A series of experiments aimed at clarifying the mode of action of barley straw in cyanobacterial growth control.

For over 25 years it has been known that rotting barley straw can be used to prevent the development of blooms of cyanobacteria and algae in freshwater bodies, although its effectiveness can be variable. The mode of action is still not understood, although a number of hypotheses have been suggested, many of which are supported by little or no experimental evidence. Here, we provide the first experimental confirmation that microbial activity is responsible for the release of either the growth inhibitory fraction, or its precursor, from whole straw, after three or more weeks of decomposition. However, a much more rapid release of inhibitory components was achieved by fine chopping of fresh straw. In bioassays of straw activity the choice of both the cyanobacterial test strain and the assay temperature affected the outcome. The inhibitory activity of straw was greater when decomposition was carried out in the presence of UV-supplemented visible light and this activity was reduced in the presence of catalase, implying that straw activity may in part involve hydrogen peroxide. A better understanding of straw decomposition is required to clarify the mode of action of straw and allow the optimisation of its use in the field.