9.Why do you think that red and brown algae out compete green algae in most marine
environments?
10.Explain why the way we classify certain organisms has changed so much in the last twenty
years? What do you see as the importance of classifying organisms?
V. Narration for “Kingdom Protista: The Origins of Eukaryotic Diversity”
Of all the kingdoms of life, kingdom Protista is perhaps one of the most diverse and
interesting. Kingdom Protista spans from single-celled organisms with various means of
locomotion and obtaining energy, to huge kelp, hundreds of feet in length that create expansive
underwater forests that are home to a wide variety of marine life.
But what do these diverse forms of life have in common? First, the cells of all Protists
have a nucleus and other complex organelles, unlike the members of the kingdoms Eubacteria
and Archaebacteria which have neither nuclei or complex organelles. Secondly, all the members
of kingdom Protista have one or more characteristics that prevent them from being classified in
kingdom Fungi, Animalia or Plantae.
For example, though there are protists that bear a striking resemblance to fungi molecular
analysis reveals that the cell walls of these protists are made up cellulose, not the chitin found in
all fungi. Another example is that, though a form of green algae now classified in kingdom
Protista is believed to have given rise to the members of kingdom Plantae the method of
reproduction nevertheless differs significantly between algae and plants. Algae release both male
and female gametes into the water where they unite and form a zygote unprotected by either
parent, while in plants fertilization occurs in a protective structure provided by one of the
parents. Finally, though there are animal-like protists which are believed to have given rise to all
the larger organisms found in kingdom Animalia, these single celled protists aren’t classified as
animals simply because by definition animals are multi-cellular.
Kingdom Protista-Three Modes of Nutrition
All three major modes of nutrition are represented in kingdom Protista: the algae of
kingdom protista trap solar energy through photosynthesis; predatory protists like amoeba ingest
their food; and parasitic forms, like some flagellates which, absorb nutrients from their
surroundings.
The Classification of Protists
As alluded to earlier, the ancestors of modern protists are believed to have given rise to
fungi, plants, and animals. To the confusion of taxonomists, many protists (such as Euglena) fit
equally well into animal-like or plant-like categories. Euglena for example, have photoreceptors
and can swim toward a stimulus, features commonly associated with animals, but they use these
abilities to seek light levels appropriate for photosynthesis, a process associated with plants.
Thus, protistan taxonomy is still the subject of revision and controversy. Here we group
members of the kingdom Protista into three categories: the fungus-like slime and water molds,
animal-like protozoa and the plant-like unicellular and multi-cellular algae.
Fungus-like Protists: Acellular and Cellular Slime Molds, Filamentous Molds
There are three major categories of molds: the acellular, or plasmodial, slime molds of
phylum Myxomycota; the cellular slime molds of the phylums Dictyostelida and Acrasida; and
the water and other filamentous molds of phylum Oomycota.
Fungus-like Protists: Acellular Slime Molds- No Cell Membranes
The acellular or plasmodial slime molds of phylum Myxomycota form a streaming,
multinucleate structure of indefinite shape and size called a plasmodium. Although the
plasmodium contains thousands of nuclei, the nuclei are not confined to discrete cells surrounded
by cell membranes, as in most multi-cellular organisms. This structure explains why plasmodial
slime molds are described as "acellular" or without cells.
Acellular slime molds crawl around in amoeboid fashion powered by the interaction of
proteins found in our own muscle cells. As a plasmodium oozes through decaying leaves and
rotting logs, it engulfs food such as bacteria, yeasts, fungi spores and bits of organic material
along the way. If food is plentiful and conditions are right a bright yellow or orange acellular
slime mold may spread over an area of several square meters.
Dry conditions or starvation stimulate the plasmodium to form a stalked fruiting body, on
top of which a structure called a sporangia forms. Inside the sporangia spores are produced.
Eventually, the sporangia dries and releases its spores to the environment. These spores
germinate into flagellated gamete cells that in favorable conditions swim and unite with another
gamete cell to form a zygote that will give rise to a new acellular slime mold.
Fungus-like Protists: Cellular Slime Molds-Cells Form Fruiting Body
The cellular slime molds of the phyla Dictyostelida and Acrasida live in soil as
independent haploid amoeboid cells, extending their pseudopods to engulf food such as bacteria.
