Aquatic humic substances, or chromophoric fraction of dissolved organic matter (СDOM) in natural water have characteristic optical properties. Considerable effort has been devoted to investigate CDOM optical properties and to improve understanding of biogeochemical carbon cycles in the northern regions. However, due to rare observations up to date there is a limited amount of data available on CDOM research in the sub-Arctic zones. This work summarizes the spectral-optical properties of CDOM from several locations along two coasts of the White Sea. The absorption and fluorescence emission spectra with different excitation wavelength were measured for water sampled in August- September 2020 in the expeditions to several locations of the Karelian and Tersky Coasts, differing in geomorphology, hydrology and anthropogenic influence. The fluorescence was found excitation-dependent; we observed the “blue shift” of emission maximum along with rising the excitation wavelength for all studied CDOM samples (surface and deep coastal water). The absorption spectra are similar in shape for all studied CDOM samples, however, absorbance values reflect concentration of humic substances in some samples. In the Lobanikha Bay we did not find the significant difference in deep and surface CDOM, but in the relict lagoon in Eastern Porya Bay the CDOM was concentrated towards bottom. Those findings are important for understanding the mechanisms of the formation of optical properties of natural water with CDOM of various origins.
The cultures of filamentous fungi in aquatic medium release fluorescent metabolites (FM) with emission spectra that closely match the typical fluorescence bands found for soil extracts and aquatic fluorescent dissolved organic matter (FDOM). FM released from some fungal cultures show as well comparable values of fluorescent quantum yield, the blue shift of emission spectra excited in the UV, and a very close match of ultraviolet–visible absorbance spectral curves related to soil and aquatic FDOM, further strengthening the similarity of fluorophores in those aquatic material. Given the importance of microscopic filamentous fungi in the global carbon cycle, our results indicate that filamentous fungi are likely to be important sources of aquatic and soil FDOM of microbial origin.
Soil fungi are actively involved in the processes of humic substances synthesis, transformation and mineralization due to production of extracellular nonspecific oxidative enzymes. The work was aimed to evaluate using spectral methods transformation dynamics for the humic product (HP) from lignosulfonate (HPligno) by filamentous soil fungal cultures Alternaria alternata and Trichoderma harzianum. Experiments showed that direct spectroscopic study of HPligno introduced into the nutrient medium and its transformation during fungal growth is challenging due to strong absorption of light by nutrient medium, development of absorbing fungal metabolites, partial utilization and destruction of HP by fungi and therefore due to the need to register tiny changes in overlapping bands. To accomplish that task we proposed a novel algorithm for processing the absorption spectra, which has not previously been used to study fungal cultures. We calculated the second-order derivative in respect to wavelength for absorption spectra measured during fungal growth and found characteristic "patterns" for introduced HP: a maximum at 270-285 and a minimum within 290-300 nm. The spectral index determined from amplitudes in the second-order derivative spectrum reflects the relative content of HP in the nutrient medium in presence of other absorbing components. We resume that two fungal strains utilized HPligno in the 0,02 and 0,1% concentrations better at 30 g/L sucrose than at 3 g/L in the medium. Thus the second-order differentiated absorption spectra helped to quantify degradation of the HPligno during fungal growth.
Hydrophobic components of cromophoric dissolved organic matter (CDOM) extracted from water samples and sediments taken in several relic basins located on Karelian shoreline of the White Sea were analyzed using spectroscopic techniques. Those water reservoirs exist at various stages of isolation from the White Sea and represent complex stratified systems of fresh and marine water layers not completely mixing trough the year. Basins separating from the White Sea are the unique natural objects for investigations of properties CDOM, its transformation in the process of turning the marine ecosystem into freshwater environment. CDOM occurring in all types of natural water represents a significant reservoir of organic carbon and plays a key role in the carbon cycle on the Earth. However, aquatic CDOM and nonliving organic matter in sediments from relic separating basins still have not been studied. The target of this work was to study absorption and fluorescence spectra of hydrophobic components of aquatic CDOM from different water depth and sediments in several separated basins of the Kandalaksha Gulf of the White Sea located near the N.A. Pertsov White Sea Biological Station.
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