Scientists at the Max Planck Institute of Limnology at Plön, Germany, have been successful in demonstrating how populations of small planktonic crustaceans (Daphnia) adapt to changing environmental stress by shifts in the frequencies of resistant genotypes. Nelson G. Hairston jr., guest researcher from Cornell University, NY, and Winfried Lampert, director at the Max Planck Institute of Limnology (MPIL), and colleagues report in the issue of 30 September 1999 of Nature that during the course of eutrophication of Lake Constance, the genetic composition of the Daphnia population shifted towards genotypes, which were less sensitive to toxic cyanobacteria (blue-green algae). Daphnids play an important role in the ecosystem as they clear the water by filtering out algae and are also a major component of fish food.
The occurrence of cyanobacteria, some of them being toxic, is a nuisance when lakes are enriched with nutrients (eutrophication). The first symptoms of eutrophication of Lake Constance from nutrient input through waste water were detected in the 1960s. The peak of the eutrophication with algal blooms was reached around 1980 before the lake responded to extensive efforts to reduce nutrient input, which resulted in the presently high standard of water quality. Although the large Lake Constance never had massive cyanobacteria blooms like other eutrophic lakes, blue-greens were present at measurable quantities. Even low proportions of toxic cyanobacteria in their food algae inhibit growth and reproduction of Daphnia or may even kill them. However, research at the MPIL showed that these animals are not helpless, but can adapt to the changing food resources.
Although such genetic shifts have been demonstrated in the laboratory, it is hard to demonstrate their relevance under field conditions. as genotypes that lived before the environmental change occurred cannot be studied physiologically in retrospect. The scientists at the MPIL used a trick to circumvent this problem. From time to time, Daphnia produce resting eggs covered with a resistant shell, that sink to the lake bottom, get burried in the sediment and can live for many years as dormant stages. Lake Constance has a sediment with fine annual layers that can be dated. Sediment cores taken from the lake were dissected in the laboratory at Plön and resting eggs from the layers with known age were isolated. It was then possible to hatch these eggs and "revive" populations of Daphnia that had survived in the sediment for up to 40 years. The scientists could now test populations which had been living in the lake before the eutrophication, at its peak and after the restoration. They added small amounts of Microcystis, a toxic cyanobacterium that had been isolated from the lake in 1972, to the animals' food and measured the performance of different genotypes.
Populations living before the eutrophication showed considerable genetic variation with respect to sensitivity against the blue-greens. Growth of some genotypes was strongly inhibited while others were only slightly affected. Sensitive genotypes diappeared during eutrophication but the resistant ones became abundant. Within ten years, the population had adapted to the new food conditions.
This discovery has basic significance for our understanding of the adaptation of organisms to changing environmental conditions (e. g. climate change) by rapid microevolution.
Contact: Wilfried Lampert
Max Planck Institute of Limnology,Plön/Germany
Phone: (+49 4522) 763 - 270
Fax: (+49 4522) 763 - 270
Journal
Nature