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Protists includes a huge variety of organisms, mostly one-celled, mostly microscopic. Protist means "first," as in first animals, first plants.

The classification of organisms is of enormous interest to biologists, who argue about it interminably. At one time, any organism that was not an animal, a plant, or a fungus, was lumped willy-nilly into the Protista. Later, as their cells were recognized to be fundamentally different, bacteria were excluded.

Currently Kingdom Protista is being subdivided into many genetic lineages, which are too arguable to introduce here. The traditional division is two large groups, the Algae and the Protozoans. But there are many kinds of protists that do not neatly fit those categories.


Algae (singular "alga") are autotrophs, they are photosynthetic, thus feed themselves. Algae are the primary producers in the ocean, the roots of most ocean food webs, just as green plants are on land. A by-product of photosynthesis is oxygen. Algae are responsible for 73 to 87 percent of the net global production of oxygen. In other words, the oceans are essential to the health of all other life, if it is to breathe.

Algae are marine eucaryotes that pioneered symbiosis with cyanobacteria, which eventually led to the development of vascular plants. One-celled algae are usually smaller than protozoans. In a gram of topspoil there are likely to be around 30,000 protozoa. In a gram of moist topsoil there can be 400,000 one-celled algae.

Over 5,000 species of algae float in the upper sunlit layers of the ocean. They are tiny, but when upwellings of nutrients occur near continental coasts, they can suddenly reproduce so fast they can be seen by satellites. Algal "blooms" are sometimes toxic (think red tide) but most are not. They do make life difficult for other plankton who could use the same nutrients, but are overwhelmed by sheer numbers. Here are some algal blooms as seen from satellites:

Algal bloom off Ireland coast
Algal bloom in Barents Sea

Large algae or macrolgae are familiar at every ocean shore. People call them seaweed. Kelp “forests” are biodiversity hotspots along coasts of most continents.

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Algae Lifestyle and Habitat

Aquatic Lifestyles

Phytoplankton float free in all aquatic habitats, salt and fresh. The total phytoplankton biomass outweighs that of all marine animals (zooplankton, fish, cetaceans,invertebrates) put together, and phytoplankton productivity is one of the primary forces in regulating our planetary climate, through infuences on  carbon dioxide levels in the atmosphere.

Many algae are motile.
One-celled algae often have flagellae and move in a spiralling manner.

chlamydomonas X 10,000
electron microscope
chlamydomonas light microscope

The most important alga in the ocean by sheer numbers is little Emiliana huxleyi , often just called Ehux. It is a coccolithophore, a shelled alga that is the major calcium fixing organism in the ocean.

Coccoliths,along with diatoms, remove carbon from the atmosphere and sink some of it into the seabed. Fossil coccoliths made the white cliffs of Dover, England. Coccolith shells have an astonishing beauty.

Emiliana huxleyi

Some aquatic algae do not float around. They live on the beds of shallow lakes and near shore seas. Most do not have flagellae but secrete a sticky substance which attach them to the substrate, enabling them to glide along leaving a trail of slime behind. These algae are part of the benthos, the life community of seabeds and lakebeds.


Not all algae are aquatic. Many algae such as diatoms thrive in moist soil or on vegetation. When soils dry out, soil algae survive by encysting.


Algae can reproduce asexually by fragmentation, by binary fission and by spore formation.  Filamentous algae, which often grow in large green visible masses, often break apart. Each part continues life as a new alga. One-celled algae reproduce by dividing (binary fission)  In the ocean, which is a stable environment, spores are a means of dispersal, rather than  a resting stage, or a survival strategy.

Some algae also reproduce sexually.

Many algae exchange genetic material through conjugation. This may be rejuvenating, but is not reproduction.

