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Aquarium.Net A Discussion of Algae

Sept. 1996 , Dr. Shimek talks about several different algae found frequently in reef tanks. Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Slime, Goo, and Hair - A Discussion of Algae

by Ronald L. Shimek

In one way or another, one of the topics most on the minds of reef aquarists is the control of algal populations in their systems. At the same time algal organisms are amongst the least understood organisms that inhabit most reef tanks. In large part this is probably due to the sheer diversity of algae. The various types of algae that seem to most concern aquarists are red slime algae, brown slime algae, diatoms, hair algae, coralline algae, and the symbiotic zooxanthellae found in many tropical organisms. Brown slime algae and zooxanthellae are similar in many regards and are closely related. But all of the others belong to separate, very distinct groups with little in common except photosynthesis.

So... What is an alga? (The singular of the word is alga, the plural is algae). Commonly algae are considered to be all of the photosynthetic organisms growing in water or water films that are not vascular plants. Vascular plants are the plants we commonly see on land, flowering plants, conifers, ferns, and few others. Vascular plant diversity is very low in the sea, only a few species of grass, and possibly mangroves can be found growing in this environment. While these are often very important ecologically, relatively few aquarists bother trying to grow either sea grasses or mangroves. Virtually all other photosynthetic organisms in marine ecosystems have been or are called algae.

Photosynthesis is the process by which light energy is used by organisms to turn carbon dioxide and water into sugar. Light is captured by a molecule of chlorophyll and with the use of an enzyme system the energy in the light is used fuse carbon dioxide and water into glucose or another simple sugar. Different methods of light capture and different kinds of chlorophyll characterize the different kinds of algae. Nonetheless, they all have the same basic requirements: light, water, and dissolved carbon dioxide gas. Additionally they all use other organic and inorganic nutrients, such as phosphate, nitrate, or sulfate to produce proteins and other chemicals. This similarity of requirements is really at the root of the problem of algal control. The basic question is, "How to limit growth of pest algae, while maximizing the growth of beneficial algae?" One of the ways we can approach the problem of control is by an examination of the various types of algae in general way to see if there are unique controls for any of the algal groups.

Algal Diversity

The diversity of algae is truly amazing; algae can be found in two (and recent evidence indicates maybe more) kingdoms of life. Red slime algae are essentially photosynthetic bacteria or microbes and are placed in the kingdom Monera. Traditionally the remaining algae have been placed in the plant kingdom, but the more recent trend is to place them into the kingdom protista, a kingdom containing mostly unicellular organisms. Finally, recent comparative examination of the genetic material, DNA, indicates that a few groups, such as the red algae and including coralline algae, may be sufficiently distinct to be considered in their own kingdom.

CYANOBACTERIA - the red (and sometimes green, blue-green, purple, or black) slime algae. There are at least several dozen, and perhaps a few hundred, species. The name literally means blue green algae, but most marine forms are other colors.

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These are simple one-celled organisms and are generally very tiny, although Nostoc , commonly called Mare's eggs, found in fresh water streams can be as big as walnuts and may be the largest prokaryotic or bacterial-type cells in the world. Their distinctive colors are due to accessory pigments which assist the chlorophyll in light capture. The accessory pigments found in cyanobacteria are unique to that group. In nature, cyanobacteria are common, but typically not obviously dominant organisms. They are often characteristic of polluted or highly organic environments. All single-celled algae are capable of rapid growth, but the cyanobacters have the fastest growth rates. In optimal culture, their populations can double every 20 minutes or so. They typically secrete a mucoid layer around themselves, and when this, and the enclosed cells, builds up to a visible thickness, we call it "red slime algae", as the most common varieties in hobbyist tanks is a pinkish to red color.

In reef aquaria, a cyanobacter outbreak generally means an excess of nutrients, primarily phosphates, nitrates, and dissolved organic material, generally from over-feeding or some undiscovered mortality. Unfortunately, the cure is not simple. Antibiotics can be used to kill the bacteria, but these don't cure the underlying problem and some can have undesirable side effects, such as significantly effecting the benficial bacteria do the biological filtration. The most pragmatic solution seems to be to vacuum out as much of the slime as possible and do a water change to lower the concentration of the nutrient levels. This may have to be done several times in sequence over several weeks to lower levels sufficiently to limit the growth of the bacteria. Additionally, it may be necessary to cut back on feeding either in amount or in frequency.

