Abstract
Archaea are widespread in marine sediments and play important roles in the cycling of sedimentary organic carbon. However, factors controlling the distribution of archaea in marine sediments are not well understood. Here we investigated benthic archaeal communities over glacial-interglacial cycles in the northern South China Sea and evaluated their responses to sediment organic matter sources and inter-species interactions. Archaea in sediments deposited during the interglacial period Marine Isotope Stage (MIS) 1 (Holocene) were significantly different from those in sediments deposited in MIS 2 and MIS 3 of the Last Glacial Period when terrestrial input to the South China Sea was enhanced based on analysis of the long-chain n-alkane C31. The absolute archaeal 16S rRNA gene abundance in subsurface sediments was highest in MIS 2, coincident with high sedimentation rates and high concentrations of total organic carbon. Soil Crenarchaeotic Group (SCG; Nitrososphaerales) species, the most abundant ammonia-oxidizing archaea in soils, increased dramatically during MIS 2, likely reflecting transport of terrestrial archaea during glacial periods with high sedimentation rates. Co-occurrence network analyses indicated significant association of SCG archaea with benthic deep-sea microbes such as Bathyarchaeota and Thermoprofundales in MIS 2 and MIS 3, suggesting potential interactions among these archaeal groups. Meanwhile, Thermoprofundales abundance was positively correlated with total organic carbon (TOC), along with n-alkane C31 and sedimentation rate, indicating that Thermoprofundales may be particularly important in processing of organic carbon in deep-sea sediments. Collectively, these results demonstrate that the composition of heterotrophic benthic archaea in the South China Sea may be influenced by terrestrial organic input in tune with glacial-interglacial cycles, suggesting a plausible link between global climate change and microbial population dynamics in deep-sea marine sediments.
Highlights
Long-term carbon sequestration in the form of organic matter (OM) deposited in marine sediments plays an important role in climate regulation
The sediment core from the Pearl River Submarine Canyon region of the South China Sea (Figure 1) represented three Marine Isotope Stage (MIS), which were correlated with changes in sedimentation rate, total organic carbon (TOC) and the terrestrial n-alkane C31 (Figure 2)
The significant positive correlation between organic carbon and sedimentation rate (Supplementary Figure 3) suggests that the former is the more important, fine-grained and reducing environments could contribute to the preservation of organic matter (Müller and Suess, 1979)
Summary
Long-term carbon sequestration in the form of organic matter (OM) deposited in marine sediments plays an important role in climate regulation. An estimated 7.8 × 1022 grams of carbon are stored in marine sediments, including both terrestrial and marine sources (Mackenzie et al, 2004). The contributions of these distinct organic pools to marine sediment vary with climate, geologic time, and geographic location (Stein, 1990; Schubert and Calvert, 2001; Yamamoto and Polyak, 2009). Bathyal sediments below 2000 m water depth comprise the majority of the sea floor (Dunne et al, 2007) and are generally oligotrophic, with low organic carbon content and low sedimentation rates (Seiter et al, 2004; Dunne et al, 2007). Coastal sediments are commonly rich in organic matter, which can consume those electron acceptors within millimeters to centimeters of the sediment depth
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