Grazing of the copepod Diaptomus connexus on purple sulphur bacteria in a meromictic salt lake

Abstract

A meromictic lake ecosystem (Mahoney Lake, BC, Canada) was investigated to elucidate the significance of chemocline bacteria in the total carbon cycle under natural conditions. In this lake, primary production by oxygenic phototrophs was insufficient to support the observed net secondary production of the calanoid copepod Diaptomus connexus and the rotifer Brachionus plicatilis, indicating the presence of additional food sources for consumers. Mahoney Lake harbours the densest population of phototrophic sulphur bacteria ever reported in a natural body of water. This layer is located at the interface between oxic and anoxic water layers and is dominated by the purple sulphur bacterium Amoebobacter purpureus. The transfer rates of A. purpureus carbon to D. connexus determined in stratified mesocosms were very low (0.71 ngC copepod(-1) day(-1)) and accounted for only 0.6% of the observed net biomass increase in the zooplankter. Stable stratification within the mesocosms prevented an upwelling of A. purpureus into the oxic part. However, measurements of carbon fluxes, infrared fluorescence microscopy and stable carbon analysis provided cumulative evidence that, under in situ conditions, the cell carbon of purple sulphur bacteria indeed enters the aerobic food chain via the grazing activity of D. connexus. Based on a two-source isotopic mixing model, A. purpureus represents at least 75-85% of the diet of D. connexus. Autumnal upwelling into oxic water layers and aggregation of A. purpureus cells appear to be the main factors determining the high carbon flux from purple sulphur bacteria to zooplankton under natural conditions, and most probably also play a key role in other aquatic ecosystems. Through this pathway, over 53% of the reduced organic matter of purple sulphur bacteria trapped in anoxic bottom waters is returned to the oxic realm.

An Overview of Purple Bacteria: Systematics, Physiology, and Habitats

“…Sulfi de triggers the growth of purple sulfur bacteria, which develop in specifi c zones of the water column where light and sulfi de are optimal (Pfennig, 1967(Pfennig, , 1975(Pfennig, , 1978a. If cell numbers are suffi ciently high, the lake water itself will become pigmented red, purple, or reddishbrown (Pfennig 1978a(Pfennig , 1989Overmann et al, 1994Overmann et al, , 1996Overmann et al, , 1999. When this occurs, it is often possible to identify the major genera of purple bacteria present by simple microscopic examination.…”

“…In certain habitats particularly favorable for their development, purple bacteria have been shown to be signifi cant primary producers (Czeczuga, 1968;Takahashi and Ichimura, 1968;Overmann et al, 1994Overmann et al, , 1996Overmann et al, , 1999. However, in most illuminated sulfi dic habitats the role of purple bacteria as H 2 S consumers is probably more important than any contribution they make to primary production; H 2 S is a highly poisonous substance for plants and animals and also for many bacteria.…”

Feeding spectra of Arctodiaptomus salinus (Calanoida, Copepoda) using fatty acid trophic markers in seston food in two salt lakes in South Siberia (Khakasia, Russia)

“…In the first of two experiments conducted with the seston of Lake Shunet, we used the water from 5.0 m depth, where ciliates and flagellates were abundant; the second experiment was conducted with seston from 5.2 m depth, and 109 diluted with lake water from 3 m. The objective of this experiment was to measure feeding of A. salinus on sulfur purple bacteria. This resource might be important for calanoids in meromictic lakes as it has been reported by Overmann et al (1999).We did not use natural concentrations of bacteria because of their very high density. However, preliminary observations revealed that the calanoids can exploit this food resource by either migrating into the ''purple layer'' for a short time or when bacteria are brought into the upper layers due to turbulence.…”

Vertical gradient of nutrients in two dimictic lakes ? influence of phototrophic sulfur bacteria on nutrient balance

“…A part of this bacterial biomass might be incorporated into 254 U. Selig et al Phototrophic sulfur bacteria in dimictic lakes zooplankton biomass by grazing (Gophen et al, 1974;Overmann et al, 1999). The degradation of phototrophic sulfur bacterial biomass can take place under either anoxic or oxic conditions.…”