Cyanobacteria.

Cyanobacteria are always mentioned along with algae (they used to be called "blue-green algae") because they are photosynthesizing organisms, as algae are, and they create similar problems for us...

"Later for that!" you cry; "Just tell me how to eliminate the repellant crud!"

Know your enemy. Knowing what kind of creature it is will help you control it.

About cyanobacteria. Cyanobacteria are procaryotes, photosynthesizing bacteria. The gulf that separates the procaryotes from the eucaryotes is the most fundamental division of all living organisms. Cyanobacteria fall into the vast first group: the single-celled organisms that lack a central nucleus enclosed within a membrane, containing chromosomes of DNA. All the genuine algae, even the simplest single-celled ones, have a central nucleus that contains their genes; that feature puts them among the eucaryotes. Their cells incorporate discrete photosynthesizing organelles (the chloroplasts), which appear to have evolved from symbiotic photosynthesizing prokaryotic bacteria that were originally engulfed by a larger bacterium but failed to get digested. To put it another way and simply, cyanobacteria are not related to algae; they're related to the chloroplasts of algae.

Some cyanobacteria exist as unicellular forms, some of which are free-floating, others as cellular aggregates that form mucilaginous sheets and films, and still others as filaments.

Cyanobacteria preceded the first algal cell by hundreds of millions of years. They weren't the very first inventors of photosynthesis--— some anaerobic bacteria beat them to it by many more hundreds of millions of years-- but they were the first organisms to evolve a photosynthetic process--— a "metabolic pathway"--— that released oxygen as a byproduct. The cyanobacteria were hugely successful on the early earth, pearling away in every still saltwater lagoon, building up in slippery mats, exhaling oxygen into an atmosphere that still scarcely had any. For many millions of years, the freed oxygen was quickly taken up by the still-unoxidized iron at the land's surfaces and the soluble iron that stained the sea's waters brown. Finally, though, these buffers were exhausted, and free oxygen began to build up in the air. The "oxygen pollution" that resulted caused a catastrophic extinction of the earliest bacterial life, which had originally been entirely anaerobic.

I try to remember this gift of atmospheric oxygen, when I'm siphoning the slimy cyanobacterial sheets off my gravel, and I try to feel grateful. But that was then, and this is now.

Nitrogen fixing. At a certain point, probably before there was any free oxygen in the air, some filamentous cyanobacteria, such as Spirogyra, learned to scavenge nitrogen from the atmospheric dinitrogen gas (N₂) dissolved in water. This was another important metabolic trick. Nitrogen is absolutely necessary to living organisms. But, though nitrogen makes up four-fifths of the atmosphere, it is locked away. The molecule of atmospheric dinitrogen consists of two nitrogen atoms bound together so strongly that only a few kinds of bacteria have the ability to capture the stable gas, using enzymes that are collectively called "nitrogenase." Nitrogenase molecules are huge and complex, giants among enzymes, built of two twisted and balled-up proteins, that combine and recombine to convert a molecule of N₂ to two molecules of usable ammonia, NH3. Though nitrogenase enables conversion of atmospheric nitrogen so that it can be employed in life processes, it has a fatal weakness; it fails in the presence of oxygen. To protect the nitrogenase from oxygen, many nitrogen-capturing cyanobacteria have evolved special nitrogen-fixing cells encased in thickened cell walls. Certain filamentous cyanobacteria are able to fix nitrogen gas dissolved in the water within these protective calls, called "heterocysts." Consequently, we might manage to reduce nitrate to unmeasurable levels without daunting cyanobacteria.

Recent research suggests that filamentous cyanobacteria don't resort to this energy-expensive metabolic technique as long as there's some free ammonia available. Since ammonia is unlikely ever to entirely run out in your aquarium, chances are that very little nitrogen fixing is going on in there, it now seems. One way or the other, nitrogen isn't a candidate when we search for a limiting factor that will frustrate cyanobacterial growth.

Cyanobacteria are astonishingly resourceful in other ways. After all, they've had 3.5 billion years to develop survival schemes. In response to stress, a cyanobacterial cell can contract into a spore, a dry dormant sphere protected by a tough coat, which will blow into any available water. Or, out of water, cyanobacteria in symbiotic partnership with protective, drought-resistant fungus have formed a whole range of lichens. Cyanobacteria have lodged themselves in some pretty specialized environments: one of the cyanobacteria that can draw nitrogen directly from the atmosphere lives only in the intercellular structure of floating Azolla plants, and nowhere else; the symbiosis of Azolla and its cyanobacteria supplements the plant's nitrogen supplies. Azolla doesn't thrive in aquarium conditions, but I'd hesitate to have it in my garden pond if I were trying to keep water-borne nitrate levels low.

