Introduction

Marine ceramics recovered from the sea serve as significant carriers of ancient maritime trade and cultural exchange. However, these artifacts have often been buried in complex marine environments, resulting in interfacial bio-geochemical degradation issues. This presents a substantial scientific challenge in the fields of cultural relic conservation and materials research. In recent years, advancements in underwater exploration and excavation technologies have led to the discovery of an increasing number of large shipwreck sites. Globally, significant discoveries from shipwreck sites continue to emerge. The “Belitung Shipwreck” (Tang Dynasty 618-907 CE) in Southeast Asian waters has yielded over 60,000 pieces of Changsha ware porcelain1. The Shinan shipwreck (Yuan Dynasty 1271-1368 CE) in South Korea produced more than 20,000 Longquan celadon pieces2. In China’s southeastern waters, major shipwreck sites excavated in recent years include the “Nanhai I“3, “Nan’ao I“4, “Dalian Island” shipwreck5, and “Xiaobaijiao I” shipwreck6, et al. These findings provide invaluable material evidence for research on the Maritime Silk Road.

During the Song and Yuan dynasties (960-1368 CE), the southeastern coastal region of China, particularly Fujian, emerged as a crucial hub along the Maritime Silk Road, facilitating global maritime trade and cultural exchange. The waters of the Haitan Strait in Fujian were a vital connection in this ancient network and now host some of the abundant underwater cultural heritage in China. Since 2005, more than 20 underwater cultural relic sites have been discovered in the Haitan Strait, dating from the late Tang Dynasty to the Qing Dynasty (755-1912 CE). Currently, the majority of recovered artifacts are ceramics7, many of which display significant bryozoan colonization and partial encrustation by mollusks on their surfaces.

In the field of marine bio-geochemical degradation of ceramics, most of the existing research focuses on identifying species within biological communities and examining the structural characteristics of both biological and secondary damage areas, drawing from biological and materials science perspectives. For example, Rao Ding et al.8 utilized OM and SEM-EDS, among other techniques, to reveal that the surface of Song Dynasty soy sauce porcelain from the Dalian Island shipwreck was primarily covered with a solitary coral and other biological deposits, while also characterizing its calcareous mineral composition. Similarly, Bao Chunlei et al.9 identified the biological deposits on the surface of the bluish-white porcelain recovered from the “Huaguang Reef I” shipwreck were identified as coral and shell through SEM-EDS, XRF, and other detection methods. Additionally, Liu Xiaoqing10 and Zhang Huan11 examined the biological deposits on ancient ceramic relics from the “Nan’ao I “ shipwreck, identifying various species such as tubeworms, corals, shells, and barnacles, and provided characterizations of these deposits. However, the mechanisms of bio-geochemical degradation arising from the synergistic interaction between bio-sediment and microorganisms are still not fully understood. Biological adhesion not only leads to mechanical damage5 of the ceramic glazes but also induces localized electrochemical corrosion8 through the influence of metabolites and mineralization processes. These factors significantly accelerate the deterioration of the ceramic matrix. The interplay between biomineralization and microorganisms during the attachment process poses considerable challenges to the long-term preservation of cultural relics.

This study examines the Yue kiln celadon (FL-3) retrieved from the Fenliuweiyu shipwreck, situated in the Haitan Strait off the coast of Pingtan, Fujian. This location is along a crucial segment of the Maritime Silk Road. Archaeological evidence indicates that Yue kiln celadon from the late Tang and Five Dynasties periods (875-979 CE) is relatively scarce in southern Fujian, Guangdong, and Hainan. In contrast, significant quantities have been found on Indonesian islands such as Java and Sumatra12. The discoveries from the Intan shipwreck (930–970 CE) and the Cirebon wreck13 allow us to reconstruct the export route: from the Yue kiln sites through Mingzhou Port, Pingtan Fenliuweiyu, Xisha Islands, and then to Java.

The Yue kiln was established during the Eastern Han dynasty (25-220 CE) and thrived during the Tang and Five Dynasties (618-960 CE). It is renowned for its “Mise porcelain” (secret-color ware). Classical texts such as Lu Yu’s “The Classic of Tea”14 and Lu Guimeng’s “Secret-Colored Yue Ware”15 commend its distinctive “emerald hues from a thousand peaks.” This investigation into its biodeterioration carries significant implications for the conservation of southern celadon systems.

This study comprehensively analyzes the composition, spatial distribution, and microstructure of bio-geochemical degradation products resulting from bryozoans, barnacles, red coral, and Rhizocaulus on the celadon interface. Employing advanced characterization techniques such as SEM-EDS, XRD, and ATR-FTIR to investigate these factors in detail. Furthermore, this study assesses the potential role of the biofilm community in bio-geochemical degradation by integrating microbiome techniques into the analysis.

