compounds were detectable in all four sets of experiments, evidence for reversibility of reactions among the compounds. After 72 h, reaction ratios between the model compounds converged in all four sets of experiments, consistent with approaches toward metastable equilibrium. Reaction ratios for ether and aldehyde formation reactions were also observed to group within a relatively small range, but without a clear convergence pattern, suggesting other non-redox reactions may approach metastable equilibrium. The approach to metastable equilibrium among the initial organic reactants could be observed and quantified even in the presence of competing redox reactions whose mechanisms are less understood, including dibenzylimine and toluene formation, which did not appear to reach steady-states. These findings identify classes of organic compounds and reactions that can reflect the conditions at which they last equilibrated and should be targeted for analysis in natural systems.

Roebuck, J.A., Seidel, M., Dittmar, T., Jaffé, R., 2020. Controls of land use and the river continuum concept on dissolved organic matter composition in an anthropogenically disturbed subtropical watershed. Environmental Science &Technology 54, 195-206.

https://doi.org/10.1021/acs.est.9b04605 About 250 Tg of dissolved organic carbon are annually transported from inland waters to coastal systems making rivers a critical link between terrestrial and ocean carbon pools. During transport through fluvial systems, various biogeochemical processes selectively remove or transform labile material, effectively altering the composition of dissolved organic matter (DOM) exported to the ocean. The river continuum concept (RCC) has been historically used as a model to predict the fate and quality of organic matter along a river continuum. However, the conversion of natural landscapes for urban and agricultural practices can also alter the sources and quality of DOM exported from fluvial systems, and the RCC may be significantly limited in predicting DOM quality in anthropogenically impacted watersheds. Here, we studied DOM dynamics in the Altamaha River watershed in Georgia, USA, a fluvial system where headwater streams are highly impacted by anthropogenic activities. The primary goal of this study was to quantitatively assess the importance of both the RCC and land use as environmental drivers controlling DOM composition. Land use was a stronger predictor of spatial ∼ variation ( 50%) in DOM composition defined by both excitation - emission matrix - parallel factor analysis (EEM - PARAFAC) and ultrahigh-resolution mass spectrometry. This is compared to an 8% explained variability that can be attributed to the RCC. This study highlights the importance of incorporating land use among other controls into the RCC to better predict the fate and quality of DOM exported from terrestrial to coastal systems.

Rontani, J.-F., Belt, S.T., 2020. Photo- and autoxidation of unsaturated algal lipids in the marine environment: An overview of processes, their potential tracers, and limitations. Organic Geochemistry 139, 103941.

http://www.sciencedirect.com/science/article/pii/S0146638019301780 This paper provides an overview of results obtained in the course of recent studies dealing with abiotic degradation of algal lipids in the marine realm. In the first part, the photooxidation of the main lipid components 5 (chlorophyll phytyl chain, caroteno ids, Δ -sterols, unsaturated fatty acids, alkenones and unsaturated alkenes) in senescent phytoplanktonic cells is examined. Type II photosensitized oxidation processes (i.e. involving singlet oxygen) play a key role in the photooxidation of these compounds. The second part of the paper describes free radical oxidation of the main lipids in senescent phytoplanktonic cells. These processes, which are believed to be induced by homolytic cleavage of photochemically produced hydroperoxides, involve the direct reaction of molecular oxygen in its triplet ground state with organic compounds under relatively mild conditions. Particular attention is given to the characterisation of pathway-specific tracers of the individual abiotic reactions described. Some of these compounds appear to have the potential to provide qualitative or semi-quantitative information about photooxidation, autoxidation, or the total abiotic degradation state of individual or groups of organisms. On the other hand, some other primary oxidation products are likely too susceptible to further degradation to permit reliable quantitative estimates, but can, nonetheless, provide a qualitative indication of photooxidation and autoxidation.