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Embracing Diversity:  Green Algae

By Mark E. Evans

In my neighborhood, there’s a store which has an aquarium as its front counter. Situated low to the ground and stocked with fish to match the movie “Finding Nemo”,  the aquarium is designed for small children, in accordance with the store’s goods. Even when the store is closed, the aquarium remains illuminated and visible thru the window. Anybody strolling the sidewalk can stop to view the fish. Well, during a leisurely walk one evening with a friend, I paused to watch the tank’s Yellow Tang (Zebrasoma flavescens) swim in and out of the aquarium’s rock wall. Unfamiliar with marine life, my friend asked me why saltwater aquariums aren’t stocked with green plants. I explained, to her surprise, that most aquatic plants are actually of freshwater origin, and that marine aquarists for the most part are limited to keeping algae if they want a display of greenery.

the generalized structure of green algae, with frond, stolon, and rhizoid

Thinking of this moment reminded me that no matter how much we appreciate our home aquaria, the natural world is a far more complicated place. As humans we tend to compartmentalize our thoughts, which prevents us from imagining complex systems at work. A real marine environment is a community of interactive species that is ultimately affected by the surrounding physical environment. For example, in the micro world, minute zooxanthellae  embedded in the tissues of coral polyps provide food for their hosts when exposed to sunlight. And in the macro world, entire coral reefs exchange energy, sediments, nutrients, and organisms with mangrove swamps. The point being that no living thing is an island, not even in the confines of a marine aquarium. But applying this perspective to the aquarium hobby is not always easy, especially when we have to balance the requirements of fish and invertebrates. However, there are other organisms that provide great benefits - like natural filtration - to our aquaria, and some of them are from the group known as the green algae. 

Contained in the phylum Chlorophyta, the green algae, are represented by about 7000 species with only about 10 percent capable of existing in a salty marine environment. However, they come in a wild array of sizes and forms, from the microscopic to the macroscopic, from the single-celled to the multi-cellular, from the fleshy-lobed to the calcareous, and from the filamentous and grassy to the frond-like. But make no mistake, green algae aren’t true plants, though they are thought to be ancestral to the plants. So naturally, green algae posses some plant-like characteristics such as pigments - chlorophyll A and B & carotenoids - as well as the ability to photosynthesize and to store energy as starch. But unlike true plants, the cells of green algae are not specialized into complex organs and tissues like roots, leaves, flowers, or vascular networks for the distribution of nutrients. The macro-sized green algae, the ones most beneficial and practical for the marine aquarist, and the ones with a superficial similarity to freshwater aquatic plants, are actually simplified structures in comparison.

Macro species of green algae from the genus Caulerpa, the one most popular among marine aquarists, are of a basic structure consisting of a frond, a stolon, and a rhizoid. The entire “plant” body is called the thallus, and among Caulerpa species it may appear differentiated like a true plant, but it is actually made up of a single cell with multiple nuclei. (Species of Caulerpa are in fact, some of the largest single-celled organisms on our planet, though I mention them here only as a generalized example. Macro green algae vary greatly morphologically). But the tubular structure, or stolon, grows horizontally over to the substrate and contains the liquid innards of the cell. Below the stolon are rhizoids or holdfasts that adhere to the substrate like roots. The primary location for photosynthesis, the frond, is the leaf like structure that extends up vertically from the stolon. Depending on the species, there is an enormous variety of frond shapes, from ones like ferns, to ones that look almost identical to green bubble coral, to ones shaped like little fingers. (It is extremely important to note that several species of Caulerpa are considered invasive species, and are harmful to native habitats, the most infamous being Caulerpa taxifolia, which has been banned in California along with others. So, check your state law to make sure you comply with any restrictions).

the species Halimeda discoidea, with its cactus-like growth of discs

Besides the fleshy green alga, there are hardened calcareous species that are attractive but not too demanding for the dedicated aquarist. Many have the appearance of small disc-shaped corals. For example, there is Halimeda discoidea, which looks like a pile of joined plates or a tiny cactus. Native to Hawaii and other parts of the Pacific, it is a slow grower and unlikely to become a nuisance, but it does reach a size of about 12 cm. With its rhizoids it can anchor itself to rock or “root” itself into deep sandbeds. Another interesting species, Udotea flabellum, is native to the Caribbean, and reaches a size of about 7 inches. Reinforced by a calcified “skeleton”, its wrinkled frond extends off the end of the stolon, and expands outwards like a fan. Both calcareous algae species are generally not recommended for the novice, as they do demand extra care. Specifically, they require the maintenance of alkalinity and calcium levels, as well as supplements of iodine. In addition, intense lighting is necessary to keep these calcareous species thriving. But it well worth the work, considering their beauty and capacity to rid aquarium water of excess nutrients. 

The bio-diversity and structure of green algae are educational subjects, but only up to a point for the marine aquarist. The functionality of keeping algae is a far greater concern for someone maintaining a large, complicated, and expensive marine system. The utilitarian view of green algae, as a tool for maintaining water quality is what’s important here. And green algae fits the bill nicely. For photosynthesis, as a process and chemical reaction, is the exact opposite of animal respiration. What is produced as waste products by invertebrates and fish is taken up by algae. The algae are after all the primary producers, and they are part of the food web and the circulation of nutrients in marine ecosystems. The growth processes of green algae therefore, powered by photosynthesis, require the input of major nutrients like nitrogen, phosphorous, sulfur, and of course, one of the most vital, carbon dioxide, as well as several micro-nutrients like iron, calcium, and magnesium, to name but a few.

