In Situ Determination of Sulfide Turnover Rates in a Meromictic Alpine Lake

Abstract

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...

Phototropic sulfur and sulfate-reducing bacteria in the chemocline of meromictic Lake Cadagno, Switzerland

“…1) suggesting the presence of both sulfideproducing as well as sulfide-consuming populations in the bacterial plume. This is supported by in situ analyses of sulfide turnover rates showing net sulfide consumption during the day and net production during the night during maximal seasonal turbidity (Lehmann et al 1998;Lüthy et al 2000). Net day-time sulfide consumption is attributed to the large activity of phototrophic sulfur bacteria, while nocturnal net sulfide production may be due to increased activity of sulfate-reducing bacteria and anaerobic respiration of internal storage compounds of phototrophic bacteria (Del Don et al 1994;Mas & van Gemerden 1995).…”

“…1) (Tonolla et al 1999;Lüthy et al 2000;Camacho et al 2001). Highest bacterial numbers are generally found at depths where sulfide becomes detectable and concentrations increase with depth to up to 3-8 mg l -1 (Fig.…”

Long-Term Population Dynamics of Phototrophic Sulfur Bacteria in the Chemocline of Lake Cadagno, Switzerland

“…In these aggregates, they are associated with sulfate-reducing bacteria that are also able to grow by disproportionation of inorganic sulfur compounds (26). Aggregates can provide distinct but relatively stable microenvironmental conditions (29) and thus a growth advantage in a habitat with rapid changes of environmental conditions (e.g., intensity of light and sulfide concentrations) (9,15,21). Potential limitation for sulfide in these aggregates could be overcome by the synergistic association with the sulfate-reducing bacteria that presumably resembles a source-sink relationship for sulfide between the sulfate-reducing and the purple sulfur bacteria (26).…”

“…This lake is characterized by a compact chemocline with high concentrations of sulfate; steep gradients of oxygen, sulfide, and light; and a turbidity maximum that correlates to large numbers of bacteria (up to 10 7 cells ml Ϫ1 ) (5,21,31). Molecular methods such as 16S rRNA gene clone library analysis and subsequent in situ hybridization with specific fluorescent probes identified the most abundant bacteria in the chemocline as large-and small-celled purple sulfur bacteria.…”

Microbial efficiency in a meromictic reservoir

“…According to Luthy et al (2000) sulphide in the oxic mixolimnion is spontaneously oxidized by oxygen, the rate of oxidation depending on the oxygen concentration. Sulphide oxidation by chemolithotrophic bacteria may occur at the oxicline, and this cannot be distinguished from spontaneous chemical oxidation in the aerated mixolimnion.…”