Protists and Protist Organisms
|Biology - The Kingdoms of Life|
The word protist may bring to mind an amoeba, a diatom, or another microorganism. But the protist kingdom also includes larger organisms such as seaweeds. Even though these resemble plants in some respects, they lack the structural complexity of plants and so are grouped with their closer relatives, the uni-cellular protists.
The boundaries of the protist kingdom are probably more blurred than those of any other. All of the three strictly multicellular kingdoms, the plants, fungi, and animals, developed from various ancient protists. Some protists share features with plants, some with animals, and some with fungi.
Taxonomists sometimes differ about where to separate the protists from these other kingdoms. The feature thought to indicate the common ancestry of all four of these kingdoms is eucaryotic cells. A eucaryotic cell has a distinct, membrane-bounded nucleus. Protists, fungi, animals, and plants have such cells. Only the monerans, or bacteria, are prokaryotes, cells without distinct nuclei.
The protists are currently defined as all of the unicellular eucaryotes and those multicellular ones that are directly related to them. For example, the Chlamydomonas is a unicellular green alga. Ulva, or sea lettuce, is a common multicellular green alga. Even though sea lettuce superficially resembles a plant, it shares ancestry more directly with the Chlamydomonas. One indication of this shared ancestry is that during one stage of its lifecycle, the Ulva produces flagellated cells very similar to Chlamydomonas. Many other multicellular green algae do the same.
Many protists are autotrophs, producing their food through photosynthesis as most plants do. Cell organelles called plastids contain the photosynthetic pigments of eucaryotic cells. In green algae, as in plants, these plastids are chloroplasts. This shared feature is one indication that the plant kingdom probably evolved from an ancient green alga.
Life cycles vary greatly among algae and among protists in general. This is a filament of Derbesia, a green alga found in freshwater ponds. The enlarged sporangium contains flagellated zoospores. After a zoospore is released, it swims to an underwater surface, where it attaches and develops. There it takes on a completely different growth form. This globular structure produces gametes, or sex cells, and releases them into the water, where pairs of them fuse to form zygotes. A zygote develops into the filamentous form of the alga, and the process repeats itself.
Volvox is very different. Usually referred to as a colonial alga, a Volvox consists of hundreds or thousands of flagellated cells occurring in a spherical skin surrounding a jellylike matrix. The cells are interconnected by threads of protoplasm, and all their flagella point outward from the sphere. The coordinated waving of these flagella causes a Volvox colony to roll through the water. Volvox may reproduce sexually, but like many other algae, it also reproduces asexually.
Certain cells begin to push inward, and by repeated cell divisions they form smaller, daughter colonies within the large colony. Eventually, the original colony ruptures, releasing the newer ones. A large group of green algae is characterised by a mode of sexual reproduction called conjugation. These algae have no flagellated cells at all. Many of them form filaments.
During sexual reproduction, two filaments of opposite mating types line up alongside each other and form connecting tubes between cells. The genetic material in the cells of the donor filament passes into the cells of the receiving filament, where zygotes are formed. A widespread conjugating alga is Spirogyra. Spirogyra is the sort of algae that most people think of when they picture algae; it often forms scummy mats in ponds or sluggish rivers. Algae don't always live in water, though. Protococcus is a common green alga that lives on tree bark, soil, or even brick walls.
As with the green algae, most algae are classed into phyla primarily according to their pigmentation. The plastids of the brown algae contain xanthophyll along with their chlorophyll, giving them a dull, olive colour. The brown algae include the largest seaweeds. Most undergo sexual reproduction. At the ends of the flat blades of this Fucusare inflated receptacles. Eggs and motile sperm are produced in the tiny holes on these receptacles. Fucus is very common on rocky coasts in the intertidal zone, where it attaches to rocks.
The red algae have a complex blend of pigments in their plastids. They're also characterised by sexual reproduction, but without motile, flagellated gametes. Generally, the male gametes are released into the water near the female structures, much as some plants release pollen into the wind. Many red algae, like this Polysiphonia, are feathery seaweeds. The red alga Corallina forms creeping filaments on underwater or intertidal surfaces. Its cell walls are thickly impregnated with calcium carbonate. Over time its growth produces great accumulations of lime, contributing to the formation of coral reefs. There are photosynthetic protists besides the algae.
