Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude

Marie-Aimée Galeron,Olivier Radakovitch,Bruno Charrière,Frédéric Vaultier,John K Volkman,Thomas S Bianchi,Nicolas D Ward,Patricia M Medeiros,Henrique O Sawakuchi,Suzanne Tank,Philippe Kerhervé,Jean-François Rontani

doi:10.1016/j.orggeochem.2017.10.013

Abstract

There exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autoxidative damage.

Highlights

Riverine particulate organic matter (POM), which consists in part of highly degraded residues from terrestrial higher plants, has long considered to be refractory compared with marine-derived POM (de Leeuw and Largeau, 1993; Wakeham and Canuel, 2006)
Re-examination of suspended particulate matter (SPM) samples from Rhône estuary In order to identify the processes causing the induction of autoxidation in coastal waters, total lipid extract (TLE) from different SPM samples previously collected (Galeron et al, 2017) from the Rhône and along a transect in its estuary (Fig. 1) were re-examined
Unusual profiles of C16:1 7 acid oxidation products dominated by 10-hydroxyhexadec-8(E)-enoic and 8hydroxyhexadec-9(E)-enoic acid were observed at stations R1 and R7 close to the river mouth (Fig. 2)

Summary

Introduction

Riverine particulate organic matter (POM), which consists in part of highly degraded residues from terrestrial higher plants, has long considered to be refractory compared with marine-derived POM (de Leeuw and Largeau, 1993; Wakeham and Canuel, 2006). Autoxidation, largely ignored for the coastal zone, proceeds by a radical chain reaction and acts mainly on organic compounds possessing C=C bonds or C-H bonds whose bond energy is relatively low (e.g. allylic, tertiary, to oxygen, etc.; Fossey et al, 1995). It can act on unsaturated lipids [e.g. sterols, unsaturated fatty acids (FAs), chlorophyll phytyl side chain, alkenes, tocopherols and alkenones; Rontani, 2012], and on amino acids (Seko et al, 2010), sugars (Lawrence et al, 2008) and polyphenols (Hathway and Seakins, 1957). It can affect biopolymers (Schmid et al, 2007) and kerogen, inducing ring opening and chain cleavage

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Discussion

Conclusion