Determining the Origin and Fate of Particulate Plant-Derived Organic Matter in the Rhone River (France) : A Lipid Tracer Review

A number of lipid tracers including fatty acids, hydroxyacids, n-alkanols, sterols and triterpenoids were used to determine the origin and fate of suspended particulate organic matter (POM) collected in the Rhone River (France), with a main focus on phytosterols, such as sitosterol, desmosterol, brassicasterol and cholesterol. This seasonal survey (April 2011 to May 2013) revealed a year-round strong terrigenous contribution to the plant derived particulate organic matter (POM) with significant algal inputs observed in March and attributed to phytoplanktonic blooms likely dominated by diatoms.

Specific sitosterol and cholesterol degradation products were quantified and used to estimate the part of biotic and abiotic degradation of POM within the river. Plant-derived organic matter appears to be mainly affected by photo-oxidation and autoxidation (free radical oxidation), while organic matter of human origin, evidenced by the presence of coprostanol, is clearly more prone to bacterial degradation. Despite the involvement of an intense autoxidation inducing homolytic cleavage of peroxy bonds, a significant proportion of hydroperoxides is still intact in higher plant debris. These compounds could play a role in the degradation of terrestrial material by inducing an intense autoxidation upon its arrival at sea.

Although sitosterol has been commonly used as a tracer of the terrestrial origin of POM in rivers, we show here that is it also found in phytoplankton, which highlights the need to use different tracers to determine the origin of POM in rivers. As part of the set of tracers we use, we have identified betulin to be an interesting candidate, although limited to a number of angiosperms species. Not only can we trace betulin to an unequivocal terrestrial origin, we also identified its specific degradation products, allowing us to trace the degradation state of angiosperm particulate debris in rivers, as well as the type of degradation undergone.