Abstract

Abstract. Elevated organic matter (OM) concentrations are found in hadal surface sediments relative to the surrounding abyssal seabed. However, the origin of this biological material remains elusive. Here, we report on the composition and distribution of cellular membrane intact polar lipids (IPLs) extracted from surface sediments around the deepest points of the Atacama Trench and adjacent bathyal margin to assess and constrain the sources of labile OM in the hadal seabed. Multiscale bootstrap resampling of IPLs' structural diversity and abundance indicates distinct lipid signatures in the sediments of the Atacama Trench that are more closely related to those found in bathyal sediments than to those previously reported for the upper ocean water column in the region. Whereas the overall number of unique IPL structures in hadal sediments contributes a small fraction of the total IPL pool, we also report a high contribution of phospholipids with mono- and di-unsaturated fatty acids that are not associated with photoautotrophic sources and that resemble traits of physiological adaptation to high pressure and low temperature. Our results indicate that IPLs in hadal sediments of the Atacama Trench predominantly derive from in situ microbial production and biomass, whereas the export of the most labile lipid component of the OM pool from the euphotic zone and the overlying oxygen minimum zone is neglectable. While other OM sources such as the downslope and/or lateral transport of labile OM cannot be ruled out and remain to be studied, they are likely less important in view of the lability of ester-bond IPLs. Our results contribute to the understanding of the mechanisms that control the delivery of labile OM to this extreme deep-sea ecosystem. Furthermore, they provide insights into some potential physiological adaptation of the in situ microbial community to high pressure and low temperature through lipid remodeling.

Highlights

  • The deep ocean has been classically considered a vast “biological desert” (Danovaro et al, 2003) due to the attenuation of organic matter (OM) fluxes with increasing depth (Wakeham et al, 1984; Martin et al, 1987; Hedges et al, 2001; Rex et al, 2006)

  • According to Xu et al (2018), the main sources of OM to the hadal zone include (1) the vertical sinking of particulate OM (POM), (2) the carrion falls of dead bodies, (3) inputs of terrestrial OM, (4) downslope transport of OM from continental slopes, and (5) in situ chemosynthetic production associated with cold seeps or hydrothermal vents

  • The Atacama Trench is located in the eastern tropical South Pacific (ETSP) along the Peru–Chile margin, and it underlies the eutrophic and highly productive Humboldt Current System (Angel, 1982; Ahumada, 1989), which includes the fourth largest oxygen minimum zone (OMZ) in the world (Schneider et al, 2006)

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Summary

Introduction

The deep ocean has been classically considered a vast “biological desert” (Danovaro et al, 2003) due to the attenuation of organic matter (OM) fluxes with increasing depth (Wakeham et al, 1984; Martin et al, 1987; Hedges et al, 2001; Rex et al, 2006). OM availability is considered the major factor controlling the abundance, biomass, and diversity of life in the deep ocean (Danovaro et al, 2003; Ichino et al, 2015), whereas hydrostatic pressure appears to be an important and additional factor controlling biological activity in hadal trench systems (Jamieson et al, 2010; Tamburini et al, 2013). POM fluxes measured at 4000 m in the North Pacific Subtropical Gyre reveal that a seasonal export pulse can exceed the mean annual flux by ∼ 150 % (Poff et al, 2021) It is unknown whether such pulses reach the hadal sediments (6000– 11 000 m). The abundance of prokaryotes in hadal depths can be influenced by dynamic depositional conditions (Schauberger et al, 2021), which in turn may be influenced by the intensity of propagating internal tides (Turnewitsch et al, 2014) All these factors likely alter the deposition, distribution, and composition of OM present in trench sediments

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