Many dinoflagellates are photosynthetic. The dinoflagellates are a large phylum of unicellular protists classed together because of several features such as a peculiar arrangement of two flagella. A longitudinal flagellum trails downward, while a transverse flagellum lines a groove around the equator of the organism. When this flagellum undulates, the dinoflagellate rotates through the water. Most dinoflagellates are planktonic, but some are photosynthetic symbionts in the cells of marine animals such as some corals.
The diatoms are another huge phylum. Most are photosynthetic, and many are important producers in aquatic food chains. Diatoms secrete intricate cell walls called tests. Pores in these silica tests allow contact between the diatom's cytoplasm and its environment. Diatoms occur in many variations of two basic shapes: some are bilaterally symmetrical, and some are radially symmetrical. The bilateral diatoms tend to crawl along a substrate while the radial ones tend to live as plankton. Diatom tests occur in two pieces, one of which forms a base, and the other a lid.
As the cell divides, the halves separate and each forms a new base. Consequently, diatoms get smaller and smaller with each division. When a diatom reaches a certain critical size, it releases its contents in the form of an auxospore, which secretes a full-sized test, starting the process over. Some groups of protists contain both photosynthetic and heterotrophic organisms. The phylum of euglenoid flagellates is an example.
Many Euglena species contain chloroplasts and photosynthesise food. Others depend on dissolved food in the surrounding water. Some euglenas may either photosynthesise or absorb food. Even with these differences in nutrition, Euglena species' common structural features clearly indicate their shared ancestry. A Euglena cell is covered by a pellicle formed of spirally bound elastic protein filaments. Beside the conspicuous flagellum is a smaller flagellum that somehow works in conjunction with an eye spot to control the movement of euglenas in reaction to the location and intensity of light. Euglenas' locomotion once inspired some taxonomists to classify them with animals.
Some other protists are even more animal-like. Many of these were once considered unicellular animals and are still collectively called protozoans. A familiar example in the rhizopod phylum of protists is the amoeba. An amoeba uses flexible cellular extensions called pseudopodia to pull itself along surfaces and to surround bits of food and pull them into the cell. This amoeba has engulfed another protist, a Paramecium, which is contained in a food vacuole inside.
Some amoebas have tests. Unlike diatoms, they do not secrete these tests; rather, these shelled amoebas glue together tiny sand grains and bits of debris to form their tests. Snail-like, the amoeba protrudes through an opening to move and feed. Similar to rhizopods are the actinipods. This protist phylum includes the freshwater heliozoan seen here. It is a planktonic pincushion, with silica spines called axopods extending from the centre. Cytoplasm extruding from pores in the central disk makes a froth in which particles of food are caught and pulled back into the centre of the cell.
The actinopod phylum also includes radiolarians. Like the diatoms, radiolarians form intricate shells of silica, and they also comprise a significant portion of the ocean's plankton. But, unlike diatoms, radiolarians are heterotrophs, or consumers. Like heliozoans, they extend bits of cytoplasm through pores and pull food particles back within the silica wall. Another phylum of shelled protists is the form aminiferans. Their shells are usually made from calcium carbonate, rather than silica compounds. Many of them live on ocean sediments, but the free-swimming planktonic forms, shown here, are most prolific. These globular shells drop to the ocean floor and slowly accumulate to great thicknesses, sometimes forming limestone or chalk deposits.
The zoomastigotes are a diverse phylum of heterotrophic protists. Some are symbiotic within other organisms. Those shown here live in the intestines of termites. Their metabolism contributes to the ability of the termites to digest wood. Others may be parasitic, even causing diseases. The trypanosomes, shown herein human blood, cause African sleeping sickness. The parasite is transferred to humans in the bite of blood-sucking tsetse flies. This disease continues to be a major health hazard in parts of Africa.