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Some Kinds of Algae


Green algae, or Chlorophytes, form a huge and diverse group of some 8,000 species. Like green plants, green algae contain two forms of chlorophyll, which they use to capture light energy to fuel the manufacture of sugars, but unlike plants they are primarily aquatic. Most green algae or Chlorophytes are microscopic and one-celled, but some are macroscopic "seaweeds." The chlorophytes are generally regarded as being ancestral to land plants.

three Chlorophyte micro-algae, some colonial, some filamentous

Codium, green, with holdfasts and floats
Ulva, sea lettuce, green
filamentous floating, Cladophera

three Chlorophyte macro-algae, often called seaweed


Another huge group of algae are the diatoms, which secrete glass shells they make from dissolved silica in water. There may be as many as 100,000 species. Some are phytoplankton, some cover the surface of the seabed.

three Diatoms, showing their silica glass shells.
Some diatoms secrete oil to say afloat so they can get lots of light

Red Algae

Another group of algae of some 7,000 species, mostly macroalgae, are the Rhodophytes, red algae. They reflect red light and absorb blue light, which penetrates water more deeply than other wavelengths, so red algae exploit niches at greater depths than green and brown macroalgae.

three Rhodophytes, red macroalgae

Brown Algae

Yet another kind of algae wear the impossible name of Heterokontophytes, also known as brown algae. Diatoms are lately classified as heterokontophytes. The most well known brown algae are the macroalgae kelps.


sargassum, with floats
giant kelp, which can be 100 meters long
an intertidal brown alga

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Ecological Roles of Algae

Marine algae are the number one producer organisms on Earth. This makes them the tasty beginning of almost all ocean food webs. Freshwater lake and wetland algae are the primary producers in those biomes. Of organisms who live in water, algae, combined with cyanobacteria, nourish all the others. They are the base of the food pyramid.

One marine group of algae are called coralline algae. They are red algae with unique abilities. Like corals, they can secrete calcium carbonate. In effect, each cell has a skeleton of calcium carbonate. One crucial role is to cement coral reefs together and protect them from breaking apart in storm waves, which in turn helps protect the land beyond. In the Hawaiian Islands many of the coral reefs are surfaced more with coralline algae than with actual corals.If you have visited a rockly coast, you have seen coralline algae. They are the pink patches on rocks in the splash zone, looking almost like a coat of paint.

These crusts both above and under water are often knobby, which creates living places for larval mollusks. The corallines give off chemicals which recruit abalone larva to live on them to be safe. Adult abolone are herbivores that graze on green algae which attach to the coralline crust and take their light. The algae and abalone live in an intergenerational symbiosis which is both chemical and behavioral. In a way, the algae are farming abalones.

Corallines also frequently slough off their surface layers of cells, a novel ability which gets rid of unwelcome settlers such as green algae and restores sunlight to the clean coralline.

Beyond the purely aquatic species, there are also many algae that live in topsoil. These algae are also the base of food production in their communities. When the soil dries up, these soil dwellers can become spores and live in suspended animation until they get wet again.

Many algae also have the astonishing ability to limit herbivore populationss. See Defense, next.

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Defense and Communication

Planktonic communities are fluid in membership, but will always contain unicellular diatom and chlorophyte algae, and will therefore contain herbivores that eat them. This arrangement exists wherever algae are found. All natural communities have limits on growth, some through grazing, or competition, some through predation, or through exhausting nutrients, but algae have demonstrated some unique defense abilities that fit communities with rapid reproduction rates, like the plankton.

Algae have chemical defense systems which are two-pronged.

•They give off chemicals which directly repel the creature eating them, and

• They give off chemicals which interfere with that creature’s reproduction.

This second ability has only recently discovered. If diatoms, for example, are being bitten by copepods, they release chemicals that will kill copepod eggs or larva (below), thus reducing grazing pressure on the local diatom populations. Researchers don’t know yet (in 2007) how this chemical release can be effective in such a dilute environment, but they know it is. The local population is protected by this defense tactic, but the injured alga is not.

For an example with larger players,  if sea urchins are grazing heavily on a population of Chlorophyte macroalgae, the algae will give off a chemical that kills sea urchin sperm. Such chemicals are also shown to kill bacteria, kill immature conchs and deter herbrivorous fish.

Brown and red macro-algae also defend against small herbivores such as amphipods, isopods, and juvenile sea urchins.

Another chemical tactic: The common macroalgae known as sea lettuce, Ulva, deters grazers with dopamine, which is an animal neurotransmitter related to adrenaline.  Dopamine reduces feeding by echinoderms, molluscs, and arthropods.

In sum, some algae have developed the incredible ability to attack both animal brain chemistry and reproductive success, as well as the old standby of just tasting bad when bitten.

Emitting chemicals is a form of communicaton, as with pheromones. Chemical signals released in water can’t be aimed at a recipient, and they can’t be turned off. The signals may be sloppy, but it’s clear that many algae have effective ways to say, “No!” to creatures that want to eat them (and to their progeny as well).