Methods

Research object

This study examines a Yue kiln celadon bowl (Artifact number: FL-3) recovered from the Five Dynasties shipwreck at Fenliuweiyu, located in the southern waters of the Haitan Strait in Pingtan County, Fujian Province. The shipwreck site is ~100 m north of Fenliuweiyu. The site was first discovered in late 2009 when looters were caught illegally salvaging artifacts. Following this, the Fujian Underwater Cultural Heritage Archaeological Survey Team conducted investigations from April to late May 2010, utilizing information provided by local fishermen. The seabed at the site primarily consists of a silt-sand substrate, with some localized areas of pure sand deposits. In proximity to Fenliuweiyu, there are sections of exposed bedrock. All artifacts recovered from the site were porcelain, predominantly celadon bowls and dishes16.

The specific celadon bowl analyzed in this study was collected in June 2024 from the shipwreck site, ~100 m north of Fenliuweiyu, at a depth of 11.6 m, and stored under frozen conditions at −20 °C. Archaeological evidence suggests that the Fenliuweiyu shipwreck dates to the Five Dynasties period, specifically during the early to middle Wuyue Kingdom era (mid-10th century CE), with the celadon artifacts dating between 939 and 942 CE16.

The bowl presents a standardized form with minor rim damage. It is characterized by a flared mouth, a slightly curved inner base, and a flat outer base. Crafted from fine, dense gray clay, the bowl has a thin body. The glaze is predominantly grayish-green, with some areas exhibiting greenish-brown tones and densely distributed micro-black spots. Its dimensions are as follows: a rim diameter of 16.2 cm, a foot diameter of 8.3 cm, and a height of 9 cm. The rim consists of five fragmented sections. Both the interior and exterior surfaces of the bowl are extensively colonized by a variety of marine organisms. The exterior features significant populations of bryozoans, red coral, barnacles, coralline algae, tube worms, and Rhizocaulus colonies. The interior surface contains large patches of bryozoans, along with red coral, barnacles, and sparse tube worms (Fig. 1). Of particular interest are the barnacles, red coral, Rhizocaulus, and bryozoans, which display pronounced bio-geochemical degradation on the glaze surface, making these areas the primary focus of the study.

Fig. 1: Yue Kiln celadon bowl (FL-3) from the Five Dynasties shipwreck at Fenliuweiyu.
figure 1

a An Interior view of the celadon bowl and rim fragments. b Exterior view of the celadon bowl and rim fragments.

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Sample pre-treatment and testing methods

Prior to sample pretreatment, initial observations were conducted under optical microscopy (OM) to characterize the specimen’s surface condition. Subsequently, to investigate the bio-geochemical degradation of celadon glaze surfaces caused by biological deposits, the marine biological deposits on the surface of the artifact samples were first removed using physical methods. The specific procedure was as follows: sterile scalpels and cotton swabs, along with warm water (40–50 °C), were used to gently scrape or wipe off the deposits. During this process, the operating force and temperature were strictly controlled to avoid mechanical damage to the artifact surface. Subsequently, the bio-geochemical degradation state of the deposit-attached surface and the celadon glaze was examined microscopically. Multiple instruments were then employed to analyze the corroded regions.

The surface morphological characteristics of biofouling residues and bio-geochemical degradation products on the celadon ware interface were analyzed using an XTZ-4KHD optical microscope (OM) from Shanghai Optical Instrument Factory in China. Comparative analysis of pre- and post-treatment surface morphology was conducted as shown in Fig. 2. This analysis provided macroscopic bio-geochemical degradation features and interfacial bonding information between the biological deposits and the celadon glaze surface.

Fig. 2: Optical micrographs showing the distribution characteristics of biodeposits and corroded areas on the celadon surface of the glaze.
figure 2

a Comparative OM before and after barnacle removal. b Comparative OM before and after red coral removal. c Comparative OM before and after Rhizocaulus organism removal. d Comparative OM before and after bryozoan removal. e Imaging area of OM on the sample surface.

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The microscopic morphology and elemental distribution of the corroded regions on the sample were analyzed using a Thermo Scientific Verios G4 US field-emission scanning electron microscope from FEI Company (U.S.A.), equipped with an Oxford Instruments 80 mm2 energy-dispersive X-ray spectrometer (EDS). Small fragments and fresh cross-sectional sample were extracted from the celadon shard. Then the samples were coated with a 3 nm layer of platinum (Pt) before the analysis. The instrument operated at voltages of 2 kV and 10 kV, with a working distance ranging from 4.6 mm to 5.2 mm, yielding secondary and backscattered electron images to analyze the surface morphology, providing critical evidence for identifying the bio-geochemical degradation products.