Another protist phylum consists exclusively of animal parasites. The sporozoans, named for the infective spores they form at some stage of their complex life cycle, include the malarial parasite. Its life cycle is completed within two separate hosts, a mosquito and a person. The life cycle of the malarial parasite involves several stages. An infected mosquito infects a person with sporozoites. These enter liver cells and become merozoites, which infect red blood cells and produce gametocytes. These are sucked up by an attacking mosquito. They form pouches called oocysts on the outer wall of the mosquito's stomach. Here, new sporozoites are produced. After they migrate to the salivary glands, they are ready to infect the next person the mosquito bites.
There are many other sporozoan parasites with equally complex life cycles. The protists most like animals in their behaviour and complexity are the ciliates. Almost all are unicellular. The Paramecium illustrates the complexity of a typical ciliate cell. Its cilia are embedded in kinetosomes beneath the cell membrane. These kinetosomes are connected, and the cilia work in concert to move the Paramecium rapidly through the water. Like most ciliates, the Paramecium has an oral groove that functions as a cell mouth, as well as an anal pore through which undigested food particles pass.
The ciliates share so many structural features that it's clear they're related. This Stentor doesn't look much like a Paramecium, but it does have an oral groove and a band of cilia as well as the overall complexity of a typical ciliate. And Epistylis grows as a large colony attached to underwater surfaces, but each individual still has the cilia bands and cell mouth associated with the free-swimming ciliates. Some of the most difficult protists to classify have been the slime moulds, common but inconspicuous organisms that live on moist logs, rotting leaves, and similar habitats. They're sometimes classed in the fungus kingdom, but their strange behaviour seems less out of place with the protists than with the more clearly defined fungi.
There are two phyla of slime moulds, cellular and plasmodial, with different life cycles. A cellular slime mould occurs in several stages. Separate amoeboid cells creep along over debris, feeding on bacteria and bits of organic matter and reproducing by cell division. When the food source is depleted, the cells are attracted toward one another, gradually forming a slimy mass as shown here. This mass of cells pulls itself together tightly and begins to move along on a track of slime. For obvious reasons, the slime mould is called a slug at this stage. The slug stops moving and gradually begins to stand up on its end. Finally, a long stalk with a sorus develops. Within the sorus, some of the cells change into tough reproductive spores and are released.
If environmental conditions are favourable, a spore develops into an amoeba again. All the changes since the cells' aggregation have been through cell differentiation. The aggregated amoebas simply took on a variety of shapes and functions. A plasmodial slime mould often appears as a network of brightly coloured filaments on or under the bark of rotting stumps and logs. This network is a plasmodium. Unlike the cellular slime moulds, a plasmodium is syncytial. This means that many nuclei exist within the same cytoplasm; they're not separated by membranes into individual cells.
Spores are produced in sporangia at the tops of stalks rising from the plasmodium. Slime moulds are most conspicuous when they form their sporangia. Some resemble small fungi. Released spores produce either amoeboid cells or flagellated swarmer cells like these. The sexual fusion of two of these cells produces a zygote. Then, successive nuclear divisions produce a new plasmodium.
The plasmodiophorans are somewhat similar to slime moulds but are parasitic. One called Spongispora causes the serious powdery scab disease of potatoes. The oomycetes such as Saprolegnia, the common water mould, are sometimes included in the fungus kingdom. Like fungi, they feed by absorptive nutrition. Filaments penetrate a food source like this hemp seed, release chemicals that digest it, and then absorb the nutrients.
Unlike typical fungi, however, the oomycetes produce flagellated zoospores, shown here being released from the zoosporangium of a water mould. Many taxonomists now place the oomycetes with the protists based on their production of flagellated cells similar to those found among many algae. The oomycetes are named for their other method of reproduction. In addition to motile zoospores, they have sexually produced oospores. Like many other protists, their life cycles are complex and involved. We've seen protists in some ways similar to fungi, some more similar to plants, and some to animals.
Some of these similarities may indicate evolutionary relationships, or some may be examples of convergent evolution, biological coincidences. There are certainly also vast differences between these protists and the multicellular kingdoms, as well as within the protist kingdom itself.
Taxonomists are refining more precise ways to determine ancestral relationships, such as analysing and comparing proteins or the sequences of nucleotides indifferent organisms' DNA or RNA. When genetic similarities are directly compared between many groups, protist taxonomy may be rearranged. For now, some protistan relationships will have to remain something of a mystery.