The contribution of algal activity to the origin and composition of organic matter in river sediments (Vrbas River, Bosnia and Herzegovina)

This study analyses sediments of the Vrbas River from its source to the Sava River confluence, assessing the origin of bitumen-type organic substances and the presence of anthropogenic oil-type pollutants. It focuses particularly on the impact of algae on the composition of organic matter. Saturated hydrocarbons were isolated by Soxhlet extraction, and group composition was determined by column chromatography. n-Alkanes, terpanes and steranes, alcoholic and fatty acid fractions were analysed by gas chromatography-mass spectrometry. The samples contain lower amounts of bitumen and higher amounts of NSO compounds and asphaltenes over the hydrocarbons, suggesting the presence of native organic substances. However, the bimodal distribution of n-alkanes suggests two precursors: higher terrestrial plants responsible for native, immature organic matter and another precursor accountable for the marked dominance of even low-chain n-alkanes. Sterane and terpene distribution could potentially point to the algal precursor, but more than that, they are hard evidence of oil-type pollution in sediments. The homologous sequence of fatty acids in the sediments indicates the presence of microorganisms, bacteria, and algae. However, cis-9-hexadecenoic acid, a product of algae biosynthesis, is another evidence of algal precursor. Cholesterol and its homologues were detected in the alcoholic fraction, indicating the presence of low-chain fatty acids with a regular sequence, which is more likely to originate from algae than bacteria. Considering that the investigated sediments also contain an oil-type pollutant, it can be assumed that normal alkanes with the specified range and distribution are biosynthesized by algae developed on the petroleum base.