You should have a working knowledge of the following terms:
- accessory pigment
- alternation of generations
- convergent evolution
- lichen | * mycolaminarin
- psuedopodium (pl. pseudopodia)
Introduction and Goals
This tutorial will cover the last three protist kingdoms. They have members that are mostly photosynthetic; additionally, many members have a profound influence on humanity. Several links have been added for your interest; however, you will not be tested on material outside of the text within this tutorial. We will begin with a discussion of Kingdom Stramenopila, which includes the diatoms, water molds, and brown algae. Next we will examine Kingdom Rhodophyta (the red algae), and will conclude with Kingdom Chlorophyta (green algae). By the end of this tutorial you should have a fundamental understanding of:
- The life histories and classifications of Kingdoms Stramenopila, Rhodophyta and Chlorophyta, and Phyla Bacillariophyta, Chrysophyta, Oomycota, and Phaeophyta
- The organisms that caused the "Great Potato Famine" in Ireland, and the outbreak of downy mildew that nearly destroyed the French wine industry
- The basic characteristics of algae, seaweeds, and kelps
- The mode of sexual reproduction known as alternation of generations
- The relationship between plants and photosynthetic protists
Introduction to Kingdom Stramenopila
Stramenopilans are a remarkably diverse, yet monophyletic group that includes: planktonic diatoms, whose microscopic glassy shells comprise diatomaceous earth; large multicellular marine seaweeds (brown algae or phaeophytes); and water molds, which include the pathogen responsible for the potato blight that drove millions of people in Ireland hungry in the nineteenth century. Although this group is diverse, they all share a common ancestral trait. Namely, the presence of hair-like projections on their flagella; "stramen" means flagellum in Latin, and "pilos" means hair; hence, stramenopilans.
Figure 1 (Click image to enlarge)
Some stramenopilans are heterotrophic (e.g., the oomycetes), whereas others are photoautotrophic (e.g., the phaeophytes).
Kingdom Stramenopila: Phylum Bacillariophyta (Diatoms)
Figure 2 (Click on image to enlarge)
Diatoms are very important from a global perspective. Marine and freshwater plankton are full of photosynthetic diatoms. Some estimates suggest that they may be responsible for over 20% of annual global carbon fixation. Diatoms are encased in a glass-like silica shell, lined with perforations to allow gas exchange at the cell surface. These shells, invisible to the naked eye, are exceptionally beautiful structures. Each one consists of two components that fit together, like the bottom and lid of a pillbox. After the death of individual diatoms, their microscopic shells sink and gradually form thick layers of sediment (diatomaceous earth). Living diatoms avoid sinking by regulating their cellular ion concentrations.
Diatomaceous earth, which essentially consists of microscopic fragments of glass, has numerous applications. For example, it is commonly used as a diatomaceous earth pesticide; the sharp edges of diatom shells scrape arthropod exoskeletons and gut linings, causing water loss and eventually death by desiccation. It also damages the tissues of worms and other small creatures, and often is incorporated into animal feed to reduce the occurrence of intestinal parasites. It has myriad other uses (e.g., giving scouring power to cleaning products, and allowing ultrafine filtration for scientific applications and water-purification systems).
Silica shells are fairly inert, and diatomaceous earth tends to stay where it settles. Therefore, scientists can look at the assemblages of diatom species in soil cores taken from diatomaceous earth, where the deepest layers of the sample are the oldest and represent the most distant past. They can date the samples and examine changes in the number of warm-climate diatom species and cool-climate species over time, gaining clues about ancient climatic patterns. Actually, it is not quite this simple. Other factors, besides temperature, can influence the diatom species found. Also, scientists do not know the temperature preferences of extinct diatom species discovered in samples.
Figure 4 (Click on image to enlarge)
Diatoms are mostly photosynthetic, but there are also heterotrophic species.
Cell division in diatoms was discusssed in the tutorial on Mitosis-Cell Cycle Regulation
Kingdom Stramenopila: Phylum Chrysophyta (Golden Algae)
Figure 5 (Click image to enlarge)
Figure 6 (Click image to enlarge)
Chrysophytes are planktonic organisms that possess a rich golden color due to the presence of carotenoids and xanthophylls. These accessory pigments allow them to expand the range of light wavelengths they can use during photosynthesis. Many golden algae are predators (as well as being photoautotrophic), using pseudopodia to prey on smaller organisms (e.g., diatoms and bacteria). Some possess silica scales, similar in composition to the shells of diatoms; silica also coats the quiescent cysts that form under unfavorable conditions, allowing chrysophytes to remain dormant for decades. Most chrysophytes are unicellular, but some species are colonial and quite elaborate in structure (e.g., Synura sp. colony pictured below).