The identification of the crystal structures of the bio-geochemical degradation products on the celadon ware samples was non-destructive and was analyzed using the Bruker D8 Discover micro-focus two-dimensional X-ray diffractometer (XRD) manufactured by Bruker in Germany. Its maximum tube voltage was 60 kV, the maximum tube current was 80 mA, the power of the X-ray tube was 2.2 kW (with a Cu target), and the angle reproducibility was (±0.0001°). The Vantec 500 two-dimensional area detector was used, with a test spot diameter of 0.5 mm and an integration time of 1200 s.

Under ultraviolet light with a wavelength of 365 nm, residual biofluorescence on the sample surface was observed. To assess the bio-geochemical degradation layer and the biological deposits adhered to the surface of the glaze, a Thermo Scientific Nicolet iS50 attenuated total reflectance Fourier-transform infrared spectrometer (ATR-FTIR, USA) was used. Micro-destructive testing was performed on minute powder samples carefully collected from the bio-geochemical degradation area. The test range of the spectrometer is from 400 to 4000 cm−1, with a spectral resolution of better than 4 cm−1, and a wavenumber accuracy of better than 0.01 cm−1, with 32 scans per measurement.

Sediment samples collected from the Haitan Strait underwent 16S rRNA amplicon sequencing and a comprehensive bioinformatics analysis at Novogene Co., Ltd. in Beijing, China. The analysis process unfolded as follows: initially, genomic DNA was extracted from the sediment samples using a commercial DNA extraction kit. The quality of the extracted DNA was assessed through amplification detection. After passing quality control, the PCR products were pooled and purified through several steps, including end repair, A-tailing, adapter ligation, and magnetic bead-based size selection. The purified products were then normalized and utilized to construct sequencing libraries, which were subsequently sequenced on the Illumina platform. In bioinformatics processing, the raw sequencing reads were merged using FLASH to create Raw Tags. These Raw Tags were then processed with Cutadapt to identify and trim residual primer sequences. Quality filtering was conducted using fastp to obtain clean tags. To maintain data integrity, chimeric sequences were identified and removed by aligning the Clean Tags against the Silva reference database, resulting in Effective Tags appropriate for downstream analysis. Denoising was performed using the DADA2 module or deblur within the QIIME2 pipeline to derive ASVs. Taxonomic annotation of the ASVs was subsequently carried out using the Silva 138.1 database, yielding species abundance tables across multiple taxonomic levels, including Kingdom, Phylum, Class, Order, Family, Genus, and Species. In this study, the composition of the microbial community was primarily analyzed and interpreted at the phylum and class level.

Results

Optical microscopy analysis of bio-geochemical degradation morphologies

Optical microscopy revealed distinct bio-geochemical degradation morphologies on the celadon glaze surface induced by four types of marine biofoulings (Fig. 2e). The residual black patches left by barnacles exhibited irregular cloud-like patterns with serrated edges (Fig. 2a). Red coral remnants displayed characteristic dendritic structures, where yellowish deposits extended along coral skeletal branches, forming radial textures (Fig. 2b). The bio-geochemical degradation features caused by Rhizocaulus colonies were particularly distinctive, manifesting as parallel-aligned linear striations with uniform spacing and consistent orientation (Fig. 2c). Bryozoan-affected areas demonstrated unique honeycomb-like pit arrays, where each bio-geochemical degradation pit maintained a regular circular shape with periodic spatial distribution (Fig. 2d).

Microstructural morphology and compositional analysis of bio-geochemical degradation products in different regions

The combined analysis of microscopic morphology and elemental distribution revealed distinct characteristics of bio-geochemical degradation on the surface of the glaze induced by marine biofouling, which varied by type. In this study, four representative biological deposits are categorized as Ba (Barnacle), RCo (Red coral), Rh (Rhizocaulus), and Br (Bryozoan). The analysis encompassed both area scanning regions, designated as M, and point scanning regions, labeled as P. The results indicated that each type of biological residue produced unique bio-geochemical degradation morphologies.

SEM-EDS analysis revealed that the primary elemental composition of the uncorroded glaze layer (Fig. 4a, G-1) consisted of Si (28.2%), Al (10.8%), and Ca (2.3%), along with trace elements such as Mg (0.1%), Fe (0.3%), and K (0.1%). The body region (Fig. 6f, B-1) was primarily composed of Si (30.9%), Al (6.5%), and Ca (16.3%), accompanied by elements including Mg (1.8%), Fe (1.7%), K (1.8%), and Mn (0.3%). Having characterized the original elemental composition of both the glaze and the body, the analysis focused on the black bio-geochemical degradation areas resulting from barnacle (Ba) attachment (Fig. 2a). The area were enriched with organic membranes and sieve-like diatom remnants (Fig. 3). EDS analysis (Table 1) indicated that in the Ba-M1 bio-geochemical degradation region (Fig. 3a), elements C, N, O, and S showed correlations, with co-distribution characteristics of Na and Cl elements. The diatom remnants were enriched with Si, Fe, and I elements. The Ba-P1 and Ba-P2 regions (Fig. 3b) exhibited high organic content (C: 37.6%, 38.8%; N: 7.6%, 13.4%) along with S (0.7%, 1.1%). The sieve-like diatoms (Ba-P3) distributed in this region (Fig. 3c) showed the presence of Si (19.5%) and Fe (1.1%). XRD analysis confirmed the presence of pyrite (FeS2) (Fig. 7a, d).