Experienced aquarists are able to hold back many algae feeding nutrients from ever entering an aquarium in the first place with the use of RO filtered water, but only up to a point. Once an aquarium’s animal inhabitants are fed, metabolic processes are making more “algae food” available. Fortunately, several aspects of green algae make them the perfect natural aquarium filter. First, there is their capacity to utilize nitrogen waste in the form of ammonia. Primarily, green algae prefer ammonia as a nitrogen source over nitrate. (In fact, too much nitrogenous waste will kill delicate species). Ammonia as a dissolved gas is excreted directly via fish and invertebrate gills and is immediately assimilated by green algae. Stored nitrogen can later be utilized in tissue growth. (Blue green-algae on the other hand, which are more like bacteria, can fix atmospheric nitrogen, and are therefore not generally hampered by low nitrogen levels). 

the species Caulerpa racemosa - usually grape-like in shape, the fronds elongate under poor lighting conditions


Phosphate levels, the other parameter aquarists worry over, is not so cut and dry when algae filtration is involved. First, the best method in forestalling high phosphate levels is the use of RO water as a base for water changes and for top off water, (or not allowing excess phosphate to enter an aquarium in the first place). After that, in order to inhibit nuisance algae, phosphate levels must be kept to a minimum. However, we must be realistic in our quest for an aquarium free of nuisance algae. A low phosphate environment favors the minute blue-green algae over larger green algae. Also, the larger green algae has the capacity to store phosphate for later use. This is where pruning come in handy. Green algae grown in a sump or refugium ideally should be harvested and fed to the main tank’s inhabitants or discarded. (But please do not discard aquarium algae into your local waters. Just allow it to dry out completely and throw in the garbage. And do not flush it down the toilet either.)       

Finally, green algae helps in stabilizing pH, a bonus for marine aquaria considering that the toxicity of ammonia is directly proportional to pH, the higher the pH the more toxic the type of ammonia, the innocuous ammonium ion (NH4) versus the deadly ammonia molecule (NH3). But the benefits of algae are best explained along with nitrification, the process in which beneficial bacteria feed on ammonia and nitrite. During the chemical processes alkalinity is weakened and nitrate is produced. Add this together with the metabolites excreted by animals, and a lot of acids are produced, lowering pH and alkalinity. (Remember that alkalinity buffers pH in both directions and that an up swing in pH makes ammonia more toxic). Photosynthesis not only competes with bacteria, reducing nitrate production (a definite plus for reef aquariums), but it is also a base producing process, which increases alkalinity.  

I hope I have demonstrated that green algae are not foreign organisms separate from the fish and invertebrates we house in our marine aquariums. They are part of the complex and interactive ecosystems in which all marine animals are born. That’s why introducing them into our captive marine systems makes our aquarium waters more natural, healthy, and balanced, not less. So, whether your want to keep green algae in your sump, refugium, or directly in your prized show tank, the benefits are apparent and worthwhile. Anyway you view it, green algae is a great start to achieving better environmental stability.


Adey, Walter H. and Loveland, Karen; Dynamic Aquaria: Building Living Ecosystems; Academic Press Inc., 1991

Castro, Peter and Huber, Michael E. ; Marine Biology; McGraw-Hill College, 3rd Edition, 1999

Debelius, Helmut and Baensch, Hans; Marine Atlas: Volume One; Mergus Publishing (Microcosm Press, U.S.A.), 1997

Moe Jr., Martin A. ; The Marine Aquarium Reference; Green Turtle Publications, 1992

Raven, Peter H., Evert, RayEvert, and Eichhorn, Susan E. ; Biology of Plants; W. H. Freeman, 6th Edition, 1999

Spotte, Stephen; Captive Seawater Fishes: Science and Technology; John Wiley & Sons Inc., 1992 

Tudge, Colin; The Variety of Life: A Survey and a Celebration of All the Creatures That Have Ever Lived; Oxford University Press, 2000


Green Algae, the Chlorophyta on WWM

Related FAQs: Green Algae, Green Macro-Algae 2Green Macro Algae 3Green Macro-Algae 4, Chlorophyte Identification, Green Macro-Algae ID 2, Green Macro-Algae 2Green Macro-Algae 3, Green Macro-Algae 4, & Chlorophyte Behavior, Chlorophyte Compatibility/Control, Chlorophyte Selection, Chlorophyte Systems, Chlorophyte Nutrition, Chlorophyte Disease, Chlorophyte Reproduction/Propagation, Green Algae Control 1, Green Algae Control 2, Green Algae Control 3, Green Algae Control 4, Green algae Control 5, Green Algae Control 6, Green Algae Control 7, & CaulerpasCaulerpa 2, Caulerpa 3Caulerpa 4Caulerpa Identification, Caulerpa Behavior, Caulerpa Compatibility/Control, Caulerpa  Selection, Caulerpa Systems, Caulerpa Nutrition, Caulerpa Disease, Caulerpa Reproduction/Propagation, Green Algae Control 1Marine Algae ID 1, Marine Algae ID 2, Marine Algae Control FAQs II, Marine Algaecide Use, Nutrient Limitation, Marine Algae Eaters, Culturing Macro-Algae; Controlling: BGA/Cyano, Red/Encrusting AlgaeBrown/Diatom Algae

Related Articles: Caulerpas, Refugiums, Macro-Algae of the Caribbean and FAQs about themAvoiding Algae Problems in Marine System, Algae Control, Marine Maintenance, Nutrient Control and Export, Marine Scavengers, Snails, Hermit Crabs, Mithrax/Emerald Green Crabs, Sea Urchins, Blennies, Algae Filters, Ctenochaetus/Bristle Mouth Tangs, Zebrasoma/Sailfin Tangs, Skimmers, Skimmer Selection, Marine Algae, Coralline Algae, Green Algae, Brown Algae, Blue-Green "Algae"/(Cyanobacteria)Diatoms, Brown Algae


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