Kingdom Stramenopila: Phylum Oomycota
Figure 7 (Click image to enlarge)
Oomycetes include the water molds and downy mildews. Some are unicellular, and many others are colonial. Most are heterotrophic decomposers that feed on dead and decaying organic matter in aquatic and terrestrial environments, but a number attack living plants and animals. The name "oomycete" means "egg fungi," which is a reference to the reproductive structures of sexually reproducing oomycetes.
Although oomycetes are not closely related to fungi, the two groups have some similarities. In particular, both groups are heterotrophs that break down food externally and then absorb nutrients from their surroundings. Additionally, many of the multicellular oomycetes form hyphae, which are very similar to fungal hyphae although the structures are not identical.
However, there are more differences than similarities. For example, the cell walls of oomycetes are composed of cellulose, not chitin (the compound found in the cell walls of fungi). Unlike true fungi, which are haploid in the feeding stage, the hyphae of most oomycetes are composed of diploid cells. Furthermore, the presence of the storage compound mycolaminarin, along with molecular evidence and the presence of flagellated cells similar to those of other stramenopiles, indicate that oomycetes belong within Kingdom Stramenopila.
The resemblance between fungi and oomycetes is an example of convergent evolution, the process by which unrelated organisms that occupy similar environments evolve similar functional traits. For example, oomycetes and fungi are decomposers and therefore stand to gain an advantage from maximizing their surface area for the absorption of food; thus, the filamentous growth form might have been favored by independent evolution in both lineages.
Oomycetes Impact on Humans
Figure 9 (Click on image to enlarge)
Oomycetes have been responsible for a number of catastrophic historical events, including the outbreak of downy mildew that nearly wiped out the French wine industry in the late nineteenth century. Plasmopara (left), a native of the New World, was inadvertently brought to France from the United States in 1870 in a shipment of American grape root stock. It quickly became a devastating problem. A mixture of lime and copper sulfate, the first chemicals used to combat a plant pathogen, solved the problem.
Figure 10 (Click on image to enlarge)
Figure 11 (Click on image to enlarge)
Figure 12 (Click on image to enlarge)
The oomycete Phytophthora infestans, or potato blight, was responsible for another cataclysmic event, the Great Potato Famine, which killed nearly one-million people in Ireland in the late 1800s, and drove one-and-a-half-million more out of the country. Phytophthora (top left) secretes enzymes that break down leaf and stem tissue (middle top), killing plants very rapidly. Tubers can become infested, turning soft and black, seemingly overnight. Potato fields in Ireland became infested with Phytophthora, which thrived in the cool damp climate of Ireland and wiped out nearly the entire country's crop in one week. In nineteenth-century Ireland, potatoes were the primary food source of the poorest classes, who often grew nothing other than potatoes. The loss of this staple crop led to mass starvation.
Kingdom Stramenopila: Phylum Phaeophyta (Brown Algae)
Figure 13 (Click image to enlarge)
Phaeophytes, or brown algae, include the largest of the protists, with some growing over 100 feet in length. The giant multicellular species that comprise "kelp forests" in temperate marine waters belong to this group. The edible alga "kombu," which is harvested by the Japanese, is also a phaeophyte. It is particularly rich in minerals, as are other marine algae. Like the chrysophytes, phaeophytes contain specific accessory pigments that give them their characteristic colors.
Figure 14 (Click image to enlarge)
Along with the two kingdoms that we will discuss next, Rhodophyta and Chlorophyta, Phylum Phaeophyta includes a number of seaweeds. Although it is not a taxonomic term, the word "seaweed" is a useful way of distinguishing large intertidal algae from other species (e.g., planktonic algae). Many types of seaweed, including those within the Phylum Phaeophyta, have complex structures that are reminiscent of plants. The thallus refers to the entire body of any seaweed that is plant-like in appearance. The thallus consists of three main parts: a stipe, which is analogous to the stem of plants; a holdfast, which secures the seaweed to a substrate; and leaf-like blades, which provide extensive surface area for photosynthesis, much as leaves do for plants.
Figure 15 (Click on image to enlarge)
Another word used informally to describe a specific type of alga is "kelp," which refers to giant seaweeds that grow in the deeper waters outside of the intertidal zone. All kelps, which form vast "forests" that support thriving marine ecosystems, are phaeophytes. As one can see (left), these organisms can reach very impressive sizes.
