Formation and biogeochemical potential of ferromanganese oxide deposited on dead sponges

Ferromanganese oxides that coat dead marine biological remains such as porous sponges, fish bones, and cold-water corals are widespread in deep-sea environments. However, their mineralization mechanisms and the intrinsic connections between ferromanganese oxides and biological remains are poorly understood. In this study, we examine ferromanganese oxide coatings (FMC) on five dead sponge specimens collected from seamounts in the Central Basin of the South China Sea (SCS) and in the West Mariana Ridge (WMR). Sponge FMC are friable and loosely attached precipitates grown onto the reticulate skeletal structure and they have a narrow range of thicknesses. High-resolution microscopic observations show that sponge FMC are composed of porous, spheroidal, and relatively compact sheath-like Mn oxides with variable oxidation state, which possibly imply a different formation pathway from general ferromanganese deposits (FMD). New observations further reveal the position of sponge FMC spatially overlapping with sponge collagen fibers, as well as reveal widespread presence of microbial cells, Nitrogen-enriched OM with abundant amide groups, and nanoscopic apatite in sponge FMC. The observations provide a new insight into the precipitation of sponge FMC that is related to the decomposition of skeleton-associated proteins of the sponge. The degradation of organic matter contributes to localized enrichment of metal cations and micro-scale oxic-suboxic transitions. Meanwhile, small-molecule carbon compounds derived from protein decomposition are favorable to the activity of heterotrophic microorganisms, which modulates localized physicochemical conditions. These redox variations in microenvironments are proposed to trigger the oxidation and reduction of metal cations and their precipitation into FMC around sponge remains. While the composition of sponge FMC resembles that of hydrogenic crusts, their rare element and yttrium (REY) patterns display notable discrepancies with hydrogenic processes, including weak negative or significantly positive Ce anomaly. The discrepancies reflect the unique geochemical characteristics of sponge FMC, possibly attributed to differences in the activity of oxic-suboxic transitions and in the redox behavior of Mn and Ce metals. Biological remains in the deep-sea have a widespread occurrence and most of them similarly serve also as FMC substrates, which may point to a broader implication for marine polymetallic cycling than previously considered.