Protists
Learning Objectives
- Describe the main characteristics of protists
- Describe important pathogenic species of protists
- Describe the roles of protists as food sources and as decomposers
Eukaryotic Origins
The fossil record and genetic evidence suggest that prokaryotic cells were the first organisms on Earth. These cells originated approximately 3.5 billion years ago, which was about 1 billion years after Earth’s formation, and were the only life forms on the planet until eukaryotic cells emerged approximately 2.1 billion years ago. During the prokaryotic reign, photosynthetic prokaryotes evolved that were capable of applying the energy from sunlight to synthesize organic materials (like carbohydrates) from carbon dioxide and an electron source (such as hydrogen, hydrogen sulfide, or water).
Photosynthesis using water as an electron donor consumes carbon dioxide and releases molecular oxygen (O2) as a byproduct. The functioning of photosynthetic bacteria over millions of years progressively saturated Earth’s water with oxygen and then oxygenated the atmosphere, which previously contained much greater concentrations of carbon dioxide and much lower concentrations of oxygen. Older anaerobic prokaryotes of the era could not function in their new, aerobic environment. Some species perished, while others survived in the remaining anaerobic environments left on Earth. Still other early prokaryotes evolved mechanisms, such as aerobic respiration, to exploit the oxygenated atmosphere by using oxygen to store energy contained within organic molecules. Aerobic respiration is a more efficient way of obtaining energy from organic molecules, which contributed to the success of these species (as evidenced by the number and diversity of aerobic organisms living on Earth today). The evolution of aerobic prokaryotes was an important step toward the evolution of the first eukaryote, but several other distinguishing features had to evolve as well.
Endosymbiosis
The origin of eukaryotic cells was largely a mystery until a revolutionary hypothesis was comprehensively examined in the 1960s by Lynn Margulis. The endosymbiotic theory states that eukaryotes are a product of one prokaryotic cell engulfing another, one living within another, and evolving together over time until the separate cells were no longer recognizable as such. This once-revolutionary hypothesis had immediate persuasiveness and is now widely accepted, with work progressing on uncovering the steps involved in this evolutionary process as well as the key players. It has become clear that many nuclear eukaryotic genes and the molecular machinery responsible for replicating and expressing those genes appear closely related to the Archaea. On the other hand, the metabolic organelles and the genes responsible for many energy-harvesting processes had their origins in bacteria. Much remains to be clarified about how this relationship occurred; this continues to be an exciting field of discovery in biology.
The early eukaryotes were unicellular like most protists are today, but as eukaryotes became more complex, the evolution of multicellularity allowed cells to remain small while still exhibiting specialized functions. The ancestors of today’s multicellular eukaryotes are thought to have evolved about 1.5 billion years ago.
Protists
Eukaryotic organisms that did not fit the criteria for the kingdoms Animalia, Fungi, or Plantae historically were called protists and were classified into the kingdom Protista. Protists include the single-celled eukaryotes living in pond water (Figure 10.13), although protist species live in a variety of other aquatic and terrestrial environments, and occupy many different niches. Not all protists are microscopic and single-celled; there exist some very large multicellular species, such as the kelps. During the past two decades, the field of molecular genetics has demonstrated that some protists are more related to animals, plants, or fungi than they are to other protists. For this reason, protist lineages originally classified into the kingdom Protista have been reassigned into new kingdoms or other existing kingdoms. The evolutionary lineages of the protists continue to be examined and debated. In the meantime, the term “protist” still is used informally to describe this tremendously diverse group of eukaryotes. As a collective group, protists display an astounding diversity of morphologies, physiologies, and ecologies.
Characteristics of Protists
There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live in symbiotic relationships with other organisms and these relationships are often species specific, there is a huge potential for undescribed protist diversity that matches the diversity of the hosts. As the catchall term for eukaryotic organisms that are not animals, plants, fungi, or any single phylogenetically related group, it is not surprising that few characteristics are common to all protists.
Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Several protist species are parasites that infect animals or plants. A parasite is an organism that lives on or in another organism and feeds on it, often without killing it. A few protist species live on dead organisms or their wastes, and contribute to their decay.
Protist Structure
Protist cells are incredibly diverse, ranging from simple, microscopic single-celled organisms to complex, multicellular or even multinucleate forms. Their sizes vary widely, from less than a micrometer to the enormous 3-meter cells of seaweed like Caulerpa. These cells may be protected by a variety of coverings, such as animal-like membranes, plant-like walls, or even silica-based shells and protein-based pellicles. In terms of movement, protists are mainly motile and employ a range of techniques: flagella for rotation, cilia for coordinated swimming, or pseudopodia for a “crawl and anchor” approach. Some even have light-sensing organs to move toward light sources.