Besides the characteristic bluish ("cyan") photosynthesizing pigment, some cyanobacteria have a range of auxiliary photosynthesizing pigments, which make them very flexible about which wavelengths of light they can use. The pigments can color the blue-green cyanobacteria yellowish to dark reddish brown to blackish. Don't let these color disguises fool you when you're identifying that slimy film with the rank stagnant-pond odor.

Links. UCal at Berkeley maintains a good brief introduction to the lifestyles and ecology of cyanobacteria at their Museum of Paleontology website. Purdue University maintains a whole "Cyanosite" devoted to cyanobacteria. It's got a huge photo gallery of cyanobacterial candids, plus a video of a filamentous cyanobacterium withdrawing within its clear sheath to elude a grazing ciliate!

Prof. Cox at Miami offers a one-page illustrated description of cyanobacteria and their ecology, like an extended definition.

The ecology of freshwater cyanobacteria might be interesting to you; it's a concern of the Soil Water Conservation Society of Metro Halifax, N.S., who maintain a homepage.

At the height of the Wall Street "initial-public-offering" market bubble, TheOnion.com headlined "Species of blue-green algae announces IPO" as Anabæna went public!

Controlling cyanobacteria. In controlling true algae, the first thing people have told you to do is to use a timer to cut down on the photoperiod: not to reduce the intensity of the light available, merely to limit the number of "daylight" hours per day to fewer than ten. They were right: get a timer.

This action will be demystified if you know about the circadian rhythm of plants. Circadian ("sir-KAY-de-an") is derived from Latin circa "about" and dies "a day" and describes the biological daily clock common to cyanobacteria, true algae and other photosynthesizing single-celled organisms and plants. (Animals also have a circadian rhythm.) Circadian rhythm regulates many cellular processes. In plants circadian rhythm regulates not only photosynthesis but also root and stem growth and the timing of flowering.

The daily rhythm is set by parts of the genome that are similar in bacteria, protists and all multicellular organisms. They code for two proteins, designate them Period and Timeless, which accumulate slowly in cells. When they reach a certain concentration, they suppress the production of two other proteins--— call them Clock and Cycle. But Clock and Cycle are in fact precursors of the first two; without them, the proteins Period and Timeless cannot be made. Thus as Period and Timeless disappear from the cell, Clock and Cycle can be produced again. The resulting ebb-and-flow rhythm takes place in an approximately twenty-four-hour cycle. The biological clock is constantly re-set by day length.  So, after a certain number of hours bathed in light bright enough to trigger photosynthesis, the process shuts down. For a brief but thorough, illustrated explanation of circadian rhythm, from a botanist's perspective, go to the page "Circadian movements"at Botany onLine.

Fish and plants benefit from a consistent light/dark cycle. Perhaps strong plant growth is what actually represses algal growth, when the lights are set by a timer.

You're generally told to remove as much of the cyanobacteria as you can, for a start. Attempting physical removal often just spreads shreds of filmy cyano-gunk around the tank. Your "aquarium use only" toothbrush, held against the end of a siphon tube with your thumb, can help you eliminate the film as you siphon away the remnants that your brush dislodges.

Simply increasing water circulation in the tank often seems to have some effect in repressing cyanobacteria. Are cyanobacteria sensitive to their own metabolic wastes, including oxygen? Possibly. Most organisms are. Having said so much, I was gratified recently to read in Christel Kasselmann's Aquarium Plants (2003, p. 59):

"The oxygen content of several aquariums was artificially increased by using oxidators. The aquariums were then observed over several years. Not only was a visibly improved plant growth registered, but another positive side effect developed, namely the growth of blue-green algae being completely terminated in a few cases."

Hydrogen peroxide and potassium permanganate both work as oxidizing agents that degrade the unprotected cell walls of cyanobacteria. You have to be careful with these, as their action is unselective. Nitrifying bacteria may be damaged too. Using these oxidizers takes a cautious hand and plenty of common sense, so don't come whining to me if you get yourself in trouble with H₂O₂. A concentration that appears ineffective in water that is high in organics will damage fishes' gills in very clean water. In a fishless planted tank you might use as much as 2 oz. of a standard 3% solution of hydrogen peroxide per 10 gallons. If you are using potassium permanganate, the water should never be more that a light pink.

