Page 62 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 3. Metal corrosion protection potential of methanogenic communities
investigated the community structure and metabolic potential of the natural CPL microbial community, the surrounding sediment more distant to the CPLs, and the top sediment in conjunction with the geochemical composition of the sediments. The microbial community of the deposit layer was studied in detail to unravel the microbial interactions that could potentially lead to the corrosion protection of natural CPLs.
Sediments are rich in organic matter and contain large amounts of calcium and magnesium oxides
Samples were taken in duplicate at the surface for bulk sediment (BS), at 3-m-deep adjacent sediments (AS), and deposit layers (DL) (see Fig. 4). Sediment properties were assessed by X- ray fluorescence and total carbon and sulfur analysis. All sediment samples at 0- and 3-m depths were rich in organic matter, with an average of 75.7%. The AS and DL differed in total C and S content: the sediment attached to the DL contained on average 13.6% C and 0.52% S, whereas AS contained on average 22.8% C and 0.85% S. This indicates a higher carbon turnover around the deposit layers. High percentages of CaO (average, 6.2%; normal soil value, 1%) and MgO (average 1.0%), the dominant cations in most soils and sediments, indicate that the system was buffered by groundwater fluxes (see Table S1 in the supplemental material) (Donahue, Miller and Shickluna 1977; Roy et al. 2006). Both cations are important in precipitative layer formation and thus crucial in CPL dynamics. Through changing alkalinity and carbon dioxide concentrations, calcium and magnesium can rapidly form, precipitating mineral oxide and carbonate complexes (Irving 1926).
Iron corrosion in freshwater ecosystems leads to the formation of iron-oxide complexes and iron carbonates
Under low sulfate conditions, iron oxidation mainly proceeds via the formation of ferrous hydroxide that is transformed into magnetite and hydrogen via the Schikorr reaction:
1) 3 Fe + 4H2OàFe3O4 + 4H2
In buffered, calco-carbonated freshwater systems corrosion additionally leads to the
formation of a wide variety of minerals, including wustite (FeO), limonite [FeO(OH)·nH2O], and the ferrous carbonates siderite (FeCO3) and chukanovite [Fe2(OH)2CO3] (Saheb et al. 2013; Kip et al. 2017). The minerals that are formed during biocorrosion in anoxic environments are
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