Abstract
Iron reduction in subseafloor sulfate-depleted and methane-rich marine sediments is currently a subject of interest in subsurface geomicrobiology. While iron reduction and microorganisms involved have been well studied in marine surface sediments, little is known about microorganisms responsible for iron reduction in deep methanic sediments. Here, we used quantitative PCR-based 16S rRNA gene copy numbers and pyrosequencing-based relative abundances of bacteria and archaea to investigate covariance between distinct microbial populations and specific geochemical profiles in the top 5 m of sediment cores from the Helgoland mud area, North Sea. We found that gene copy numbers of bacteria and archaea were specifically higher around the peak of dissolved iron in the methanic zone (250–350 cm). The higher copy numbers at these depths were also reflected by the relative sequence abundances of members of the candidate division JS1, methanogenic and Methanohalobium/ANME-3 related archaea. The distribution of these populations was strongly correlated to the profile of pore-water Fe2+ while that of Desulfobacteraceae corresponded to the pore-water sulfate profile. Furthermore, specific JS1 populations also strongly co-varied with the distribution of Methanosaetaceae in the methanic zone. Our data suggest that the interplay among JS1 bacteria, methanogenic archaea and Methanohalobium/ANME-3-related archaea may be important for iron reduction and methane cycling in deep methanic sediments of the Helgoland mud area and perhaps in other methane-rich depositional environments.
Highlights
The involvement of microorganisms in electron-accepting processes (EAP) in marine sediments results in the formation of redox zones which may overlap with one another (Canfield and Thamdrup, 2009)
At the sulfatemethane transition (SMT) of marine sediments sulfate reducers belonging to the Desulfococcus/Desulfosarcina (DSS) group and methanotrophic archaea (ANME-1, 2, and 3) are often found in relatively higher proportion (Lloyd et al, 2006; Harrison et al, 2009; Knittel and Boetius, 2009) compared to deeper methanic zones
The observation of elevated concentrations of dissolved iron in the methanic zone of marine sediments has been a matter of interest in subsurface geomicrobiology for some time, and it is still not known which microbial population might be involved in the reduction of iron
Summary
The involvement of microorganisms in electron-accepting processes (EAP) in marine sediments results in the formation of redox zones which may overlap with one another (Canfield and Thamdrup, 2009). Jorgensen et al (2012) have recently shown that the depth-wise distribution of archaeal populations, Marine Group I, are linked to nitrate profiles of deep sediments from the arctic mid-ocean ridge, buttressing their suspected role in the nitrogen cycle (Durbin and Teske, 2011). Iron reduction is a major electron-accepting process in marine surface sediments and organisms known to be involved in this process have been studied (Roden and Lovley, 1993; Vandieken et al, 2006b; Nickel et al, 2008; Vandieken and Thamdrup, 2013). Based on most-probablenumber (MPN) cell counts, members of the Desulfuromonadales were the most abundant iron-reducing bacteria (65% of the total bacteria population) in surface sediments from Aarhus Bay (Vandieken and Thamdrup, 2013)
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