## 3 Experimental Results
### Microstructure and Micro-Area Composition of the Attached Marine Organism Remains
#### 3.3.3 Biological Overlapping Phenomenon between Coralline Algae and Red Coral  
The crusted surface of the coralline algae provided an ideal substrate for the attachment of red coral larvae, resulting in a biological overlapping phenomenon between the two organisms, as depicted in Figure 8. Figure 8a illustrates the distribution of pink-white, red coral skeletons, yellow coralline algae skeletons, and brick-red sediments from top to bottom. Figure 8b provides a close-up of the boxed region in Figure 8a. The group skeletons of red coral reached a thickness of up to 100 \(\upmu\)m and exhibited noticeable interspaces with the coralline algae. The coralline algae skeletons were approximately 50 \(\upmu\)m thick, with their lower edges tightly bound to the body surface of sample DL-3. Foreign pollutants were deposited within the pore channels of the coralline algae skeletons and the surface layer cracks of the body. Based on the contrast in the scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) data (see positions P7 and P8 in Table 3), these foreign pollutants were primarily iron rusts, which were deposited without reacting chemically with the body, resulting in the brick-red region shown in Figure 8a. Figure 7: Morphology of the red coral attached to the outer wall of sample DL-2: (**a**) optical microscopy (OM) image showing the cross-section of the red coral attached to the outer wall; (**b**) scanning electron microscopy (SEM) image of the close-up region indicated in (**a**), highlighting the section where red coral and residual glaze are present; (**c**) SEM image displaying the 3D network structure of the red coral skeletons; and (**d**) SEM image providing a close-up view of the region indicated in (**c**). The cross-sectional element mapping images, as shown in Figure 9, reveal a distinct layered structure. Specifically, the uppermost layer of red coral group skeletons was predominantly composed of calcium (Ca), magnesium (Mg), and oxygen (O), indicative of calcite skeletons formed through bio-mineralization. The foreign pollutants filling the cracks in the coralline algae skeletons and the body mainly consisted of iron (Fe), magnesium (Mg), and silicon (Si), which may also include iron rust, sea sand, sea mud, and sediments derived from Mg\({}^{2+}\) in seawater. The surface observation reveals a biological overlap between the red coral and coralline algae. Figure 10a shows that the inner wall of sample DL-3 is entirely covered with foreign pollutants and remains of organisms. Figure 10b highlights the remains of coralline algae skeletons on the surface of the pollutant sedimentary layer. It is speculated that coralline algae initially attached to the sedimentary layer of pollutants, continuously grew, and mineralized, thus covering a significant portion of the sediment. Subsequently, red coral accumulated and adhered to the crusted surface of the coralline algae, growing and forming its skeletons. Pollutants transported by seawater were further deposited on the coralline algae skeletons, filling the pore channels. External mechanical damage led to partial fragmentation and detachment of the coralline algae group skeletons, exposing the underlying pollutant sedimentary layer. Figure 8: Morphology of the cross-section of sample DL-3 where coralline algae and red coral are attached: (**a**) optical microscopy (OM) image showing three distinct layers of different colors; and (**b**) scanning electron microscopy (SEM) image of the boxed region in (**a**), highlighting the clear boundary between red coral and coralline algae, large interspaces, tight bonding between coralline algae and the body, and pollutants deposited in the microcracks of the coralline algae skeletons and the body. Figure 9: Cross-sectional element mapping images corresponding to the position shown in Figure 8b.