Algae, Our Faithful Friend

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An algae seascape at Cape Ann, Massachusetts, with southern kelp dominating the foreground. (Ralph Fuller photo)


ALGAE COMES IN EIGHT GAZILLION DIFFERENT FORMS, from tiny little slimy green stuff to giant kelp, and to most of us it seems obnoxious and a thing to be ignored, if not despised.

Except that algae is the foundation of the food chain, a pioneer in the evolution of life, and essential to our existence.  

A mob of blue tang rush along a reef in Curacao, razing on micro-algae.

A mob of blue tang rush along a reef in Curacao, razing on micro-algae. (Ralph Fuller Photo)


Algae uses sunlight to photosynthesize carbon and hydrogen into the simple sugars that are nutrients for many animals. The beautiful parrotfishes that adorn the reefs, the little damselfishes that challenge us when we get too close to their territories, the black spiny sea urchins we hope to avoid, all directly survive on algae. Even the kelps that thrive in colder waters provide food and shelter for herbaceous fishes and invertebrates.

Beyond this — well beyond it — that initial algal growth begins the process in which nutrition is transferred from smaller organisms to larger ones on up the food chain until it reaches apex predators, quite possibly ending up on our dinner table in the form of a tuna steak. Granted, a more direct example would be the grasses that a cow consumes before becoming sirloin tips (it cuts out a lot of middlemen, or, more properly, middle organisms).

As a byproduct, it produces much of the planet’s oxygen.


Algae regularly get referred to as “plants” in casual conversation, but in fact they are not plants but protists, organisms separate from animals and plants, with some characteristics of each. As such, they are located in a separate kingdom from the animal, plant and fungi kingdoms and from the bacteria and archaea domains. Many protists are single-celled organisms but some, like seaweeds, are multicellular.

Brown Algae Bon 0722

Brown algae, this bunch at Bonaire. (Ralph Fuller Photo)


On land, it’s grasses, trees and other plants that are the engines of photosynthesis. Marine grasses are photosynthetic, of course, but the real work underwater is done by algae and bacteria.

In the aggregate, this transformation of carbon and hydrogen is referred to as primary production – a measurement of the food produced. It’s estimated that at least 50 percent of all the earth’s primary production still takes place in the oceans – some scientists put the oceans’ share to be as high as 90 percent.

Without algae in the oceans, the tree-and-grass world we know wouldn’t have happened, not even sirloin tips. Algae led the way in the move from the oceans to dry land. Some 420 million years ago, when there was life in the seas and none whatever on the earth’s landmasses, green algae were the pioneers that made the jump to land-plant organisms, developing impermeable walls for internal water reservoirs that let them live outside the oceans.



(Ralph Fuller Photo)

•   Kick-starting the Food Chain

A broad range of fishes graze on algae, from the parrotfish that are constantly scraping dead corals in search of algal sustenance to the rabbitfish that hang out on reefs throughout the Pacific basin. Surgeonfishes earn their livings with algae. Those mobs of blue tangs that rush from one reefish place to another, like hungry little Labrador retriever puppies, are mostly grazing on algae.

Creatures as diverse as snails, chitons and limpets, shrimps, crabs and other marine animals  dine earn their livings consuming algae.

Black spiny sea urchins sweep the reef at night, vacuuming up algae. Since 1983, reef health in the Caribbean has been set back immensely due to a massive disease-caused die-off of urchins. The increased algal populations that have resulted from loss of urchin algal-grazing, especially in competition for coral larval settlement sites, are a factor in declining coral populations.

Curacao Mushroom Forest 17

Reef-building corals get a huge boost from zooxanthellae. Here: Mushroom Forest at Curacao. (Ralph Fuller Photo)

•   Reef-Building

One of the most important ways algae benefits the oceans is its role in reef-building. Reef-building coral polyps (technically called hermatypic corals) live in a symbiotic relationship with a type of algal organisms called dinoflagellates (whip-like appendages that can help them sort of swim).

More specifically referred to as zooxanthellae, these algae are embedded in the tissues of the polyps, living a protected existence within the corals calcium-carbonate corallites. There, they proceed with a relentless program of photosynthesis, producing the simple sugar glucose – and sharing it with their coral polyp hosts. Hermatypic polyps do extend their tentacles out into the water current to filter passing plankton, as coral polyps are supposed to do, but they get 80 percent of their nutrition from their embedded zooxanthellae. It’s this boost that allows them to reproduce and create fantastic reefs for us to dive on.