In the best-studied genus, Dictyostelium, individual cells release a chemical signal when food
becomes scarce. This signal attracts nearby cells into a dense aggregation that forms a slug-like
mass called a pseudo plasmodium ("false plasmodium"), since it consists of individual cells. The
pseudo plasmodium then behaves like a multi-cellular organism. After crawling toward a source
of light, the cells in the aggregation take on specific roles, to form a fruiting body. Haploid
spores formed within the fruiting body are dispersed by the wind and germinate directly into new
amoeba like individuals.
Funguslike Protists: Oomycetes -Water Molds and Plant Parasites
While slime molds are motile and feed by endocytosis, the 700 or so species of
oomycetes and are made up of immobile, filamentous cells that feed by absorption. A multinucleated cytoplasm streams through the filamentous bodies of water molds and terrestrial
oomycetes. Water molds such as Saprolegnia, the white cottony filaments of which can often be
seen radiating off dead fish and insects in the water are all aquatic and all feed on dead organic
matter. Most terrestrial oomycetes are harmless or helpful decomposers, but a number are serious
plant parasites that have in the past had profound economic impacts on human society.
For example, one species of terrestrial oomycetes attacks grapes. Its inadvertent
introduction into France from the United States in the late 1870s nearly destroyed the French
wine industry. Another type of terrestrial oomycetes has destroyed millions of avocado trees in
California, while still another is responsible for "late blight," a devastating disease of potatoes.
When accidently introduced into Ireland, these oomycetes destroyed nearly the entire potato
crop, on which most of the Irish lived. The resulting famine of 1845 to 1847 reduced by half the
Irish population, from 8 to 4 million. Over I million starved outright, while the rest emigrated
(many to the United States) or fell victim to diseases spreading through the weakened populace.
In the past, oomycetes, like acellular and cellular slime molds, have been considered
fungi. However, oomycetes differ significantly from true fungi. For example, the cell walls of
oomycetes, contain cellulose, while those found in fungi contain only chitin. Further, oomycetes
reproduce using actively swimming, flagellated cells called zoospores. Swimming spores like
those produced by oomycetes aren’t found in the life cycle of any fungi. A final distinction is
that unlike fungi, oomycetes are diploid (instead of haploid) throughout most of their life cycle.
Animal-like Protists: Protozoa Differ in Locomotion
The animal-like protists or Protozoans are unicellular, most are non-photosynthetic
organisms that absorb or ingest their food. They are widely distributed in soil and water; some
are parasitic. They include the zooflagellates of class Mastigophora; the amoeboids of class
Sarcodina; the parasitic sporozoans of class Apicomplexa; and the predatory ciliates of class
Ciliophora. As we will see one of the features differentiating these groups is their method of
locomotion.
Animallike Protists: Zooflagellates- Flagellum Propel Zooflagellates
Zooflagellates all possess at least one flagellum. This versatile organelle, may propel the
organism, sense the environment, or ensnare food. The zooflagellates are a diverse group
believed to be ancestral to the other protists. In addition one group of flagellates, the
choanoflagellates, are believed to be the protists most closely related to animals. One of the
pieces of evidence that suggest this is that choanoflagellates bear a striking resemblance to the
cells most characteristic of sponges, the most primitive animals.
Many zooflagellates are free-living, inhabiting soil and water, while others are symbiotic,
living inside other organisms in a relationship that may be either mutually beneficial or parasitic.
One symbiotic form can digest cellulose and lives in the gut of termites, where it helps them
extract energy from wood. Members of the genus Trypanosoma are responsible for African
sleeping sickness, a potentially fatal disease, which is transmitted to mammals such as livestock
and humans by tsetse flies feeding on their blood. Trypanosoma then develops in the host. A new
cycle of infection begins as another tsetse fly bites and draws blood from the infected host.
As suggested earlier euglenoids are particularly difficult for biologists to classify.
Structurally and molecularly euglenoids are clearly related to the zooflagellates, but unlike other
zooflagellates many euglenoids are capable photosynthesis. The ability of euglenoids such as
Euglena to carry out photosynthesis led many biologists to originally classify euglenoids as
unicellular algae. However, many biologists now believe that euglenoids should be classified as
zooflagellates, since the evidence suggests that they are simply zooflagellates that have evolved a
permanent endosymbiotic relationship with chloroplasts ingested by their protozoan ancestors
millions of years ago.
No matter how they are classified euglenoids are interesting organisms. They consist of a
single, complex cell that moves by whipping its flagellum through the water. Its simple lightsensing organelles consists of a photoreceptor or eyespot, located at the base of the flagellum,
and an adjacent patch of pigment. The pigment shades the photoreceptor only when light
impinges from certain directions. This allows euglenoids to determine the direction of a light
source. Using information from the photoreceptor, the flagellum propels the protist toward light
levels appropriate for photosynthesis. Most euglenoids live in fresh water, and in contrast to
diatoms and most dinoflagellates, they lack a rigid outer covering. This allows some to move by
wriggling as well as by whipping their flagellum.
