Abstract
Hadal trenches have higher microbial carbon turnover rates as compared to adjacent abyssal plains. However, the source of organic carbon in the trench remains enigmatic. In this study, we show that a fraction of organic carbon is possibly derived in situ and correlated with chemoautotrophic communities supported by the fluid discharge of water-rock interaction in the trench wall, based on analysis of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, including archaeal isoprenoid GDGTs (IsoGDGTs) and bacterial branched GDGTs (BrGDGTs), in sediments and rocks of the Mariana and Yap Trenches, northwest Pacific Ocean. These trench sediments contained relative higher BrGDGTs ratios, which was a rare observation in the open ocean. The BrGDGT-to-IsoGDGT ratios ranged in 0.02–0.88 (mean = 0.10 ± 0.11) in sediments and 0.09–0.38 (mean = 0.17 ± 0.13) in altered rocks. The calculated values of branched and isoprenoid tetraether (BIT) index ranged from 0.02–0.73 (mean = 0.18 ± 0.11) in sediments and from 0.16–0.9 in altered rocks (mean = 0.37 ± 0.27). Moreover, these GDGTs exhibited similar characteristics to those of altered basalt rocks, indicating inputs of organic carbon from the trench subsurface environment. Thus, in addition to organic-rich material settling, we propose chemoautotrophic activity in oceanic crust could be an additional source of organic carbon in the deepest part of the ocean, with an important role in deep-sea carbon cycles.
Highlights
Hadal trenches, which are located on the axis of subduction zones, are covered by water depths ranging from 6,500 m to 11,000 m and represent some of the most remote and least-explored regions on Earth (Jamieson et al, 2010; Jamieson et al, 2017, 2018)
Several previous studies have found that relatively high branched GDGTs (BrGDGTs) content presented in some chemoautotrophic systems, such as cold seep (Zhang et al, 2020), hydrothermal fields (Hu et al, 2012; Lincoln et al, 2013; Pan et al, 2016), and serpentinite-hosted ecosystems (Newman et al, 2020). These findings indicated that the high relative abundance of BrGDGT in the sedimentary environment of Open Ocean may be an indicator of the chemoautotrophic ecosystem
BrGDGT was 3.87 ± 1.38 (n = 67) in the trench sediments (Supplementary Table 2), indicating little contribution of terrestrial soil inputs (Xiao et al, 2016). This conclusion was consistent with two recent studies, which reported that BrGDGTs in the Mariana trench were characterized by high cyclopentyl rings (Ta et al, 2019) and predominance of hexamethylated 6-methyl BrGDGT (Xiao et al, 2019)
Summary
Hadal trenches, which are located on the axis of subduction zones, are covered by water depths ranging from 6,500 m to 11,000 m and represent some of the most remote and least-explored regions on Earth (Jamieson et al, 2010; Jamieson et al, 2017, 2018). It has recently been revealed that there is a significantly higher microbial carbon turnover rate in hadal sediment than that in adjacent abyssal plain (Glud et al, 2013; Luo et al, 2018) This environment is known to Organic Carbon in Trench Environment sustain a diverse array of metazoan organisms (Jamieson et al, 2010) and heterotrophic microbial populations (Nunoura et al, 2015). The existence of fluid discharge features, including fluid discharge points and associated pockmarks, as well as chemosynthetic microorganisms, in the outer rise region was directly observed by submersible investigations in the southern Mariana trench (Du et al, 2019) Around these fluid discharge features, iddingsite-rich muds with high hydrogen and methane concentration were close association with augite, indicating the occurrence of iddingsitisation in these altered basalts (Du et al, 2019). The potential contribution of carbon fixation by these chemosynthetic microbial communities, which related to water-rock interaction, in the trench bottom remains enigmatic
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