Abstract
Marine fjords with active glacier outlets are hot spots for organic matter burial in the sediments and subsequent microbial mineralization. Here, we investigated controls on microbial community assembly in sub-arctic glacier-influenced (GI) and non-glacier-influenced (NGI) marine sediments in the Godthåbsfjord region, south-western Greenland. We used a correlative approach integrating 16S rRNA gene and dissimilatory sulfite reductase (dsrB) amplicon sequence data over six meters of depth with biogeochemistry, sulfur-cycling activities, and sediment ages. GI sediments were characterized by comparably high sedimentation rates and had “young” sediment ages of <500 years even at 6 m sediment depth. In contrast, NGI stations reached ages of approximately 10,000 years at these depths. Sediment age-depth relationships, sulfate reduction rates (SRR), and C/N ratios were strongly correlated with differences in microbial community composition between GI and NGI sediments, indicating that age and diagenetic state were key drivers of microbial community assembly in subsurface sediments. Similar bacterial and archaeal communities were present in the surface sediments of all stations, whereas only in GI sediments were many surface taxa also abundant through the whole sediment core. The relative abundance of these taxa, including diverse Desulfobacteraceae members, correlated positively with SRRs, indicating their active contributions to sulfur-cycling processes. In contrast, other surface community members, such as Desulfatiglans, Atribacteria, and Chloroflexi, survived the slow sediment burial at NGI stations and dominated in the deepest sediment layers. These taxa are typical for the energy-limited marine deep biosphere and their relative abundances correlated positively with sediment age. In conclusion, our data suggests that high rates of sediment accumulation caused by glacier runoff and associated changes in biogeochemistry, promote persistence of sulfur-cycling activity and burial of a larger fraction of the surface microbial community into the deep subsurface.
Highlights
Arctic fjords with marine-terminating glaciers constitute an important interface for freshwater and sediment influx from land into the sea, thereby influencing the physical and chemical conditions in the coastal marine ecosystems (Svendsen et al, 2002; Etherington et al, 2007)
Three dsrB-operational taxonomic units (OTUs) that positively correlated with sediment age were affiliated to the family
In the NGI sediments, steep gradients of sulfate reduction rates (SRR) (Figure 2) presented here and previously (Glombitza et al, 2015), indicated that most of the labile organic matter deposited from marine primary production was mineralized near the seafloor surface, which is typically observed for marine shelf sediments (Flury et al, 2016)
Summary
Arctic fjords with marine-terminating glaciers constitute an important interface for freshwater and sediment influx from land into the sea, thereby influencing the physical and chemical conditions in the coastal marine ecosystems (Svendsen et al, 2002; Etherington et al, 2007). Increased water turbidity in close proximity to the glacier can negatively influence surface water primary production (Etherington et al, 2007; Zajaczkowski, 2008), which leads to lower organic matter availability in the underlying sediments (Bourgeois et al, 2016). Fjord sediments receive significant amounts of terrigenous organic matter as evidenced by high C/N ratios of the sediment organic matter pool (Goñi et al, 2013; Wehrmann et al, 2014) With these large inputs of both marine and terrigenous organic matter, the ultimate role of glaciated fjord sediments in the global carbon cycle depends on the extent to which the large organic matter inputs are degraded, and there is a need to better understand constraints on microbial community structure and degradation potential
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