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Algae symbioses are important. Lichens are the classic example of symbiosis,  between algae and fungi, although often the photosynthetic partner is a cyanobacterium.
Lichens are the major source of fixed nitrogen in old growth forests. Lichens are complete entities, and reproduce as one. In cross section, lichens reveal their green partners protected between tough layers of fungal mycellia.

The other famous algal symbiosis is with warm-water corals, between a cnidarian animal ( a polyp) and an alga, a dinoflagellate called zooanthellae, that live inside the coral’s body and help feed it, although the two do not create a new entity. Both can live without the other, and they reproduce separately. Corals move through several life stages, and cannot be symbiotes until living as adults, in a reef.

Global warming has heated ocean waters. Shallow coral reefs are “bleaching,” that is, the coral animals are losing their algal symbiotes. For some years, this was regarded as the death of the reefs. Some have died, puttin an end to communities that have lived for thousands of years. But it is turning out that some corals are recruiting other algae that function well in the warmer waters, and are not dying. However, there are other results of warming going on that have killed much coral, and threatens marine biodiversity, because of the many larval forms of life that are adapted to spend their early lives feeding in the reefs.

One popular algal symbiont is Chlorella, a small one-celled green alga that does well inside animal bodies, provided the animal body is transparent.

Hydra, a pondwater relative of coral, often is green, because inside its tiny body are hundreds of Chlorella. So equipped, the hydra can live quite well without catching any food in its tentacles. But it does anyway--variety is spice.

Euglenas (below) names a textbook genus of photosynthetic protists that are sometimes called algae and other times called protozoans. They contain chloroplasts rather than a separate algal partner, as do green plants, which suggest that this symbiosis is very old.

three Euglenoid algae

Some freshwater sponges also live in association with chlorella alga, which colors them green. Sea slugs (below) often host algae. Giant marine clams hold parasols of green algae to the light. Being green means not going hungry.

There are also worms that live in symbiotic union with algae. One is the little  shoreline flatworm Convoluta  which appears entirely green, and no longer even has a mouth, its dependence on its algal partner is so complete.

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Mixotrophs: Betwixt and Between

Some organisms refuse to fit neatly into human categories. Several groups hover somewhere between algae and protozoa; botanists call them algae and zoologists call them protozoa. Ecologists just call them protists. Some of these are mixotrophs—they are photosynthetic at times, and carnivorous when they need to be. Dinoflagellates are among the most common mixotrophs, and create many planktonic blooms. One is the toxic "red tide."

Some protists are photosynthetic in only part of the life cycle; in the rest, they hunt their food. In some groups with many species, like the dinoflagellates, some are heterotrophs (other feeders) and some are photosynthetic autotrophs (self feeders). (Families and relationships are often difficult to understand.)

dinoflagellate, dividing
two photosynthetic dinoflagellates

dinoflagellates secrete oil droplets for buoyancy


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Explore Further in Biosphere

Biosphere: Introduction
Biosphere as Place: Introduction
Biosphere as Ocean: Life Zones
Biosphere as Ocean Floor: Benthic Biomes One
Biosphere as Ocean Floor: Benthic Biomes Two
Biosphere on Land: Terrestrial Biomes
Biosphere on Land: Anthropogenic Biomes
Biosphere as Process: Introduction
Biosphere Process: Floating Continents, Tectonic Plates
Biosphere Process: Photosynthesis
Biosphere Process: Life Helps Make Earth's Crust
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
Biosphere Process: Marriage of Wind and Water
Biosphere Process: Gas Exchange
Biosphere as An Expression of Spirit
The Ecological Function of Art
The Earth Goddess
The Tree of Life
The Green Man
Earth Art
Biosphere as Community
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
Biosphere Microcosm: Germs
Biosphere Community: The Eucaryote Domain
Biosphere Community: Protists 1: Algae
  Biosphere Community: Protists 2: Protozoa
Biosphere Community: Plants: What's New?
Biosphere Community:Plant Diversity--Major Groups
Biosphere Community: Plant Defense
Biosphere Community: Plant Pollination
Biosphere Community: Plant Seed Dispersal
Biosphere Community: Kingdom Animals
Biosphere Community: Kingdom Fungi
Biosphere Community: Six Great Extinctions
Return to Ecology Index