ABSTRACT The occurrence of taste and odor problems in drinking water supplies is a widespread phenomenon. From a Saxonian water reservoir we isolated a cyanobacterial species which was classified as Phormidium sp. Under laboratory conditions it produced an earthy-musty smell due to the synthesis of geosmin. The only genes shown to be involved in geosmin biosynthesis are cyc2 and geoA of Streptomyces. Based on the alignment of Cyc2 with a putative sesquiterpene synthase of Nostoc punctiforme, a degenerate primer pair was designed. By PCR, we could amplify two similar genes in Phormidium sp., which we named geoA1 and geoA2. Their expression was studied by reverse transcription-PCR. This revealed that both genes are expressed at 20°C and a light-dark cycle of 12 h. Expression was not detectable at the end of a 24-h dark period. To analyze the prevalence of geoA1 and geoA2 in samples from the phytobenthos, we generated PCR fragments with the same degenerate primer pair. Fifty-five different sequences that might represent geoA variants were obtained. The GC content ranged from 42% to 67%, suggesting that taxonomically very different bacteria might contain such genes.

Der vorliegende Beitrag beschreibt Zielsetzung, Schwerpunkte und erwartete Ergebnisse eines durch das Bundeswirtschaftsministeriums (BMWi) geforderten Forschungsprojektes. Dabei wird eine Qualifizierungsumgebung fur kleine und mittlere Verlage geplant, konzipiert und realisiert, die neue internetund intranetbasierter Lernformen (E-Learning) und konventioneller Lernformen (Prasenzveranstaltungen) kombiniert.

In Bautzen reservoir, a shallow, hypertrophic water in Eastern Saxony, biomanipulation led to structural changes in the phytoplankton community but did not reduce algal biomass. To supplement the top-down management, a new type of water treatment technology was tested during two seasons (May–August 1996/1997), aiming at the bottom-up control of mass developments of the cyanobacterium Microcystis aeruginosa.The technology is based on a combined lake–internal phosphorus precipitation and a transport of hypolimnetic water rich in free carbon dioxide into the upper layers. During the treatment periods, there were found both an increase of CO2concentrations in the mixed layer and an extension of the period in which free CO2was detected in the epilimnion. The concentrations of phosphorus could be lowered drastically in the whole water body. Microcystiswas almost totally suppressed (1996) or appeared with a delay (1997) compared to the regular annual pattern observed before the treatment. In contrast to the preceding year (1995), diatoms played a major role in the summer phytoplankton during the treatment years (1996/1997). The two application periods are compared with respect to the influence of meteorologically determined variables.

We determined clearance rates and ingestion rates of Daphnia galeata on bacteria and phytoplankton in order to test if bacteria are an important alternative food resource for daphnids during periods of low phytoplankton biomass in the biomanipulated Bautzen reservoir (Germany). D. galeata was able to feed on bacteria with the same efficiency as on algae during most of the time. In spite of similar clearance rates, bacteria ingestion was usually lower than phytoplankton ingestion due to lower bacterial biomass. Only at low biomass of algae in late fall and during the clear water phase, bacteria contributed up to 87% and 42%, respectively, to total carbon ingestion of D. galeata. However, even a short period of relatively high bacteria grazing by daphnids may be important for biomanipulation. Bacteria might bridge over periods of food limitation of daphnids thus promoting the maintenance of high Daphnia biomass. Therefore, ingestion of bacteria by daphnids is thought to stabilize biomanipulation and may hold a key position in the food web of biomanipulated lakes.

To better characterize the proposed growth modulatory functions of mast cells and their products in the regulation of epidermal homeostasis, we have assessed keratinocyte proliferation by incorporation of [3H]thymidine in murine skin organ cultures after incubation with different concentrations of histamine, serotonin or TNF-alpha. Both histamine and serotonin significantly stimulate the proliferation of keratinocytes as compared to controls (P<0.001), reaching maxima within a range from micro- to nanomolar concentrations. Even at subnanomolar concentration, TNF-alpha significantly inhibits the incorporation of labelled thymidine by epidermal keratinocytes in murine skin organ culture (P<0.001). These observations support the concept of mast cell products as modulators of keratinocyte proliferation, and suggest a role for mast cells in the regulation of epidermal tissue turnover under physiological conditions.