Plant Life: protista

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Algae

Red algae

Algae comprise a diverse group of (with few exceptions) photosynthetic oxygen-producing organisms, ranging in size from microscopic single cells to gigantic seaweeds.

The study of algae is known as phycology (in Greek, phycos means "algae"). Currently, most authors place eukaryotic algae in the kingdom Protista (domain Eukarya) and prokaryotic algae in the domain Bacteria.

In the past algae were considered to be lower plants because some forms look like plants. As in plants, the primary photosynthetic pigment in algae is chlorophyll a, and oxygen is produced during photosynthesis.

Brown Algae

Seaweeds that are brown to olive-green in color belong to the phylum Phaeophyta, or brown algae, which includes between fifteen hundred and two thousand species.

Brown algae (phylum Phaeophyta) are familiar to most people as brown or dark green seaweeds. Some brown algae are microscopic in size, but many are relatively large: One giant kelp measured 710 feet in length. All brown algae are multicellular.

Appearance and Distribution

Brown algae have a body, called a thallus, which is a fairly simple, undifferentiated structure. Some thalli consist of simple branched filaments. Some brown algae have more complex structures called pseudoparenchyma because they superficially resemble the more complex tissues of higher plants.

Cellular slime molds, or dictyostelids, were originally considered to be fungi. These microscopic, multicellular organisms are easily mistaken for some of the microfungi that commonly occur as contaminants in laboratory cultures. However, cellular slime molds are more closely related to the protozoans than to fungi.

Although once thought to be fungi, the protists of the phylum Dictyosteliomycota actually have more in common with the paramecium or amoeba that can be observed in a drop of pond water when viewed under the microscope than they do with mushrooms and toadstools.

Cellular slime molds are essentially microscopic throughout their entire life cycle, and only rarely can they be observed directly in nature, as is the case for the plasmodial slime molds. Cellular slime molds must therefore be grown under controlled laboratory conditions in order to be studied.

Spirogyra

It is almost impossible not to see Spirogyra floating on the surface of a pond on a hot summer day, but most people dismiss it as pond scum. Few realize that what they are looking at is a member of the Charophyceae, a class in the phylum Chlorophyta, or green algae, and a cousin of the ancestor of the Embryophyta, or bryophytes and vascular plants.

Most Charophyceae, like Spirogyra, live in freshwater habitats, but some also occur in moist soil in terrestrial habitats. Charophyceae can live as single cells, colonies, or branched and unbranched filaments and come in a variety of shapes.

The characteristics that unite members of the class—and which link them with the embryophytes—include flagellated cells (similar to sperm cells in vascular plants), a nuclear envelope that breaks down during mitosis, mitotic spindles that persist as phragmoplasts (a type of cytoskeletal scaffolding) through cell division either by furrowing or by forming a cell plate, the presence of chlorophylls a and b and phytochrome, and the storage of starch inside plastids.

Chlorophyceae (from the Greek word chloros, meaning “green”) make up an extremely large and important class of green algae. Members may be unicellular, colonial, or filamentous. Cells of unicellular and colonial chlorophyceans may have two or more flagella.

There are about 2,650 living species of chlorophyceans. The main features of the class (and most plants) are the use of starch as the principal food reserve and the green chloroplasts with chlorophylls a and b. In spite of plant characteristics, this algal group is not directly related to early land plants.

Chlorophyceans are almost entirely restricted to freshwater and terrestrial habitats. Some members of this class have adapted to life on snow as snow algae. Snow algae cause snow to appear red-burgundy or orange in color because of high levels of unusual carotenoid pigments within the algal cells.

The Chrysophyceae, classified within the kingdom Chromista, are mostly unicellular or colonial organisms found in fresh and salt water throughout the world.

The Chrysophyceae (in some systems corresponding to the phylum Chrysophyta) are related to heterokont algae and include more than eight hundred described species that are classified in approximately one hundred genera.

