Why barley straw effects the algal population

The overproduction of algae in ponds may not only be unsightly, but may seriously effect aquaculture and have serious implications for water management authorities. It is therefore important to understand the natural control of algae. Joseph Boylan from the Iowa State University reviewed the possible reasons why barley straw was effective in the control of algal.

(Science is a self-correcting process not a belief in magic. This is such a beautiful example of how it works at finding answers.) sdk

 “Several hypotheses have been formulated; most have been disproved:

  1. The straw could be reducing the availability of some trace element or micronutrient essential for algal growth.
    • Welch et al. (1990) did not find any differences in concentrations of filterable reactive phosphorous, ammonium, or nitrate between a treated and non-treated section of a canal, yet, algal growth inhibition occurred in the treated section.
    • Gibson et al. (1990) added a substantial amount of N and P to an algal culture treated with barley liquor; algal growth inhibition still occurred.

  2. The increased invertebrate populations associated with the straw (reported in Street 1997, 1978, and 1979) could be grazing on algae, consequently acting as a form of biological control.
    • This may be true to some extent; however, this cannot explain the significant reduction in algal growth documented by several controlled laboratory studies, e.g., Pillinger et al. (1994).

  3. The effect could be due to pesticide residues on the straw.
    • Since barley straw takes weeks to months to become active, growth inhibition is not likely to be caused by pesticides, which would have an immediate impact, and would lose efficacy through time.
    • After analyzing water samples extracted from rotted barley straw that had been applied to a reservoir, Everall and Lees (1996) detected no suburea or triazine herbicides; significant algal growth inhibition occurred in this study.

  4. Inhibitors produced by fungi attached to barley straw could cause the effect.
    • Pillinger et al. (1992) found three of several species of fungi to inhibit algal growth; however, they concluded that the general and widespread anti-algal effects of barley straw are unlikely be explained by the anti-algal properties of specific fungi.
    • Ridge and Pillinger (1996) found autoclaved straw to still exhibit significant antialgal activity. However, Gibson et al. (1990) found autoclaved straw to not become active, whereas non-autoclaved straw did.

  5. Since barley straw did not become active in several studies until it had rotted, microbial decomposition could be essential; furthermore, the microbes that decompose straw could generate a naturally occurring algaecidal chemical.
    • Ball et al. (2001) found that an extract prepared from finely chopped fresh barley straw did not inhibit the growth of Microcystis, whereas an extract prepared from finely chopped decomposed barley straw did.
    • Pillinger et al. (1994) and Ridge and Pillinger (1996) concluded that since finely chopped, fresh straw also exhibited anti-algal effect, microbial decomposition is not essential; hence, it is the straw that produces the effect, not the microbes.  
    • In the field, the microbes are most likely needed to break down the straw, as the latter researchers did mechanically.

  6. Once broken down, the straw releases an anti-algal chemical, or a chemical that, through a pathway, becomes anti-algal.
    • Pillinger et al. (1994) suggested that phenolics present (tannins) in the straw could be responsible for the anti-algal effect of barley straw; barley straw does contain tannins (Brandon et al. 1982).
      • They found an authentic source of tannin, tannic acid, as well as a natural source, oak leaves, to be anti-algal.
      • When hide powder, a protein source expected to complex with tannin, was added to the tannic acid and to the oak leaf extract, the inhibitory effect of both was diminished. However, this only occurred at acidic pH values; the proteïen precipitation did not occur with tannic acid or the natural tannin source (oak leaves) under alkaline conditions (pH~8).
      • Quinones (oxidized phenolic compounds) were also tested. The study showed strong evidence that oxidized phenols are much more efficacious.
      • Increasing aeration during the decomposition of barley straw and during bioassays of rotted straw liquor and fresh, finely chopped barley straw significantly increased growth inhibition.
    • The latter authors stated that further oxidation of quinones would lead to the formation of humic acids, which perhaps would nullify the algal growth inhibitory effect. This thought fits two patterns. First, studies (e.g., Harriman et al. 1997) have reported that the effect of barley straw diminishes in about six months; perhaps by this time humic substances have formed. Secondly, Kholdegarin and Oertil (1997) reported phenolic compounds to interfere with nitrification, and Vallini et al. (1997) reported humic acids to facilitate nitrification.
      • However, recent unpublished reports as well as some published reports (e.g., Barrett et al. 1996) have speculated that these humic substances, in the presence of sunlight, create conditions suitable for the formation of hydrogen peroxide, and that this is the substance responsible for the algal growth inhibition. Hydrogen peroxide is formed in natural waters through the interaction of humic substances and sunlight (Cooper et al. 1988). Hydrogen peroxide has been shown to inhibit the growth of algae (Barrett and Newman 1992). Hydrogen peroxide inhibits carbon dioxide fixation (Ashton and Crafts 1981).
      • However, Pillinger (Irene) found barley straw to inhibit the growth of algae in the dark, which is obviously a condition that lacks sunlight to induce hydrogen peroxide formation.
    • Pillinger et al. (1995) tested another phenolic source (lignin) taken from rotted elm and sycamore wood for anti-algal properties; barley straw contains lignin (Love et al. 1998; Ball et al. 2001).
      • Both white-rotted and brown-rotted wood of both species showed growth inhibition ability.
      • Brown-rotted wood was found to be much more inhibitory; this was attributed to its higher lignin content.
      • This finding was supported in Ridge and Pillinger (1996).
    • Everall and Lees (1997) analyzed water samples extracted from rotted barley straw that had been applied to a reservoir; significant algal growth inhibition occurred in this study
      • The authors attributed the effects to a variety of phenolic compounds found in the samples. One of the compounds analyzed, 2,6-dimethoxy-4-pheno, was found to be highly inhibitory to algal growth in Pillinger (1994).
    • Ridge and Pillinger (1996) tentatively concluded that algal inhibition by barley straw is associated with the solubilization and oxidation of lignin, and furthermore, that the source of algal inhibition can be more accurately defined as oxidized polyphenolics.”


 barley straw

Written by J. D. Boylan, January 2000 – Revised Dec 2001, Department of Animal Ecology, Iowa State University.

 (Extracted by S de Kock 12/10/2015)