(A) Natural hydrous iron oxide aggregates found between 6.5 and 7.5 m in the water column (scale bar = 1 μm). The micrographs display intimate mixtures of organic fibrils naturally stained by iron oxides. (B) Silver stain on organic fibrils found at 7 m in the water column (scale bar = 200 nm). These fibrils consist of polysaccharides that may be naturally stained by hydrous iron oxides.