An interesting series of 1998-1999 posts from the Aquatic-Plants Digest concerned treatments with ordinary 3% hydrogen peroxide, delivered undiluted to the affected area with a turkey baster. That thread's archived at thekrib.com. August Eppler began it all by reporting that 2 oz of 3% H₂O₂ in a ten-gallon tank zapped cyanobacteria. Here was raised the possibility I've just mentioned, that cyanobacteria are susceptible because they don't produce the protective catalase that decomposes free radicals, which plants and animals possess.

Though many cyanobacteria can manufacture their own nitrogen, you may still be able to starve them by limiting the phosphate or potassium they also require, which is also a major technique in controlling algae. But if this technique slows plant growth, it may be counter-productive.

Ordinarily, as your ecosystem matures, green algae and the higher plants will outcompete cyanobacteria. Strongly assimilating plants in a good current flow will maintain dissolved oxygen levels near saturation-- or even above, for massed plantings can produce oxygen faster than it can be outgassed at the water's surface. Scattered spores and clusters of cyanobacteria will tend to survive only where they are protected by dense biofilm or layers of diatoms.

Erythromycin as a last resort. Sometimes cyanobacteria can get the upper hand, even when you have limited the photoperiod to ten hours, reduced the phosphate levels in the water, raised levels of dissolved oxygen, and increased water circulation. And still nothing has worked.

Knowing that cyanobacteria are bacteria rather than algae in the ordinary sense suggests a possible use at last for that ineffective erythryomycin at the back of your medication closet. Erythromycin acts by inhibiting protein synthesis in prokaryotes' ribosomes. (I'm told Kanamycin would work too.) Though it may be packaged for the aquarium trade, erythromycin is primarily effective against gram-positive bacteria; it is discredited by some professionals as largely ineffective against gram-negative bacteria, which cause almost all of the bacterial diseases in aquarium tropical fishes. So if you want to use it against cyanobacteria, but you hesitate to, for fear of encouraging bacterial resistance to erythromycin-- well, they're largely resistant already.

Still, people are getting to be too hasty in resorting to erythromycin before they've exhausted other possibilities. "The home aquarium is an ecosystem; it does not react well to toxins and antibiotics," Diana Walstad would remind you. At the recommended dosage of 200mg/10 gallons, erythromycin won't affect your plants (or any other eukaryote organism). Still, Erythromycin is not selective about which bacteria it kills. "Broad spectrum" is the polite way to describe it. You don't ever want to run erythromycin unnecessarily through your biological filter, even though I'm told that nitrifying bacteria are all gram-negative types. Before you use it, I recommend you remove any biowheel that you may be running and disconnect the sponge filter. Put the biowheel and the sponge into a separate container of aquarium water for safe-keeping for a couple of days. Yes, lack of ammonia will reduce the nitrifying bacteria, but the colonies will rapidly rebound. And if you have an undergravel filter, merely disconnect the airstones.

Be prepared for the massive cyanobacterial die-off. Dying cyanobacteria can release toxins that will stress your fish and all the other organisms in the tank and could be deadly. Cyanobacterial toxins affect the neural system and damage the liver. Liver damage will not be evident perhaps, until a victim suffers symptoms of "dropsy" long afterwards. Rinse out the filter media each day. Manually collect as much of the cyanobacterial sheets as you can in a brine-shrimp net. After 48 hours, do a 40% water change and repeat the erythromycin dosage. On the fourth day, siphon again carefully and do another 40% water change.

Be prepared too for a possible ammonia spike that you might see after nuking cyanobacteria with erythromycin. The pulse of ammonia in your water might be a result of nitrifying bacteria getting knocked back. It might also be the result of ammonia released into the system as fragments of dead cyanobacterial sheets are decomposed by other aerobic bacteria. Any pulse of ammonia is liable to bring on a small pulse of nitrite in its wake. Life can be hell.

This use of erythromycin to control cyanobacteria was worked out in the rec.aquaria newsgroups in 1992, and before you begin, you should read the recapitulation of the posts, archived at theKrib.com: http://www.thekrib.com/Plants/Algae/cyanobacteria.html