In the meromictic alpine Lake Cadagno a dense layer of phototrophic bacteria, mainly Chromatium okenii and Amoebobacter purpureus, develop annually at the chemocline at about 10 to 11 m depth. Radiometric spectral profiles of the incident sunlight demonstrate different attenuation coefficients in the mixolimnion and in the chemocline not only for the visible light effective at each depth (photosynthetically available radiation), but also for selected photosynthetically active wavelengths used by oxygenic and anoxygenic phototrophs. Phototrophic bacteria sampled from the upper part of the layer at the maximum of cell concentration were incubated in transparent bottles at the sampling depth and at a lower depth where the light intensity is only a few percent of the one at the sampling depth. Within 4 h the specific bacteriochlorophyll concentration (Bchl protein _ ' ) increased up to 50% depending on the difference in light intensity between the sampling and the incubation depth. The specific bacteriochlorophyll concentration in the upper part of the layer remained constant (53.0 mg Bchl g-' protein, S.D. = 4.8) in spite of large changes in cell concentrations in the lake water over the season. These observations illustrate the phenomenon of light-regulated pigment synthesis under natural conditions.

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Lake Cadagno is a meromictic alpine lake with a dense layer of phototrophic bacteria at about 12 m depth closely below the oxic-anoxic interface. Phototrophic bacteria are known to react by phototaxis and chemotaxis to changes in the environmental factors light, oxygen or hydrogen sulfide. To determine whether this bacterial plume undergoes diel changes in depth and density, a series of absorption and temperature sensors were positioned vertically and horizontally in this layer, allowing changes in local cell concentration and bacterial movement to be followed with high spatial and temporal resolution. The signals from the absorption sensors were proportional to the cell concentrations in the light path. In the lake, the cell concentration in the bacterial layer was highly dynamic and showed oscillations of various frequencies. The width of the bacterial layer and its depth in the water column varied also. Many of these oscillations were a consequence of internal waves and seiches induced by the frequent alpine winds and the morphology of the lake bed. Parallel to fluctuations in cell concentrations, changes in temperature were monitored. This allowed us to distinguish between physically induced displacements of the cells in the water body and the active movement of bacteria due to changes in light conditions and chemical gradients in the water column. Maximum active vertical movements observed during the daytime were about 25 cm. However, diurnal active vertical bacterial movement was not always found throughout the summer season, possibly due to the particular weather situation and to seasonal changes in bacterial community structure. KEY WORDS: Chromatium okenii · Amoebobacter purpureus · Phototrophic bacteria · Turbidity · In vivo · Oscillations · Bacterial movement Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 35: [105][106][107][108][109][110][111][112][113] 2004 fusing upwards from deeper layers and the sediment or produced by the SRB in the bacterial plume serves as electron donor for phototrophs and chemolithotrophs (Peduzzi et al. 1991, Camacho et al. 2001, DelDon et al. 2001. Although the temperature in the layer rarely exceeds 9°C, the bacteria are physiologically highly active (Joss et al. 1994, Fischer et al. 1996, Lüthy et al. 2000, Camacho et al. 2001.The organismic diversity of this specific microbial ecosystem has been characterized in recent years using molecular tools, revealing various closely related clones of Chromatium okenii, Amoebobacter purpureus and of sulfate-reducing bacteria (Tonolla et al. 1999, 2003, Bosshard et al. 2000b, Peduzzi et al. 2003a. A microscale sulfur cycle is assumed in the plume, as phototrophs and sulfate reducers form symbiotic aggregates (Lüthy et al. 2000, Peduzzi et al. 2003b.Turbidity profiles obtained when routinely monitoring the lake during past decades repeatedly showed the bacterial concentration in the layer to be highly variable in space and time (Tonolla 1987 and authors' unpubl. data). F...

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In the 19th century, rooms were frequently decorated with wallpaper containing arsenical pigments; illness and fatalities often resulted. In 1839, Leopold Gmelin described a mouse-like odor under those conditions. Much later, it was recognized that the problem was the formation of toxic and volatile trimethylarsine by fungal action. Gmelin's observation was misreported as a garlic-like odor that is characteristic of trimethylarsine. Gmelin's original article in German and an English translation are included, and possible explanations for the incorrect reporting and for the original observation are described.