In the frame of a research project concerning regrowth problems a water utility at Lake Constance is involved. For this plant it was interesting to find out which effect is exerted by organic products released from algae living in the lake water. Therefore model experiments were carried out using two different algal cultures in the stationary phase, where the algae were carefully separated by filtration. From these extracellular organic carbon solutions (exudates) dilutions were prepared using highly purified deionized water. These were tested for their ability to support bacterial growth by adding a mixed bacterial biocenosis after sterile filtration and monitoring growth curves by turbidity measurements, a method developed at the DVGW-Research-Center in Karlsruhe to judge drinking water with respect to its regrowth potential. It was shown that the exudate from a Haematococcus culture is very growth stimulating, though the substrate concentration analyzed as ∆DOC was very low. In contrast the exudate from a Stephanodiscus culture gave a very low growth response. The effect of the Stephanodiscus culture for the drinking water plant at Lake Constance is much more important because Stephanodiscus is a species common in Lake Constance whereas Haematococcus is not.

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