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Environmental monitoring of natural water bodies is important in the Arctic zone for studying their evolution under the influence of climate change and the urbanization processes in the region. In connection with the specific life conditions for anoxygenic phototrophic microorganisms their presence can indicate hydrogen sulfide contamination of the water reservoir and serve as a marker of hydrogen sulfide. This makes the problem of optical diagnostics of phototrophic bacteria very urgent and important. There is a number of spectral methods for determination of chlorophyll-containing microorganisms. The spectral properties of photosynthetic bacterial pigments, bacteriochlorophylls (BChls), are still poorly understood, however. For the first time we applied fluorescence spectra of BChl extracts to receive depth distribution of BChl d in the lake. Fluorescence emission spectra were measured using a Solar CM2203 luminescence spectrometer under excitation at wavelength of 425 nm, corresponding to the BChl d absorption peak. In September 2020 the maximal concentration of BChl d was found at the depth of 2.275 m (16700 mg/m3) in Lake Trekhtzvetnoye. The thickness of the bacterial plate did not exceed 5 cm, and the pigment distribution was found vertically asymmetric. We emphasize that fluorescence quantification of BChl d is more sensitive compared to spectrophotometric one, and it makes possible estimation of ultralow BChl concentrations without water sample pre-concentration.
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