If photosynthetic euglenoids, are maintained in darkness, they lose their chloroplasts, but
can still absorb nutrients from their surroundings. In this state they closely resemble the animallike zooflagellates from which they are believed to have evolved.
Animal-like Protists: Sarcodines- "False feet" Move Amoeboids
Also called amoeba, the members of class Sacrodine possess flexible cell membranes that
they can extend in any direction to form pseudopodia (literally, "false feet") that are used for
locomotion and engulfing food. Amoeba lack many of the specialized organelles found in
flagellates and ciliates, but their reputation as "blobs of jelly" is contradicted by their complex
internal structure and their sophisticated ability to sense and capture prey. An important parasitic
form, particularly common in warm climates, causes amoebic dysentery. Multiplying in the
intestinal wall, this parasite causes severe diarrhea, and may perforate the intestine, occasionally
causing fatal infections.
Heliozoans, a striking form of freshwater sarcodine, may be found floating in ponds or
attached by stalks to an underwater plant or rock. They have stiff, needlelike pseudopodia, each
of which is supported internally by a bundle of microtubules. Some heliozoans cover themselves
with intricate and delicate shells of silica.
The foraminiferans and radiolarians are primarily marine sarcodines that also produce
beautiful and elaborate shells. The shells of foraminiferans are constructed mostly of calcium
carbonate (chalk), while the radiolarians secrete shells of silica (glass). These elaborate shells are
pierced by myriad openings through which pseudopods extend. The chalky shells of
foraminiferans, accumulating over millions of years, have resulted in immense deposits of
limestone at various locations throughout the world.
Animal-like Protists: Ciliates- Short Cilia Move Ciliates
Ciliates which inhabit both fresh and saltwater ecosystems represent the peak of
unicellular complexity. They possess many specialized organelles, including the cilia after which
they are named. Their cilia may cover the cell, or they may be localized, as in Didinium. In the
well-known freshwater genus Paramecium, rows of cilia cover the entire body surface. Their
coordinated beating propels the cell through the water at a protist speed record of a millimeter
per second. Although only a single cell, Paramecium responds to its environment as if it had a
well-developed nervous system. Confronted with some noxious chemical or with a physical
barrier, the cell immediately backs up by reversing the beating of its cilia and then proceeds in a
new direction. Ciliates are accomplished predators. Some, including Paramecium and Didinium,
immobilize their prey with explosive darts called trichocysts embedded in the outer covering of
the cell. Prey is escorted to a mouth-like opening, the oral groove. It is digested in a food
vacuole, which forms a temporary "stomach," and is excreted by exocytosis. Excess water is
accumulated in a contractile vacuole, which periodically contracts, emptying the fluid through a
pore to the outside.
Animal-like Protists: Sporozoans- Adult Sporozoans are Immobile
Sporozoans are specialized protozoan parasites that live inside the bodies and
occasionally the individual cells of their hosts. They are named after their ability to form
infectious spores, which are transmitted from one host to another through food, water, or the bite
of an infected insect. As adults, sporozoans have no means of locomotion. Many have complex
life cycles, a common feature of parasites.
A well-known example is the malarial parasite Plasmodium. Parts of its life cycle are
spent in the stomach, and later the salivary glands, of the Anopheles mosquito. When the
mosquito bites a person, it passes the Plasmodium to the unfortunate victim. The sporozoan
develops in the liver and then enters the blood, where it reproduces rapidly in human red blood
cells. The synchronized release of spores through rupture of the blood cells causes the recurrent
fever of malaria. Uninfected mosquitoes may acquire the parasite by feeding on the blood of a
malaria victim, spreading the parasite when it bites another person, thus continuing the infectious
cycle.
Unfortunately, although the drug chloroquine kills Plasmodium, drug-resistant
populations of are rapidly spreading throughout Africa, where the disease is prevalent. Programs
to eradicate mosquitoes have also failed, because the mosquitoes rapidly evolve resistance to
most pesticides. Researchers hope to use genetic engineering to breed strains of mosquito that
kill, rather than transmit, the sporozoan parasite or to create non-disease causing forms of the
sporozoan parasite that can be used as a vaccine.