They aremost closely related to the Synurophyceae and other pigmented heterokont algae, including the Bacillariophyceae (diatoms), Eustigmatophyceae, Phaeophyceae (brown algae), and Xanthophyceae (yellow-green algae), among others.

Cryptomonads

The phylum Cryptophyta describes tiny, motile, unicellular organisms with two slightly unequal flagella bearing lateral hairs. Cryptomonads live mainly in marine and freshwater environments.

Some cryptomonads are alga-like, with bluegreen, red, and olive-brown photosynthetic pigments including chlorophylls a, c2, alpha-carotene, xanthophylls (alloxanthin, crocoxanthin, zeaxanthin, and monadoxanthin), and phycobiliproteins (phycoerythrin and phycocyanin).

Cryptomonads are found in a variety of moist places, such as algal blooms in the ocean or in fresh water, and on beaches. Some members are intestinal parasites in animals.

Diatoms

Diatoms are unicellular microorganisms of the phylum Bacillariophyta that are abundant in aquatic, semi aquatic, and moist habitats throughout the world, growing as solitary cells, chains of cells, or members of colonies.

Diatoms, algal organisms of the phylum Bacillariophyta, have more than 250 genera and about 100,000 species. A distinctive siliceous cell wall called a frustule surrounds each vegetative cell. Diatoms have an extensive fossil record, going back some 100 million years to the Cretaceous period.

Deposits of fossil diatoms, known as diatomite or diatomaceous earth, are mined commercially for use as abrasives and filtering aids. One subterranean marine deposit in Santa Maria, California, is about 900 meters in thickness. More than 270,000 metric tons of diatomaceous earth are quarried annually from a deposit in Lompoc, California.

Dinoflagellates

Dinoflagellates, phylumDinophyta, are unicellular and colonial algal organisms from the kingdom Protista named for the spinning motions that result from the movement of their flagella.

The two thousand to four thousand species that make up the Dinophyta phylum typically have two flagella. Dinokonts (Dinophyceae) have one flagellum running in a groove that cuts transversely across the cell and another flagellum, the sulcus, that runs backward in a longitudinal groove and is more or less perpendicular to the transverse one. In desmokonts (Desmophyceae), both flagella arise from a point at the front of the cell.

The motions of the flagella, which make dinoflagellates spin like a top, help give rise to the name, for dinos in Greek means “whirling,” and flagella means “whip.” Single-celled species are the most common, but colonial species exist. The largest dinoflagellate, Noctiluca, may grow as large as 2 millimeters in diameter.

Euglenoids

Organisms called euglenoids, the algal phylum Euglenophyta in the kingdom Protista, make up a large group of common microorganisms numbering between 750 and 900 known species.

Euglenoids can be found in both fresh and stagnant water. Some genera of euglenoids can also be found in marine habitats. Euglena and Phacus are representative common genera. Euglenoids are unicellular, except for those of the colonial genus Colacium.

Because euglenoids have flexible cell coverings, move about freely, and ingest their food through a structure called a gullet, many scientists have classified the euglenoids as animals. Some species of euglenoids, however, have chloroplasts and are able to supply at least some of their food needs through photosynthesis.

An algae covered rock

The green algae are a diverse group of eukaryotic organisms classified in the phylum Chlorophyta. They are considered eukaryotic because individual cells possess a prominent structural feature known as a nucleus, which houses the chemicals responsible for heredity and metabolic regulation.

The phylum is one of several algal phyla in the kingdom Protista, where algae are grouped based upon pigmentation, carbohydrate storage reserves, and cell wall composition.

Green algae are found in moist soils and fresh-water and saltwater habitats; most are believed to be freshwater-dwelling. The phylum consists of at least eight thousand species. Some estimates place this number at seventeen thousand species. Several shared characteristics support the hypothesis that these organisms and terrestrial plants derived from a common ancestor.

Haptophytes

The algal phylum Prymnesiophyta, or Haptophyta, is a monophyletic taxon that contains two hundred to three hundred extant species that are 4-40 microns in size.

