Abstract
Sulfur-containing substances with reducing properties in natural water bodies are partners in the reactions with active oxygen forms (ROS) and so are their specific antipodes. The ratio of counterflows of an oxidizer and a reducing agent sets the redox potential of the aquatic environment, is responsible for its self-purification and ultimately forms the quality of water as a habitat. At the same time, data on the formation and destruction channels of substances with the reduced sulfhydryl groups and correlation dependences of their quantity with regard to the other components’ concentrations in natural aquatic environment are fragmentary and insufficient. During the years 2015-2019, four water bodies, two lotic systems and two lentic ones were monitored. Thiols and sulfates were monitored, and it was found that in all the monitored aquatic systems the thiols content is subject to seasonal variation; therefore, its provenance is predominantly natural. To elucidate the seasonal dynamics of different sulfur forms in natural waters, the Pearson linear correlation coefficient was calculated and a positive summer correlation was attested related to the maximal biological activity. This proves that sulfate ions are used by hydrobionts as a source of sulfur for the synthesis of organic compounds, including thiols. In spring and autumn, the calculated coefficients have negative values, which denotes the dominance of chemical oxidation of the organic compounds with sulfur. These are periods with minimal biological activity. It was shown that out of two studied thiols, cysteine and glutathione, only the first one has shown the toxicity with regard to cyanobacteria.
Highlights
The ecological status of aquatic systems has become a key topic of numerous studies in recent decades, due to the increased anthropogenic impact on the environment
In our previous publication we have analyzed in detail the role of hydrogen peroxide in oxidation reactions occurring in natural waters [1]
During 2015-2019, the content of thiol compounds in four water bodies making the part of the Nistru river basin was monitored, which falls within the range 10-6-10-5 mol/L
Summary
The ecological status of aquatic systems has become a key topic of numerous studies in recent decades, due to the increased anthropogenic impact on the environment. In our previous publication we have analyzed in detail the role of hydrogen peroxide in oxidation reactions occurring in natural waters [1]. The main source of oxidative equivalents (i.e., electron acceptor) is molecular oxygen present everywhere, its involvement in the redox turnover is only possible with the participation of the so-called Reactive Oxygen Species (ROS), hydrogen peroxide being the most well-known and significant of them. Any organic substances (total organic carbon, TOC) are subjects to redox transformations in the course of radical self-purification processes in natural waters. Due to the initial chemical stability (low reactivity), they are not responsible for the redox state formation of the aquatic environment
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