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25.09.2006

To avoid water bodies being covered with duckweed, that is to avoid covering with the microalgae and cyanobacteriae film, filterer fish, silver carps, for example, are put into lakes and storage ponds. However, specialists of the Institute of Biophysics (Siberian Branch, Russian Academy of Sciences), Krasnoyarsk State University and Krasnoyarsk State Agrarian University have determined that silver carps, although readily swallow the phytoplankton, do not fully digest it. It is for this particular reason, regardless of fish’s effort, water bodies continue to remain covered with duckweed.

The point is that some cyanobacteriae and algae species go through the fish’s intestines without being damaged. Moreover, it happens so that cyanobacteriae, having gone en route through the crucian carp, increase the growth rate and fill up the entire water body. The Krasnoyarsk researchers became interested if the silver carp’s intestines possess such invigorating effect.

The subject of their inquiry was the two-year old fish caught in the Berezhskoy storage pond of the Krasnoyarsk Region. The water from the Berezhskoy storage pond filtered via the bacteriological filters was populated with contents of the silver carps’ intestines or by cyanobacteriae collected in the storage pond. The temperature, illumination and airing where microorganisms were kept were as close as possible to the natural conditions. On the fifth day, the phytoplankton spread out in all retorts, and on the seventh day, the growth and photosynthesis of cyanobacteriae from the storage pond began to decline, while the microorganisms resettled from the intestines felt well as before. After eight days of experiment, the growth of cyanobacteriae also became slower in the experiment, but the water still remained greener than that in the case of free living phytoplankton.

The investigations showed that advantage in growth was obtained only by one species of cyanobacteriae - Microcystis aeruginosa - which found themselves in the fish’s intestines. After having been inside the silver carp, these bacteria grow up so much better that they doom the struggle against duckweed to a complete failure. Being consumed does so much good to these cyanobacteriae that they fill up the entire water body which the silver carp has already released from other species of cyanobacteriae and diatomic algae.

Mechanisms of cyanobacteriae stimulation in the course of passing through fish intestines is still unknown. According to some researchers, when cyanobacteriae are going through the digestive tract, they assimilate nutrients, which are present in the intestines in relatively high concentrations, first of all, phosphorus. Nevertheless, in the water bodies covered with duckweed, the nutrients’ concentration is anyway sufficiently high, therefore, such explanation seems unlikely. However, silver carps as well as any other fish can be engaged in fighting against duckweed only after the phytoplankton specific composition has been precisely determined in a particular water body.

Nadezda Markina | Source: alphagalileo
Further information: www.informnauka.ru

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