil record of cili-ates is almost exclusively that of tintinnids, which are first found in Ordovician rocks.

Haptophytes. Haptophytes, also known as prymnesiophytes, are a group of photosynthetic eukaryotes that possess two smooth flagella, a unique organelle called a haptoneme, and a body covered by organic or calcareous scales. Haptophytes possess chlorophylls a and c as their primary photosynthetic pigments. Hap-tophytes are a predominantly marine group, and they constitute a significant component of the plankton in today's oceans. Coccolithophorids, a haptophyte group bearing calcite scales (coc-coliths), have an extensive fossil record extending back to the Late Triassic.

Stramenopiles. Stramenopiles are a morphologically diverse group of photosynthetic and heterotrophic protoctistans. Members of this group possess two flagella at some stage in their life cycle; one flagellum is smooth and the other is covered with tripartite hairs. Photo-synthetic stramenopiles possess both chlorophylls a and c. Some of the photosynthetic stra-menopile groups that are found living in the plankton include: diatoms (marine and freshwater), xanthophytes (freshwater), chryso-phytes (marine and freshwater), silicoflagellates (marine), synurophytes, eustimatophytes, raphidophytes, and phaeophytes (marine macroalgae).

Diatoms are a diverse group of stramenopiles with numerous living and fossil species. Diatoms possess an elaborate frustule, or shell, composed of opaline silica. Most planktonic species are centric forms that form radially symmetrical frustules. Living diatoms are yellowish-brown in color and possess, in addition to chlorophylls a and c, accessory photosyn-thetic pigments such as beta carotene and xanthophylls. Diatoms are most common in waters with high nutrients. Some diatoms synthesize a neurotoxin identified as domoic acid that can accumulate in shellfish without any adverse affects, but that may be life-threatening to humans (causing amnesic shellfish poisoning) and to other vertebrates.

The earliest fossil marine diatoms are Early Jurassic in age; the earliest fossil freshwater diatoms are Paleocene in age. Fossil accumulations of diatom frustules are called diatomites or diatomaceous earth, and may be quite extensive in thickness. A well-known outcropping of a marine diatomite is the Middle Miocene Monterey Formation in California, which is mined commercially.

Chlorophytes. Chlorophytes, or green algae, are closely related to land plants and include prasinophyceans, chlorophyceans, and trebouxiophyceans. Chlorophytes include both single-celled and colonial multicellular forms that are characterized by the possession of at least two flagella of equal length. Chloro-

phytes possess chlorophylls a and b as their primary photosynthetic pigments, and carotenoid derivatives as accessory pigments. Chloro-phyceans are a major component of the plankton in freshwater ecosystems, reaching high abundances in nutrient-rich waters.

Prasinophyceans are predominantly marine photosynthetic eukaryotes that may be either biflagellated or quadriflagellated. Their cell bodies and flagella are typically covered in distinctive scales. As mentioned above, prasinophyceans are found as endosymbionts in a few radiolarian taxa. Some prasino-phyceans produce a highly resistant, sporopol-lenin-containing nonmotile stage called a phycoma. Prasinophycean phycomata are found in rocks as early as the Proterozoic, extending all the way up to the Recent.

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