DIATOMS - the brown or golden slime on aquarium walls or substrata. They generally put in the Kingdom Protista , and the Division Bacillariophyceae .

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Diatoms also single-cell organisms but are significantly larger and more complicated than cyanobacteria. They have cell walls containing silica. The individual cells are yellow-green to brown. They contain two types of chlorophyll and at some stage in their life cycles have motile stages which move by the action of one or two tiny beating hairs called flagella.

Most diatoms show very limited mobility and exist primarily as groups of cells growing a film over the surface of objects. In low concentrations they probably don't have any noticeable effect in aquarium systems, and indeed they are probably always present. Although they can respond to nutrient increases in a manner similar to the cyanobacteria, they seem to be more responsive to increases in the amount of dissolved silica in the water. Silicon, as silicate, is dissolved in sea water, and these organisms use it to construct their shells. Under normal conditions, diatoms can be generally well controlled in aquaria by the use of grazers such as snails, or chitons, and by using some sort of scraper to remove the film periodically from the walls.

If the aquarist has extra-high levels of silica and excess nutrients in the water, then diatoms often "bloom" producing dense rapidly growing films which can cover every exposed surface in the aquarium. Often the excess silica seems to be caused by the use of river sand instead of calcareous sand, and the cure is easy, but tedious, and that is the complete replacement of the siliceous sand by calcareous sand. The use of silica-free deionized, distilled, or reverse osmosis water can also help reduce problem outbreaks. The latter cure will definitely assist in reducing the diatom population. It may have the unintended result of also reducing or removing sponges and perhaps limpets from the aquaria as these animals also require silica, and in relatively high amounts, to live.

DINOFLAGELLATES - the brown slime algae and zooxanthellae. They generally put in the Kingdom Protista , and the Division Dinoflagellata or Pyrrophtya .

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This group is characterized by having a rigid external skeleton or test made of cellulose, and generally possessing two flagella which move them through the water. They possess accessory pigments in addition to chlorophyll and these pigments often give them a red or brown color. Some are also brilliantly bioluminescent. Common in both fresh and salt water, dinoflagellates are of immense economic importance, causing red tides and paralytic shellfish poisoning. They are also immensely important for reef aquarists, as many tropical invertebrates harbor these algae in their bodies.

The symbiosis of the invertebrate and algae provides both with benefit as the algal cells get protection from environmental problems such as predators, and the invertebrate gets nutrition from the alga. The algal symbiont also gets some essential nutrients such as nitrogen from the animal. Reef-building corals, clams in the genus Tridacna, many sea anemones, and some sponges all harbor symbiotic dinoflagellates called zooxanthellae.

Dinoflagellates can also cause problems for an aquarist when they occur free living in the aquarium. In a manner analogous to the other algae we have discussed they can "bloom" to cause a brown slimy film that overgrows most surfaces. It appears that this slime is also caused by too much nutrient in the water, and as with the red slime, the cure is to lower nutrient levels through the use of water changes and by siphoning out the slime.

Some dinoflagellates are not capable of photosynthesis and instead are either predatory or parasitic. These seem to show up rarely in aquaria, and are generally no cause for problems.

CHLOROPHYTA - the green algae; contains hair algae, Derbesia species, some decorative algae, such as Caulerpa, and numerous other forms.

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Members of the green algae can be either single-celled or consist of many cells. Except for some carotenoids, they generally lack accessory pigments, and generally their color is a clear rich green. Many of them incorporate calcium carbonate into their bodies. The multicellular chlorophytes generally have a reproductive stage where they release thousands spores into the water. These settle and produce new individuals elsewhere.

The unicellular green algae are very small, but important as food for many suspension-feeding organisms. Most reef aquaria harbor an invisible population of unicellular green algae that becomes noticeable only if their population reach relatively high numbers, at that point they turn the water green.

