Metal Ions and Hydroperoxide Content: Main Drivers of Coastal Lipid Autoxidation in Riverine Suspended Particulate Matter and Higher Plant Debris

Abstract

Autoxidation is a complex abiotic degradation process, and while it has long been known and well studied in biological compounds, it has been widely overlooked in environmental samples and as a part of environmental processes. With recent observations showing the magnitude of the involvement of autoxidation in coastal environments, it has become critical to better understand how and why this degradative process takes place. At the riverine/marine interface, recent findings evidenced a spike in autoxidation rates upon the arrival of suspended particulate matter in seawater. In this study, we aimed at identifying autoxidation-favoring factors in vitro by analyzing suspended particulate matter incubated under different conditions. If metal ions have long been known to induce autoxidation in biological systems, we show that they indeed induce autoxidation in particulate matter incubated in water, but also that the content in photochemically-produced hydroperoxides in suspended particulate matter is crucial to the induction of its autoxidation in water.

Autoxidation as a major player in the fate of terrestrial particulate organic matter in seawater

“…This work follows another preliminary study [ Galeron et al ., ] undertaken in order to corroborate the original hypothesis formulated by Rontani et al [] on the Mackenzie, stating that the intense autoxidation observed in the river plume could be catalyzed by the release of redox‐active metal ions in the estuarine mixing zone. The aforementioned study confirmed the effects of the presence of metal ions on autoxidation rates but also highlighted hydroperoxide content as a major driver of autoxidative processes in organic matter [ Galeron et al ., ].…”

“…The induction of these autoxidative processes was attributed to the presence of redox‐active metal ions desorbed from particulate matter in the estuarine mixing zone. This assumption was backed by a recent study on the effect of metals on autoxidation of SPM in vitro [ Galeron et al ., ].…”

“…This raises once more the question of what is inducing—or favoring—autoxidative processes in the salinity gradient of river plumes? The lesser role played by photooxidation and autoxidation in organic matter degradation during riverine transport in the temperate Mediterranean area, compared to polar areas, has an impact on the amount of hydroperoxides found in SPM, lately found to play a major role in the induction of autoxidation in coastal areas [ Galeron et al ., ]. Indeed, hydroperoxide levels are much higher at higher latitudes, where photooxidation acts much more intensely, producing hydroperoxide‐rich SPM later exported to coastal areas [ Rontani et al ., ].…”

“…Building upon the results obtained in the Mackenzie estuary and during in vitro experiments [ Galeron et al ., ], we attributed this induction of autoxidative processes to the desorption of metal ions—adsorbed onto SPM during riverine transport—in the estuarine mixing zone. Upon desorption, these redox‐active metal ions could have the potential to kick‐start autoxidation, by catalyzing homolytic cleavage of hydroperoxides already present in SPM and resulting from photooxidation and autoxidation processes during its transport.…”

“…In higher plant debris, homolytic cleavage (catalyzed by some metal ions, temperature, or UV radiations) [ Schaich , ] of hydroperoxides resulting from type II photosensitized oxidation seems to be at the origin of the initiation of free radical oxidation (autoxidation) processes [ Girotti , ; Rontani et al ., ; Galeron et al ., ]. Thanks to new specific lipid tracers recently identified [ Galeron et al ., , ; Rontani et al ., ], the monitoring of the autoxidative state of terrestrial higher plant‐derived organic matter in aquatic environments is now possible.…”