Effects of Dissolved Organic Matter Fractions of Varying Molecular Weights and Cd2 + on Scenedesmus Obliquus Growth

The effects of dissolved organic matter (DOM) fractions of various sizes and Cd2+ on the growth rate of Scenedesmus obliquus has been studied. The studies were carried out both separately for DOM and Cd2+, and for both. DOM fractions (>30, 10–30, 3–10, 1–3 and <1 kDa) has been obtained by ultrafiltration; algae growth was assessed by the chlorophyll content, determined spectrophotometrically at 678 nm. It has been found that cadmium at concentrations of 5.62–22.5 μg/L stimulated and at concentrations of 56.2 and 112.4 μg/L suppressed the growth of algae. DOM fractions had different effects on S. obliquus, they both reduced and stimulated the growth of algae in the range of 0.001–1 mg C/L, and the effect on the growth rate was in the range of ±10–25%. The greatest positive response of microalgae was shown by the fraction <1 kDa, which was probably associated with the active participation of this fraction in the algae nutrition. In the presence of inhibitory concentrations of Cd2+, DOM fractions of 30, 1–3, and <1 kDa had a positive effect on the growth of S. obliquus. The probable mechanisms of such effect of DOM was its binding of Cd2+ ions (mainly on the surface of algal cells), as well as the high biological activity of its low molecular weight fraction <1 kDa. For the first time, data were obtained on the effect of heavy metals on the growth of algae in the presence of DOM fractions of different sizes. The data obtained expand our understanding of the mechanisms of regulation the number and state of microalgae in ecosystems, when affected by heavy metals.