Algae: allelopathic control

...on the other hand, "Use plants to control algae,"recommends Diana Walstad. In her book Ecology of the Planted Aquarium she explains in detail just how the "higher" vascular plants are able to outcompete algae. The old-time fishkeepers did always notice that algae seldom flourished in tanks where the plants were thriving. Allelochemicals, most thoroughly studied among land plants, are part of plants' aggressive chemistry in the struggle for a place in the sun.
Natural chemistry. Algae aren't defenseless either. Algae fight back. Algae have their own ways of keeping the upper hand. First of all, physically, they block light from leaves by overgrowing and directly smothering them. Green water has a similar effect; it absorbs useful wavelengths of light before the light reaches the plants.
Another, more subtle weapon in the algal repertory is a metabolical advantage. The algal advantage is this: when carbon dioxide is in short supply, algae are able to scavenge carbon by dissociating carbonates, in a process called "biogenic de-calcification." They do this more efficiently even than a group of plants that have independently developed a similar capability, the so-called "hard-water plants," like Vallisneria. More indirectly, intensely photosynthesizing euglenoids and algal cells in green water scarf up the available CO2, which has the result of raising the pH. At the higher pH, more free carbon dioxide is naturally converted to carbonate, so that plants are forced to compete for the carbon derived from carbonates, where algae have the edge.
Allelopathic chemistry. "Growing algae produce numerous volatile and nonvolatile organic substances, including aliphatic alcohols, aldehydes, ketones, esters, thioesters, and sulfides," notes the World Health Organization in their guidelines for drinking water quality. Eons ago, the precursors of these allelochemicals probably originated innocently enough as chance byproducts of algal metabolisms.
The extent to which allelopathic chemistry suppresses algae or affects the growth of aquatic plants is controversial. Though allelopathic chemistry is well established in laboratory flasks, it proves hard to establish allelopathic interactions in the comparatively immense volumes of natural flowing waters and large lakes. In a 1998 essay "Shallow lakes, biomanipulation and eutrification", which I'll come back to in a few moments, Brian Moss wrote, "The extent to which these substances, faced with a barrage of heterotrophic bacteria, may accumulate and be effective in natural waters on a range of many species of algae which may easily substitute for one another, is unknown and will be extremely difficult to investigate. The potential, however, undoubtedly exists." An Israeli study "Allelopathy among submerged hydrophytes" by T.H. Shaq was presented as a poster at the IX International Symposium on Aquatic weeds (Dublin 1994); in that report, among five species of aquatic plants tested, Ceratophyllum demersum (Hornwort or "Coontail") and Chara connivens repressed growth and germination of Lemna (Duckweed) and lettuce seedlings. Diana Walstad credited allelopathic interactions with algae suppression in her book The Ecology of the Freshwater Aquarium, 1999. Dr Ole Pedersen's two-part article "Allelopathy: chemical warfare," first published in The Aquatic Gardener, 2002, (archived at The Wayback Machine) was skeptical of Diana Walstad's assertions, especially that when "tanks with heavy plant growth often seem to have very little algae" allelopathic interactions were the cause. Pedersen was skeptical that allelopathic chemistry counts for much in practice, though he does cite sulfur compounds produced by Nitella, Chara and Ceratophyllum spp that apparently keep the plant itself free of algae by inhibiting algal photosynthesis. Myriophyllum demersum slowly releases phenols that partly suppress enzymes in cyanobacteria; M. demersum is a temperate-zone water plant, but perhaps the tropical Myriophyllums used in aquaria have similar capabilities.  But most toxic phenolic compounds are not released until the plant dies, Pedersen noted, and thus are not technically allelopathy. A quibble, if you're looking at the over-all effect of certain plants in an ecotope.
The more relevant studies of allelopathy use intact plants rather than extracts of cellular compounds to assess allelopathy. Pedersen's main conclusion: "Personally, I do not see much ecological relevance in these kinds of experiments. At best, they may be used to look for potential candidates of true allelopathic behavior because the studies, after all, demonstrate that the plants contain toxic compounds. However, many of these studies take the conclusion much too far and recommend using the plants for aquatic weed management or algae control without the necessary documentation for allelopathic behavior in nature."
Would allelopathic effects be magnified in the limited water volume of the aquarium? Some experienced aquarists dismiss the whole subject of allelopathy out of hand. Nevertheless, our confined aquaria seem to me much more like enclosed flasks than they are like natural waters, and I can't see how the lab results could be entirely irrelevant. Allelopathic chemistry is just one among the "buffer" factors that stabilize a planted system and keep its water clear of algae.
Using Pithophora. So a strongly allelopathic alga might also help you suppress other algae, mightn't it? One algal ally that has had some success in this "fight-fire-with-fire" technique is a filamentous alga, Pithophora, or cotton-ball algae, that  forms big surface mats in still, enriched warm-water ponds in the summertime. P. oedogonia is particularly tolerant of steamy tropical temperatures, even above F86o (C30o) and low light. It lies on the bottom, until trapped oxygen makes it buoyant. Mine was a sharp lime green, but outdoors Pithophora can be a brownish olive drab and provokes uncomplimentary remarks. In The Ecology of the Planted Aquarium, Diana Walstad reports the test results of A. P. Fitzgerald, who compared guppy tanks with and without Pithophora under 24/7 lighting, and found that the tanks without Pithophora turned green in a week, while those with Pithophora remained clear through a month's trial, even when phosphate and nitrate levels remained high. My own experience is utterly unscientific, but I find that tanks with some Pithophora are especially free of other kinds of more troublesome algae. Though it's a scourge in sunlit ponds, indoors Pithophora is so easily controlled by winding it onto a chopstick or tongue depressor that I inadvertently lost my supply of it.