Some pest algae are members of this group. These are the hair algae, Derbesia and similar species such as Bryopsis , and whorl algae such as Batophora . These are common members of normal coral reef communities. In natural systems, they are generally found in small patches and are food for many common grazers. Many of these grazers are large fish or large snails that unsuitable for aquaria. Consequently in our systems these algae often have no biological controls such as herbivores, and they can become very abundant nuisances that once established are very difficult to control. Their control is basically the same as for the various slime algae; remove as much of the alga as possible and reduce the nutrient, and if possible, the light level. Occasionally some herbivores will eat them. Abalone, Haliotis species, are often excellent controls for them. Unfortunately the abalone can become too large for a normal aquarium. Most grazing snails, while good for diatom, and dinoflagellate control, will not eat the hair algae.

Valonia ventricosa , the bubble alga, is also a chlorophyte. This is a unicellular alga which grows fastened to the substrate. They can range up to a couple of centimeters across. When few in number they can be an attractive and interesting addition to a reef tank, but under certain circumstances they can also bloom and they can over grow corals and other invertebrates. They can be removed physically with forceps or some pliers.

Many species of decorative algae are found in the chlorophyta . Perhaps the most common are various species of Caulerpa. Other algae such as the calcareous Halimeda , Rhipocephalus , and Penicillus species are often grown as well. When a large individual of Caulerpa produces spores, it can often produce enough to turn a large aquarium milky-green with them. They either settle out, get filtered out, get eaten or die within a short time, leaving the water clear again. The husk of the parent remains behind as a mass of white flaccid tissue. It should be removed. All algae are leaky and some varieties of Caulerpa have been implicated as producing chemicals that have caused some animal deaths. In small amounts these algae are generally benign, however.

RHODOPHYTA - the red algae. These are multicellular algae with very complicated life cycles often containing alternating stages with morphologies that are very different from one another. They are only distantly related to the other algae and appear to have little in common with them other than the presence of chlorophyll. They have a reddish accessory pigment that allows them to live deeper or under dimmer light conditions than other algae.

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The most common type of red algae found in aquaria is crustose coralline algae. These appear as red, pink or lavender crusts often with a white growing edge. Articulated forms are common in nature, but relatively rare in marine aquaria. These are attractive organisms and are generally desired in a reef system. Their presence generally indicates satisfactory conditions for other organisms as well. Because of the presence of alternate life stages and sexual and asexual forms coralline algae are often damnably hard to identify to even genus.

Filamentous red algae can also be found in aquaria. They are often very abundant in natural systems as well. These range from low-growing red fuzzes to larger pink or red algal clumps. These are benign and attractive organisms and their presence generally also implies a good population of small crustaceans which often live on or around them.

PHAEOPHYTA - the brown algae; contains rockweeds and kelps, also some calcareous forms. These algae contain a brownish accessory pigment, fucoxanthin, which gives them their characteristic brown color. Some species of phaeophytes are the largest algae, and can be huge. Bull kelp, Nereocystis luetkeana from the U. S. West Coast can increase in length at rates exceeding one meter per day, and the giant kelp off of California can be hundreds of feet long.

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With the exception of Sargassum species, brown algae are relatively uncommon in marine aquaria. They are not particularly hard to culture, but tend to prefer dimmer light situations then reef aquarists typically provide. Often coralline algae can outcompete them for space in an aquarium. Most brown algae are temperate organisms, and their tropical diversity is limited. Occasionally species of the calcareous Padina are found in aquaria where they can be very attractive.


There are numerous other algal groups, and some such as the coccolithophores are exceedingly important in natural coral reefs; and indeed, in all the world's oceans. However, most of these forms are either microscopic or rarely seen in aquaria and I will not discuss them.

Suggested Reading:

Sze, P. 1993. A biology of the algae. 2nd. ed. W. C. Brown. Dubuque Iowa. 272 pp.

Delbeek, J. C. and J. Sprung. 1994. The reef aquarium. Ricordea Publishing. Coconut Grove, FL. 544 pp.

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