|Burhenne, Matthias: Biotestsystem mit Bodenalgen zur ökotoxikologischen Bewertung von Schwermetallen und Pflanzenschutzmitteln am Beispiel von Cadmium und Isoproturon |
Biotests are an important device to assess the toxicity of chemicals, pesticides, polluted water, and soils because they can provide direct information about the influence of a compound on the organism level. Besides various biotests using higher plants there is only the DIN 28 692 biotest “Growth-inhibition test using fresh water algae Scenedesmus subspicatus and Selenastrum capricornutum” (DIN 28 692) also known as the OECD 201 biotest “Algal, Growth Inhibition Test” which is currently available for an ecotoxicological assessment of chemicals such as pesticides on the autotrophic organism level. This aquatic biotest is based on a single specie of fresh water algae and is increasingly applied to evaluate polluted soils and sediments. There is almost no information on aquatic biotests which are using soil algae as test organisms instead. A more comprehensive biotest system which actually combines aquatic and terrestric biotests using several soil algae species as test organisms has not been reported, yet. Thus, a biotest system was developed and subsequently evaluated by using cadmium (cadmium chloride) as a heavy metal, and the herbicide arelon containing isoproturon as the active ingredient.
Soil algae were isolated from unpolluted soil in order to obtain test organisms which are not resistant or tolerant to pollutants. The algae isolates were then cultivated, and subsequently identified. A total of 35 algae species was collected. Cell numbers of algae found in different soils ranged from 800 000 to 1 000 000 cfu/g dry soil. Even a sandy soil with low nutrient levels contained 400 000 cfu/g dry soil. Algae species used in the biotest system were Xanthonema tribonematoides, Stichococcus bacillaris, Klebsormidium flaccidum, Xanthonema montanum, Chlamydomonas noctigama. These species were chosen because they represent the broad spectrum of algae which were not only frequently isolated from soil, but did also markedly differ in cell morphology, growth rate, and the way they reproduce. In addition, the fresh water specie Scenedesmus subspicatus served as a reference algae.
Based on these different algae species a gel biotest using liquid gel medium was developed to investigate the contamination path via water, and also a soil biotest with a pre-treated soil of low sorption capacity was deviced to simulate the contamination path through gas, water, and solid phase. The gel biotest did included all six algae species but only four were part of the soil biotest. In both biotests, the sensitivity of the test organisms was tested over several generations by measuring growth inhibition during a period of 96 hours. Since culture conditions, and in part the composition of the culture medium were identically, the measured EC values could be compared directly. The input of material, time and expenses was clearly reduced compared to the DIN algae test although 4 and 6, respectively, test organisms were handled. This was accomplished by employing microplate based techniques for both tests, and further by automatic analysis of the gel biotest plates with a microplate photometer.
The evaluation of the biotest system using cadmium chloride and isoproturon did reveal that soil algae have had only low to medium sensitivity to cadmium chloride in the gel biotest. Algae sensitivity in the soil biotest was very low which was in accordance with data from
99other biotests using different soil organisms. The weak response of the algae was most likely caused by the sorption of the cadmium ions to the soil matrix what may have decreased the bioavailability of cadmium. In comparison, soil algae were very sensitive to isoproturon in both, the gel biotest and the soil biotest. Both biotests indicated almost identical sensitivities of the tested soil algae which was surprising since 30 of the isoproturon was sorbed even in soils with a low sorption capacity. There are two possible explanations. First, even the sorbed isoproturon fraction may have influenced soil algae by either the close contact of the algae cells with the loaded soil particles or by slime excreted by the cells, which may have caused a desorption of compound, and therefore an increased bioavailability of isoproturon.Second, the algae were influenced by isoproturon in the beginning of the experiment when most of the compound was still bioavailable.
Soil algae when compared to the water algae Scenedesmus subspicatus were generally 5 to 10-fold less sensitive to cadmium chloride. Only Klebsormidium flaccidum has proved to have a similar sensitivity as Scenedesmus subspicatus had, whereas Xanthonema montanum was about 20-fold less sensitive. With isoproturon, however, no differences in sensitivity could be seen between Scenedesmus subspicatus and the tested soil algae, except Stichococcus bacillaris which was about 5-fold less sensitive.
The biotest system as developed in this study has shown to be suitable for obtaining valuable information about ecotoxicological effects of chemicals on soil and water algae. Since the biotest system consists of two different test media (liquid gel and soil) it is possible to determine ecotoxicological effects on algae in both, water and soil.
A first draft of the developed biotest system has been submitted to the “Technical Committee 190 - Soil Quality” of the International Standards Organization (ISO) for review.
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