Alternation of Generations
Recall from the tutorial on Heredity and Life Cycles that life cycles were discussed. In this discussion, alternation of generations was introduced. Although there is great variety in modes of reproduction among algae, we will focus on alternation of generations (the most complex of their life cycles). It occurs in Phylum Phaeophyta and in the two kingdoms that we will discuss next, the Rhodophyta and the Chlorophyta. This type of life cycle is also seen in more complex organisms. For example, plants also alternate generations during sexual reproduction, as do fungi.
Alternation of generations refers specifically to the alternation between multicellular haploid life stages and multicellular diploid life stages. The key feature is that both the diploid and haploid stages are multicellular. Diploid forms (sporophytes) produce haploid spores, which divide and develop directly into multicellular haploid structures (gametophytes). Gametophytes then produce haploid gametes, which join with other gametes to form diploid sporophytes once again. In other words, the diploid and haploid generations alternate, over and over. In some organisms the diploid stage is the dominant form that is responsible for the majority of growth and resource acquisition, whereas in others (e.g., fungi) the haploid stage is dominant.
The brown alga Laminaria is a model for alternation of generations. Laminaria reproduces with heteromorphic alternation of generations (shown below under Chlorophyta). That is, the haploid and diploid stages exhibit different forms.
Members of Kingdom Rhodophyta differ from other eukaryotic algae because they do not have flagellated cells at any point in their life cycles. DNA sequence data (and other sources) indicate that this lineage arose independently, sometime before Kingdom Stramenopila.
Most rhodophytes are marine algae, but a number of species live in freshwater and some even inhabit terrestrial niches. Alternation of generations occurs frequently in Kingdom Rhodophyta. Like many of the photosynthetic eukaryotes, rhodophytes are characterized by accessory pigments in their chloroplasts, which endow them with unique colors. In the case of rhodophytes, also called red algae, pigments called phycobilins produce rich shades of pink, scarlet, and red that are so deep that they approach black. These red pigments, in particular the phycobilin phycoerythrin, allow rhodophytes to photosynthesize at depths where only high-energy blue and green light can penetrate. In fact, the color of rhodophyte species tends to be correlated with the depth where they commonly occur; deep-water species are often nearly black, concentrated with phycoerythrin, whereas shallow-water species can contain so few accessory pigments that they appear almost green, having few pigments to mask the green color of chlorophyll.
Figure 17 (Click image to enlarge)
Many red algae are large multicellular organisms, although they are not in the same size class as the largest of the brown algae. A number of these species are of economic importance; the odds are very good that you've eaten red algae or that you have relied on its derivatives. The seaweed used in sushi (nori) is a red alga.
Figure 18 (Click image to enlarge)
Chlorophytes, or green algae, resemble plants more closely than do any of the other photosynthetic protists. In fact, some classification systems group members of Kingdom Chlorophyta with plants on the premise that green algae actually resemble plants more closely than they resemble other protists. The chlorophyll of green algae (chlorophyll a and chlorophyll b) is strikingly similar to that of plants. Green algae also resemble plants because they store fixed carbon as starch and they possess cell walls composed of cellulose. Although the issue is contentious, most systematists continue to classify green algae separately.
There is a tremendous amount of diversity within this kingdom. Chlorophytes occupy a remarkable array of habitats, including marine, freshwater and terrestrial environments. A number are seaweeds, most are freshwater algae, and some terrestrial chlorophytes (lichens) live in a symbiotic association with fungi. The 7,000 species that comprise this kingdom range from unicellular to colonial to truly multicellular. Separate evolutionary events might have led to the evolution from single-celled green algae to colonial organisms (e.g., the beautiful Volvox), to multinucleate single-celled seaweeds, and to multicellular seaweeds (e.g., Ulva). The photos below show the diversity of this group.
Kingdom Chlorophyta also exhibits many forms of reproduction, and most species reproduce asexually and sexually. For example, Spirogyra is composed of long filaments of cells that can break apart and divide to produce new individuals. Alternatively, bridges can form between different individuals, allowing the exchange of gametes. Like the brown and red algae, many green algae also display alternation of generations during their sexual cycles, switching between haploid and diploid forms. Note, unlike the brown alga Laminaria, Ulva alternates between haploid and diploid forms that are structurally similar. This is referred to as isomorphic alternation of generations.
The protists are ecologically and economically significant organisms; they are responsible for a large portion of the Earth's fixed carbon and for a number of plant and animal diseases, some of which have caused considerable devastation to mankind. Considering their small size, their diversity is astounding. The beginnings of multicellularity in early protists paved the way for even greater structural complexity and diversity in taxa such as plants, animals, and fungi. For more information on the systematics and taxonomy of protists (and all other life forms), the web site of the University of California-Berkeley's Museum of Paleontology is an excellent resource, although their system of classification differs somewhat from the system used in this course.