The phylum Haptophyta is divided into two subclasses, the Prymnesiophycidae and Pavlovophycidae, that differ significantly from one another. The phylum is usually classified within the kingdom Chromista with other algae containing chlorophyll a and c (excluding dinoflagellates), but their exact relationship to the heterokont algae remains unclear.

Although a few freshwater species are known, most are marine, and species diversity within the phylum is greatest in nutrient-poor waters of the tropical and subtropical open oceans. Most are motile biflagellate single cells or nonmotile coccoid (walled unicells) cells. Colonial, filamentous, and palmelloid forms are also known.

Heterokonts

Heterokonts are a group of closely related phyla with flagella in pairs, one long and one short. They include oomycetes, chrysophytes, diatoms, and brown algae.

The term “heterokont” refers either to the flagellar arrangement of biflagellate cells in which the two flagella differ in length (as in anisokonts), type of motion, or ornamentation, or to those organisms (and organisms evolutionarily derived from such lineages) in which biflagellate cells with heterokont flagella are produced at some point during their life cycle.

The most common heterokont flagellar arrangement consists of a posteriorly directed whiplash flagellum and an anteriorly directed tinsel flagellum. The tubular tinsel flagellum characteristically bears two rows of stiff, glycoproteinaceous, tripartite hairs previously referred to as mastigonemes but now increasingly referred to as stramenopili. Such organisms are often referred to as stramenopiles.

Microbial Nutrition and Metabolism

The diverse metabolic activities of microorganisms make them a critical component of all the earth’s ecosystems and a source of many useful products for human industry.

Microorganisms—bacteria, fungi, algae, and protists—are found in every environment on the earth that supports life. Microorganisms have been found in hot springs where temperatures exceed 80 degrees Celsius as well as in rocks of Antarctic deserts.

To ensure survival in a variety of habitats, microorganisms have developed a fascinating variety of strategies for survival. The study of microbial ecology involves consideration of the mechanisms employed by microorganisms to obtain nutrients and energy from their environment.

Oomycetes

Oomycetes are a diverse group of fungus-like eukaryotic microorganisms that form the phylum Oomycota. First classified within the kingdom Fungi, oomycetes are now unambiguously recognized as distinct from fungi and more closely related to heterokonts, such as brown algae, phylum Phaeophyta.

The more than five hundred species of oomycetes, commonly known as water molds, white rusts, or downy mildews, are essentially saprophytic but include pathogens of plants, insects, crustaceans, fish, vertebrate animals, and various microrganisms.

Plant pathogenic oomycetes cause devastating diseases on several crop, ornamental, and native plants. Animal pathogenic oomycetes can cause severe losses in aquaculture and fisheries. Both have had a significant impact on human history.

Plasmodial Slime Molds

The plasmodial slimemolds, or myxomycetes, phylum Myxomycota, are a group of fungus-like organisms usually present and sometimes abundant in terrestrial ecosystems. However, this group comprises about eight hundred species, related neither to cellular slime molds nor to fungi.

The plasmodial slime molds have no cell walls and exist as thin masses of protoplasm, which appear to be streaming in a fan like shape, under favorable conditions.

As these masses, called plasmodia, travel, they absorb small particles of decaying plant and animal matter as well as bacteria, fungi, and yeasts. When mature, a plasmodium may weigh 20-30 grams and take up an area of 1 meter or more.

Protista

The Protista form one of the four kingdoms of eukaryotic organisms and include the algae, protozoans, slimemolds, and oomycota.

Included among the diverse organisms called protistans (kingdom Protista) are algae, protozoans, slime molds, and the oomycota. Algae were long considered to be simple plants and were assigned to kingdom Plantae but lack the more highly differentiated tissues and organs characteristic of “higher plants” such as mosses, ferns, and seed plants.

Protozoa, all of which are unicellular, were assigned to the kingdom Animalia; considered more animal-like than plant-like, they are not considered here in detail.

Plant Life