How Protists Obtain Energy
Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Photosynthetic protists (photoautotrophs) are characterized by the presence of chloroplasts. Other protists are heterotrophs and consume organic materials (such as other organisms) to obtain nutrition. Amoebas and certain other heterotrophic protists consume food through phagocytosis (Figure 10.14). In this process, the cell membrane wraps around a food particle to form an internal sac, known as a food vacuole. This vacuole merges with a lysosome, breaking down the food into molecules that enter the cytoplasm for cellular metabolism. Any leftovers are ejected from the cell via exocytosis.
Reproduction
Protists reproduce by a variety of mechanisms. Most are capable some form of asexual reproduction, such as binary fission to produce two daughter cells, or multiple fission to divide simultaneously into many daughter cells. Others produce tiny buds that go on to divide and grow to the size of the parental protist. Sexual reproduction, involving meiosis and fertilization, is common among protists, and many protist species can switch from asexual to sexual reproduction when necessary. Sexual reproduction is often associated with periods when nutrients are depleted or environmental changes occur. Sexual reproduction may allow the protist to recombine genes and produce new variations of progeny that may be better suited to surviving in the new environment.
Human Pathogens
Many protists are pathogenic parasites that must infect other organisms to survive and propagate. Protist parasites include the causative agents of malaria, African sleeping sickness, and waterborne gastroenteritis in humans. Other protist pathogens prey on plants, effecting massive destruction of food crops.
Plasmodium Species
Plasmodium species are the main culprits behind malaria, especially P. falciparum, which is responsible for most malaria-related deaths. In 2010, it was estimated that malaria caused between 0.5 and 1 million deaths, mostly in African children. The parasite’s life cycle involves both mosquitoes and vertebrates, chiefly humans. Infection leads to the destruction of red blood cells (Figure 10.15), causing severe anemia and triggering a strong immune response marked by high fever. Efforts to control malaria focus on targeting the mosquito vector, Anopheles gambiae.
Trypanosomes
Another parasite, T. brucei (Figure 10.16), causes African sleeping sickness by evading the human immune system through constantly changing its surface proteins. Without treatment, African sleeping sickness leads invariably to death because of damage it does to the nervous system. During epidemic periods, mortality from the disease can be high. Greater surveillance and control measures have led to a reduction in reported cases; some of the lowest numbers reported in 50 years (fewer than 10,000 cases in all of sub-Saharan Africa) have happened since 2009.
In Latin America, T. cruzi leads to Chagas disease, affecting the heart and digestive system and resulting in malnutrition and heart failure. T. cruzi infections are mainly caused by a blood-sucking bug. An estimated 10 million people are infected with Chagas disease, which caused 10,000 deaths in 2008.
Plant Parasites
Protists also wreak havoc on plants. Plasmopara viticola causes downy mildew in grape plants, severely affecting wine industries. Phytophthora infestans is responsible for potato late blight, which was a key factor in the Irish potato famine in the 19th century and continues to devastate potato crops in some regions today. Both parasites underscore the economic and human cost of protist infections, highlighting the need for continued research and control measures (Figure 10.17).
Beneficial Protists
Protists play critically important ecological roles as producers particularly in the world’s oceans. They are equally important on the other end of food webs as decomposers.
Protists as Food Sources
Protists play a crucial role as a food source for a variety of organisms. For example, planktonic protists are often consumed directly, while photosynthetic types like dinoflagellates serve as nutrient factories. Take zooxanthellae, a type of dinoflagellate, which lives in a mutually beneficial relationship with coral polyps (Figure 10.18). The polyps offer them a cozy home and nutrients, while the dinoflagellates return the favor by sharing their energy. This partnership is vital for building coral reefs. When this relationship breaks down, we see a phenomenon known as coral bleaching, leading to the death of corals. This also explains why you won’t find reef-building corals in waters deeper than 20 meters; there’s simply not enough light for the dinoflagellates to do their photosynthetic magic.
Protists themselves and their products of photosynthesis are essential—directly or indirectly—to the survival of organisms ranging from bacteria to mammals. As primary producers, protists feed a large proportion of the world’s aquatic species. (Compared to on land, where terrestrial plants serve as primary producers.) In fact, approximately one-quarter of the world’s photosynthesis is conducted by protists, particularly dinoflagellates, diatoms, and multicellular algae.
Protists do not create food sources only for sea-dwelling organisms. For instance, certain anaerobic species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute to digesting cellulose ingested by these insects as they bore through wood. The actual enzyme used to digest the cellulose is actually produced by bacteria living within the protist cells. The termite provides the food source to the protist and its bacteria, and the protist and bacteria provide nutrients to the termite by breaking down the cellulose.
Agents of Decomposition
Many fungus-like protists are saprobes aka saprophytes (organisms that feed on dead organisms or the waste matter produced by organisms), and are specialized to absorb nutrients from nonliving organic matter. For instance, many types of oomycetes grow on dead animals or algae. Saprobic protists have the essential function of returning inorganic nutrients to the soil and water. This process allows for new plant growth, which in turn generates sustenance for other organisms along the food chain. Indeed, without saprobic species, such as protists, fungi, and bacteria, life would cease to exist as all organic carbon became “tied up” in dead organisms.
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