By and large, it’s the myriad variations in zooxanthellae that give corals their striking colorations. When corals are subjected to sustained stress, from too-warm or too-cool water temperatures, for example, their zooxanthellae are expelled and the previously colored coral is rendered stark white. This is coral bleaching. If conditions are corrected, zooxanthellae may move back in. But if not, with its main source of nutrition gone, the corals are likely to die after an extended period.

Encrusting Coralline Red Algae

Encrusting Coralline Red Algae, here photographed on the Great Barrier Reef, is the “cement” that holds reef parts together. (Ralph Fuller Photo)


•   Reef-Cementing

A coral reef is a dynamic equation, continually being built up as its members grow and torn down as they compete for space and nourishment.  When corals die, what remains are their calcium carbonate skeletons, which tend to yield to the forces of the oceans – such as storm surge. In short, they become rubble. Some algae develop their own calcium skeletons and play an essential supporting role in reef building. Algae like encrusting red coralline algae constitute the underlying “mortar” of the reef, filling in cracks and crevices and cementing the pieces together into unified solid structures.

Some scientists, presumably algae specialists, hold that tropical reefs should rightly be called Algal Reefs – or at least Biotic Reefs – because of the role algae play in their construction and maintenance. Often, algae seem to make the reef chaotic, messy and difficult to discern. But without them there would be no coral reefs. However, any name change seems unlikely to happen.

•   Sand Production


Halemida & Red Coralline Algae

The green discs of halimeda algae conceal a calcium carbonate base. (Ralph Fuller Photo)

Halimeda Sand & Sea Cucumber

A coarse sand, “Halimeda Sand” (with a Sea Cucumber on the Great Barrier Reef) is made when Halimeda algae discs are broken up. (Ralph Fuller Photo)

In the course of their relentless pursuit of the algae growing on dead coral skeletons, parrotfishes inevitably scrape in bits of the calcium carbonate the algae rests on. The fishes’ pharyngeal teeth grind it into fine grains, exuding it as waste. It’s true: parrotfishes poop out sand – a very fine version of it. Some of the best beaches in the Caribbean, such as Grand Cayman’s Seven Mile Beach, are composed with parrotfish assistance.

Sand is also produced directly by some algae. Halimeda is a form of algae that grows disc-shaped structures formed of calcium carbonate. Reputedly, this deters animal grazing. When they die, the discs fall to the bottom to eventually  be broken up into coarse grains.

A northern lobster emerges from the lettuce leaf algae off Cape Ann.

A northern lobster emerges from the lettuce leaf algae off Cape Ann. (Ralph Fuller Photo)

•   Shelter

It’s probably more true of kelp and seaweed in cooler waters than of microalgae in the tropics, but even for animals that don’t eat algae it plays an important role in providing shelter for fishes, crabs and other marine creatures. A bottom covered with kelp constitutes a formidable hiding space.


If you like sushi you can thank algae for the nori used in its preparation. Around the world forms of algae are used as food in dishes as diverse as spinach-like dulse and in soups and salads. Algae are used as a source of nutritional supplements and stabilizers in puddings and dairy products like ice cream. Algae are also used for fertilizer.




Giant kept - a display at the Monterey Bay Aquarium. Via Wikimedia Commons

Giant kept – a display at the Monterey Bay Aquarium. Photo via Wikimedia Commons.

Southern Kelp, off Cape Ann

Southern Kelp, off Cape Ann. (This and next four photos: Ralph Fuller)

Possibly Hanging Vine Algae, Bonaire

Possibly Hanging Vine Algae, Bonaire; (Spaghetti worms also on display)

Sea Pearl - a single cell of algae

Sea Pearl – a single cell of algae; Curacao

Red algae; Folly Cove, Cape Ann

Red algae; Folly Cove, Cape Ann

Rockweed, with floats; Lane's Cove, Cape Ann

Rockweed, with floats; Lane’s Cove, Cape Ann









•   Overgrowth & Eutrophication

As noted, overgrowth of algae has been a serious problem in the Caribbean since the early 1980s mass die-off of sea urchins, which previously helped to keep algal populations in check. Among other issues, algal overgrowth diminishes the suitable settling spots for coral polyp larvae.

A similar problem is eutrophication, a phenomenon increasingly seen along coast-lines due to fertilizers and other chemicals that foster algal growth, leading to overgrowth of oxygen-depleting bacteria in bodies of water. Fishes and other animals literally smother in water that has no oxygen.