Plantlike Protists: Algae- Capable of Photosynthesis
Algae are important photosynthetic organisms in marine and freshwater ecosystems.
Algae take on a variety of forms from small single celled species that swim via a flagella to huge
kelp of tree sized proportions. The single-celled algae include the dinoflagellates of the phylum
Dinophyta and the diatoms the phylum Chrysophyta.
Often called phytoplankton (literally, "floating plants"), these small photosynthetic
protists are widely distributed in oceans and lakes. Although microscopic in size, their
importance is immense. Phytoplankton account for nearly 50% of all the photosynthetic activity
on Earth and are the primary producers in most aquatic food webs.
Plantlike Protists: Dinoflagellates- Dinoflagellates Have Two Flagella
"Dino" is Greek for "two". Dinoflagellates are so named because of their two whiplike
flagella. One flagellum encircles the cell, located within a special groove, while the second
projects behind it. Some dinoflagellates are bounded only by a cell membrane, while many are
covered with cellulose walls that resemble armor plates. Only about 50% of dinoflagellates are
capable of photosynthesis, the rest absorb or capture nutrients from the environment or live as
parasites in marine organisms. Those dinoflagellates that do carry out photosynthesis, usually
have a combination of chloropasts and pigments that gives them a more golden-brown color than
the usual bright green color of euglenoids.
Although some live in fresh water, dinoflagellates are especially abundant in the ocean,
where they are an important food source for larger organisms. Many dinoflagellates are
bioluminescent, producing a brilliant blue-green light when disturbed. Specialized
dinoflagellates, known as zooxanthellae, live within the cells of certain coral species that are
dependent on them to provide photosynthetic nutrients and remove carbon dioxide. Those
species of coral dependent on dinoflagellates are limited to relatively shallow, well-lit waters
because of the dinoflagellates dependence on light energy to carry out photosynthesis.
The green chlorophyll in dinoflagellates is often masked by red pigments that help trap
light energy. Under certain conditions, when the water is warm and rich in nutrients, a
dinoflagellate population explosion occurs. These microorganisms can become so numerous that
the waters are dyed red by the color of their bodies, causing a "red tide". Fish die by the
thousands, suffocated by clogged gills or by oxygen depletion resulting from the decay of the
bodies of billions of dinoflagellates. But oysters, mussels, and clams have a feast, filtering
millions from the water for food. In the process, however, they concentrate a nerve poison
produced by the dinoflagellates. During red tides, people or other animals feeding on these
mollusks may be stricken with potentially lethal paralytic shellfish poisoning.
Plantlike Protists: Diatoms- Silica Coverings Protect Diatoms
The photosynthetic diatoms, found in both fresh and salt water, are so important to
marine food webs that they have been called the "pastures of the sea." They produce unique glass
like coverings out silica that consist of top and bottom halves that fit together like a Petri dish
and protect their body. Accumulations of the glassy walls of diatoms over thousands of years
have produced fossil deposits of "diatomaceous earth" that may be hundreds of meters thick.
"Diatomaceous earth" is slightly abrasive substance that is widely used in products such as
toothpaste and metal polish.
Diatoms store reserve food as oil, which gives them buoyancy in the water; this buoyancy
helps keep diatoms near the surface, where the light required for photosynthesis is abundant. A
unique combination of chlorophyll molecules and other pigments gives most diatoms a brownish
color, like that of the multi-cellular brown algae of phylum Phaeophytathe with which they share
a recent common ancestor.
Plant-like Protists- Multi-cellular Algae- Classified by Color
Multi-cellular forms of algae are classified into one of three phyla based on their
characteristic color. Phylum Phaeophytathe contains largely brown colored algae; phylum
Rhodophyta is made up of algae with a reddish hue; while the members of phylum Chlorophyta
tend to have a bright green appearance. The variation in color among the different phyla results
from differences in the pigments used in conjunction with chlorophyll, to capture light for
photosynthesis. The brown and red pigments, of brown and red algae respectively, absorb the
green, violet, and blue light that most readily penetrates deep water. Green alga is perhaps less
dependent on these pigments as most green algae, as we’ll see, live largely in shallow bodies of
water.
Phylum Phaeophyta: Brown Algae-Include Giant Kelp
Brown algae are believed to have diverged from diatoms about 200 million years ago.
Brown algae increase their light-gathering ability with brownish-yellow pigments that (in
combination with green chlorophyll) produce their brown to olive green color. The 1,500
species of brown algae are almost entirely marine. Various forms of brown algae are the
dominant "seaweed" found along rocky shores in the temperate (cooler) oceans of the world,
including the east and west coasts of the United States.