Algae do prefer hard water. In general, my soft New York water encourages my lazy fishkeeping habits. But in this soft acidic water I can only culture "green water" by keeping a substrate of crushed coral aragonite to boost the carbonate. On the other hand, sometimes heroic efforts are required to keep my  black brush algae at bay.
Barley straw: is this allelopathy? Since 1990 somewhat inconclusive tests in British ponds and canals, garden pools and fairly small natural bodies of water — even in gently flowing water — have used rotting barley straw to inhibit the development of cyanobacteria and algae. Some successes have led to  the currently fashionable use of barley straw in inhibiting algae in small ponds —now being extended to aquaria. Loosened bales of barley straw applied early in the growing season and retained in sunlit, oxygenated waters near the surface were found to cinhibit the development of some kinds of algae though not others. McGill University was quick to report investigations by the Aquatic Weeds Research Unit of the UK Agricultural and Food Research Council's Institute of Arable Crops Research. In 2003 there were a number of posts about barley straw at the Aquatic Plants Mailing List (search "barley straw").The literature as of 2005 iwareported by Stan Geiger, et al."Barley straw - algae control: literature analysis".  I figure this should be reported as an allelopathic effect, since some  results from field trials and lab assays have been interpreted as effects arising from allelochemical inhibitors, which are thought to derive from the breakdown of lignin in the straw, releasing polyphenols that leach into the water. Other kinds of plant litter, including rotting wood and some kinds of deciduous leaf litter like oak leavesmay indeed release algal inhibitors. Exactly what these chemical inhibitors are and what determines the potency of different types are still unclear.
This technique has been developed especially at the IACR Centre for Aquatic Plant Management, Sonning, near Reading, Berks: there is an archived version of their article "Control of algae with straw".
Several features are worth noting. The barley straw method works on suspended, planktonic algae rather than on established algal films attached to surfaces, it's been noted. Barley straw works even in gently flowing water. Barley straw has no effect on existing phytoplankton. Barley straw isn't immediately effective: there is a lag time of several weeks. It's also been noted that the effects are more noticeable in warm (>70°F) water and that plenty of dissolved oxygen supports the water-clearing effects.
On the other hand, field trials in New Zealand in 1994 were inconclusive, even finding enhanced growth of "water net" algae in the early stages of the trial. In the most successful lab trial (no. 2) "it was found that the decomposing straw had consumed all available nitrogen in the water." That seemed like a suggestive observation to me: perhaps barley straw is part of limiting nutrients, then.
However, the explications of how barley straw works often verge on alchemical fantasy, viz: "As barley straw decomposes in water, it releases lignins. These are oxidized to humic acids in the presence of oxygen. If sunlight then shines on the humic substances, hydrogen peroxide is formed. The peroxides in turn inhibit the growth of algae. They have no effect on existing algae. Because hydrogen peroxide is dangerous in high doses and is highly unstable (it does not last long), adding hydrogen peroxide directly to the water would not work and could harm the animals and plants. Straw releases a constant supply of low levels of hydrogen peroxide which is safe and effective. You would have to sit by the pond and add a drop of dilute hydrogen peroxide every hour or so to get the same effects."
An alternative explanation of the barley straw effect on algae. My hunch links all these otherwise inexplicable side effects of barley straw. It seems more reasonable to suppose that, rather than releasing mysterious allelopathies, oxidizing lignin or catalysing hydrogen peroxide, the loose strands of barley straw, which are decaying only slowly because of their lignin content, are providing a loosely-woven physical structure that becomes a safe haven for a community of filter-feeding zooplankton, gorging on decomposers and scrounging any algae. This aspect is a feature of biomanipulation, which encourages zooplankton in order to control phytoplankton.
Chief among phytoplankton grazers in shallow planted natural waters are daphnia. The daphnids multiply faster in warmer water and require oxygen, accounting for features generally noticed by investigators. Later in the season, increasingly dense growths of water plants take up the ecological role of supplying refugia from plankton-eating fish. As for the advocates of barley straw, it may be that researchers haven't noted a rise in Daphnia populations in these clarified ponds, simply because they haven't been looking.
Marketing barley straw. A barley straw extract was soon promoted  for algae control in aquaria. Since the specific chemistry involved hasn't been unraveled, if the effects of barley straw are indeed chemical, I don't see how it can be synthesized. I don't know why you don't just dose with peat extract and hydrogen peroxide, if you still feel that those are the effective elements in all this. Another aquarium marketer was producing packets of unidentified strawy material meant for insertion in the aquarium's filter to combat algae: is this not barley straw? Following a universal principle of magic, the ecological science has been transferred to a talisman: a packet containing a symbolic strawy sampling is tucked into the filter of an aquarium. Talismans often appear to be effective: the "placebo effect." Any subsequent reduction of planktonic algae-- marine phytoplankton and even nitrates have now been added to the list-- is attributed by the believer to the effects of the talisman.
If the science of it all seems rather soft and murky, the business aspect of it all is neither soft nor murky. A report in the Pittsburgh Post-Gazette's web version quotes Wayne Davis, president and CEO of Plantabbs Products, a Maryland-based firm. "His business has been experimenting with barley straw for several years. Davis, a water lily grower, says he approached this cure with much skepticism. But he says it works. In fact, his firm imports and sells Scotch barley straw, a high-quality straw left from the production of Scotch whiskey. He sells half-pound bales of straw, in mesh netting with one weight, for $24.95 plus shipping. So why go to the expense of buying special barley straw? According to Davis, his firm has experimented with all kinds and nothing seems to work as well as the straw from barley used in the whiskey industry." That's right! The straw left over from the barley that is used to make Scotch is so much more special than common barley straw, that it was being sold for $3.25 an ounce plus shipping. Oh, maybe you should just google"barley + straw"  and judge for yourself.