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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