Brown algae are found from the intertidal zone, where they cling to rocks exposed at low
tide, to far offshore. Several types use gas-filled floats to support their bodies. The giant kelp
plants found along the Pacific coast occasionally reach lengths over 75 meters, and may grow
over 15 centimeters daily. Kelp are the largest of all protists. Their dense growth and towering
height, helps kelp form undersea forests that provide food, shelter, and breeding areas for a
variety of marine animals.
Kelp are commercially harvested largely for the gummy polymer, alginic acid they
contain. Alginic acid cements the cells and filaments of brown algae together and helps their
holdfasts stick to coastal rocks. Commercially it is used as an emulsifier in ice cream, cosmetics,
and other products.
Phylum Rhodophyta: Red Algae-Extremely Ancient and Diverse
Red algae come from an extremely ancient line of multi-cellular, photosynthetic
organisms. Ancestors of modern red algae are the first multi -cellular organisms in the fossil
record. The calcium carbonate cell walls produced by some red algae has assured their solid
presence in the fossil record. Because they have a history of over 600 million years there is much
more molecular variation among red algae there is even among plants. Another indication of the
ancient origins of red algae is that while they reproduce sexually their male gametes travel in an
amoeboid motion rather being flagellated, as they are in more recently evolved species whose
gametes travel in a liquid.
The 4,000 to 5,500 species of red algae, ranging from bright red to nearly black, derive
their color from red pigments that mask their green chlorophyll. Red algae are almost exclusively
marine. They dominate in deep, clear tropical waters, where their red pigments absorb the deeply
penetrating blue-green light and transfer this light energy to chlorophyll, where it is used in
photosynthesis. A red algal crust has been found at 270 meters where only 1/100,000th of the
light striking the ocean surface is available for photosynthesis.
Calcium carbonate or (limestone) stored in the tissues of some red algae, has contributed
to the formation of reefs through out the ocean. While some species of red algae are harvested
for food, others are harvested for the gelatinous substances carrageenan and agar, which are
extracted and used as emulsifiers in products such as paints, cosmetics, lotions, ointments, jellies
and ice cream.
Phylum Chlorophyta: Green Algae- Ancestors of Plants
The 7,000 species of green algae are diverse group. While most green algae are
multicellular, there are unicellular forms and a few species that cluster in colonies. These
colonies range from a few cells to a few thousand cells, as in members of the genus Volvox,
where rudimentary division of labor is seen. Some green algae, such as members of the genus
Spirogyra, form long chains of cells. In general individual green algae are quite small, though
they often intertwine into huge dense mats the can dominant ponds and the shallow edges of
lakes. Most larger green algae are found among 10% or so that live in the ocean, such as Ulva,
or "sea lettuce," which as leaves similar in size to that of its terrestrial namesake. Also like its
namesake “sea lettuce” is raised commercially as food in Japan and other countries
Green algae are of special interest because their ancestors are believed to have given rise
to terrestrial plants. A number of lines of evidence support the hypothesis that land plants
evolved from green algal ancestors: First, green algae use the same type of chlorophyll and
accessory pigments as do land plants. Second, green algae store food as starch, as do land plants,
and have cell walls of similar composition. Finally, unlike red and brown algae most green algae
live in fresh water.
In contrast to the nearly constant conditions of the ocean, freshwater habitats such as
ponds, swamps, streams, and lakes are highly variable. Dramatic fluctuations in temperature and
rainfall over evolutionary time probably exerted intense selective pressure on ancient freshwater
algae to withstand extremes of temperature and periods of dryness. The resulting adaptations
probably served their descendants well as they invaded the land.
Because green algae have the same type of chlorophyll and accessory pigments as do
land plants; store food as starch, as do land plants; and have cell walls of similar composition to
land plants some biologists continue place green algae in kingdom Plantae rather than in
kingdom Protista. But the point of classification isn’t the arbitrary placement of organisms in one
category or another- it is to illustrate the evolutionary relationships that exist between different
species of organisms.
All biologists no matter whether they place green algae in kingdom Protista or Plantae
agree that terrestrial plants arose from green algae. The disagreement lies in which classification
scheme best illustrates science’s current knowledge of the relationship between unicellular
protists, green algae, and terrestrial plants. As our knowledge of evolutionary relationships
increases through molecular studies and other means, classification schemes will continue to
change so as to reflect new that information and provide us with a greater insight into how the
living diversity that surrounds us came to be.