A push-pull method, previously used in groundwater analyses, was successfully adapted for measuring sulfide turnover rates in situ at different depths in the meromictic Lake Cadagno. In the layer of phototrophic bacteria at about 12 m in depth net sulfide consumption was observed during the day, indicating active bacterial photosynthesis. During the night the sulfide turnover rates were positive, indicating a net sulfide production from the reduction of more-oxidized sulfur compounds. Because of lack of light, no photosynthesis takes place in the monimolimnion; thus, only sulfide formation is observed both during the day and the night. Sulfide turnover rates in the oxic mixolimnion were always positive as sulfide is spontaneously oxidized by oxygen and as the rates of sulfide oxidation depend on the oxygen concentrations present. Sulfide oxidation by chemolithotrophic bacteria may occur at the oxicline, but this cannot be distinguished from spontaneous chemical oxidation.Lakes, like most ecosystems, are open systems in a steady state with different trophic levels. To understand the interactions between organisms and the environment, inputs and outputs of the nutrients at each trophic level must be known. Rates describe the turnover of chemical species at specific sites and therefore describe elemental fluxes in biogeochemical cycles as well as microbial activities in an ecosystem.In physiology and ecology several methods for evaluating rates have been described. In sediments microbial reaction rates are calculated from concentration-depth profiles by using flux calculations based on a modified Fick's first law; however, as incubations of several days are necessary, only a low temporal resolution is achieved. Primary production rates from phytoplankton are obtained by the use of radioactive carbon isotopes. Samples are collected in a depth profile and incubated with 14 CO 2 at the originating depth for a few hours. But such results will also not give the actual in situ reaction rates for rapidly cycling elements.In this paper we present a new approach for determining in situ reaction rates for production and consumption of sulfide in the open water. Experiments were done in a meromictic alpine lake with a pronounced biological sulfur cycle (7,11,12,15,20). Under anoxic conditions sulfide is produced by sulfatereducing bacteria, which reduce sulfate to sulfide at the expense of organic substrates. At the same time, if light is present, sulfide is oxidized by anoxygenic phototrophic bacteria, which use it as an electron donor. Concentration profiles for Lake Cadagno often show no sulfide in the layer with the highest biomass concentration of the water column and thus cannot give information on actual turnover rates of the sulfur compounds because, in a steady state, sources and sinks are balanced. The "push-pull" technique, which has been previously applied in groundwater systems (9, 10, 17) was used to obtain real in situ sulfide turnover rates with a high temporal resolution in an undisturbed ecosystem. MATERIALS...

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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