At the mouth of the Mississippi River, an area the size of Rhode Island is a dead zone for animal life because of nitrogen runoff along the river basin’s entire length. World-wide, the numbers and sizes of dead zones are growing.

Red Tide bloom off La Jolla, CA. via Wikimedia Commons

Red Tide bloom off La Jolla, CA. (Photo via Wikimedia Commons).

•   Red Tide

Red tide is the common name for what scientists call Harmful Algal Blooms (HABs), a phenomenon in which colonies of algae bloom extensively while producing toxins that can harm fish, shellfish, marine mammals, birds – and humans. Human illnesses from red tides are rare but can be serious of even deadly. HABs have occurred along the coast of every coastal state in the U.S. Even if toxins are not present, red tide can create eutrophic conditions, making the waters unlivable for marine life.

•   Ciguatera Poisoning

Ciguatera fish poisoning is the consequence to humans who eat fish contaminated with toxin called ciguatoxin. It’s produced by a tiny dinoflagellate that clings to corals or algae and are consumed by small fish. Just as the nutrition produced by algae moves up the food chain, so does the accumulation of ciguatoxin. It’s rare for the illness to be fatal but effects can linger for extended periods. Symptoms can include vomiting, diarrhea, cramps and nausea.

Ciguatoxin is said to accumulate most significantly in large fishes like grouper, amberjack and hogfish, with barracuda said to be perhaps the worst offender. Florida is currently the northern limit of ciguatera poisoning at present but with climate change the range is expected to expand.


Photosynthesis is driven by photosynthetic pigments – that is, molecular compounds whose electrons are energized by sunlight. Each reacts to specific wavelengths within the color spectrum, useful traits for efficient productivity in varying environments. The most prevalent by far are green chlorophylls, which is why there is so much green in our world, both above and below water.  More properly, green chlorophyll pigments absorb all the colors of the light spectrum except green, reflecting those light waves and making grasses, tree foliage and algae appear green to us.

Scientists have identified several different types of chlorophyll, but they named them Chlorophyll “a” and Chlorophyll “b,” etc. – not exactly useful as memory aids. Chlorophyll “a,” found in all algae and plants, is the world’s principal driver of photosynthesis. Chlorophyll “b” occurs in all plants – but among algae only in green algae, underscoring the link between it and land plants. Differences among them all include such issues as storage mechanisms – the Chlorophyll “a” in plants is better at storing energy in the form of starch than the “c” found in diatoms, dinoflagellates and other protists.

Red Algae at Cathedral Rock, Cape Ann  RF Photo

Red Algae at Cathedral Rock, Cape Ann. (Ralph Fuller Photo).


While green chlorophyll  molecules are found in all photosynthetic algae, they’re often masked by brown and red pigments, yielding colors like purples, oranges, mauves and yellows. These carotene and phycobolin proteins don’t perform photosynthesis but they do absorb sunlight and pass the energy they take in to the chlorophyll that does.

And they absorb light energy more efficiently than green pigments, resulting in fast-growing brown types like giant kelps or brown variations that survive in deeper waters. Red-pigments are even better at light capture – they appear red to us because they reflect red light waves while absorbing blue ones, the deepest-penetrating color of the spectrum.  Red-pigmented algae can be found at depths of more than 600 feet.


Converting carbon dioxide (CO2) into an organic molecule is a multi-step process called carbon-fixation. Stimulated by sunlight, chlorophyll causes the electrons in water molecules (H2O) to become excited, sparking a chain reaction that in nanoseconds unites their hydrogen atoms with carbon atoms in new molecules of glucose (C6H12O6,  if you’re really, really technical). Other inorganic nutrients also play roles.

Glucose isn’t the end product. Most of it is converted to other types of organic matter or used to fuel the transformation. A lot is converted into adenosine triphosphate (ATP), an energy-carrying compound used by all living organisms and into carbohydrates, proteins, lipids and nucleic acids used for growth and reproduction.

PRINCIPAL SOURCES:  Marine Biology, Fourth Edition, Peter Castro, Michael Huber; Nybakken, James W. Marine Biology, An Ecological Approach; National Oceanic and Atmospheric Administration; Monteray Bay Aquarium Research Institute; Maricopa Community Colleges “Photosynthesis” Web Site; Royal Botanic Garden Edinburgh,; Knott, Emily, “Asteroidea,” Tree of Life Web Project,; Arizona State University, “An Introduction to Photosynthesis and Its Applications;” Photosynthetic Pigments, University of California Museum of Palentology, Berkeley.