Dynamics of Different Sulfur Forms in Natural Waters and Their Influence on the Redox State

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.