Fig. 3: SEM images of the bio-geochemical degradation regions on the surface of the glaze caused by barnacles.
figure 3

a Barnacles-induced bio-geochemical degradation area (M1: mapping area). b Magnified image of box b in (a). c Sieve-like diatoms in the selected region of box c in (a).

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Table 1 Elemental composition of original celadon glaze/body and four types of biofoulings in the bio-geochemical degradation areas of celadon glaze (Wt%)
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SEM analysis of the yellow bio-geochemical degradation regions caused by red coral (RCo) on the glaze surface (Fig. 2b) revealed the formation of stratified bio-geochemical degradation pits with distinct morphological features (Fig. 4a). The pit peripheries and interiors contained filamentous organic membranes (Fig. 4b) exhibiting characteristic elemental compositions (RCo-P1: C: 30.1%, N: 9.0%, O: 33.1%) with detectable sulfur content (1.0%). The pit bases displayed rod-like (RCo-P2) and granular crystals (RCo-P3) encapsulated by organic matrices (RCo-P4), showing enrichment of calcium (16.7%, 26.5%) and phosphorus (5.6%, 12.0%) (Fig. 4c). XRD phase identification confirmed the presence of hydroxyapatite (Ca5(PO4)3(OH)) and aragonite (CaCO3) (Fig. 7b, d), with the latter representing characteristic biomineralized products of coral skeletons. EDS detected an iodine enrichment of 2.9% in the RCo-P4 region.

Fig. 4: SEM images of bio-geochemical degradation areas on the glaze surface caused by red coral residues.
figure 4

a Red coral-induced bio-geochemical degradation pits. b Magnified image of box b in (a). c Rod crystals in pits.

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SEM analysis of the brown bio-geochemical degradation area caused by Rhizocaulus (Rh) on the glaze surface (Fig. 2c) reveals the accumulation of Na and Cl crystals (Rh-P1, Rh-P2) (Fig. 5a, b), as well as the presence of bio-geochemical degradation pits (Fig. 5c). Additionally, the surface of the bio-geochemical degradation area exhibits flaky organic films (Fig. 5d) and remnants of sieve diatoms (Fig. 5e). EDS analysis indicated that the primary components of the organic film identified in the pits (Rh-P3) consisted of carbon (C) at 31.7%, nitrogen (N) at 12.0%, and oxygen (O) at 27.8%, along with trace amounts of sulfur (S) at 0.7%, iron (Fe) at 2.4%, and iodine (I) at 1.6%. Importantly, phosphorus (P) was absent in this area, which presents a contrasting perspective with the bio-geochemical degradation zone observed in red coral.

Fig. 5: SEM images of the glaze surface bio-geochemical degradation areas induced by Rhizocaulus.
figure 5

a The glaze surface reveals the accumulation of Na and Cl crystals. b Magnified image of the Na and Cl crystals in box b in (a). c Rhizocaulus-induced bio-geochemical degradation pits. d Magnified image of flaky organic films of box d in (c). e Magnified image of remnants of sieve diatoms of box e in (c).

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The black bio-geochemical degradation area is predominantly characterized by bryozoan (Br)-induced pits (Fig. 2d), which exhibit greater depth compared to other biofouling-affected regions. The pits (Fig. 6a–e) are abundant in diatom debris (Br-P1, Si: 10.9%; Fe: 21.6%) and mineral crystals (Br-P2, Fe: 38.2%; Mn: 2.1%; S: 1.3%). Cross-sectional observations (Fig. 6f–j) further reveal the presence of iron-rich and iron-manganese spherical particles (Br-P3, Br-P4) within the bio-geochemical degradation pit area. XRD analysis confirmed the predominant presence of goethite (α-FeO(OH)), pyrolusite (MnO2), along with a small quantity of pyrite (FeS2) (Fig. 7c-d). Furthermore, the round and flaky calcareous algae (Br-P5, Br-P6) attached to the bio-geochemical degradation zone displayed an elevated iron content, measuring 30.4% and 23.5%.

Fig. 6: SEM images of bio-geochemical degradation regions caused by bryozoans.
figure 6

a–e The glaze surface. f–j The glaze cross-section. a Bryozoans-induced bio-geochemical degradation pit on the surface of the glaze. b Magnified image of diatom debris of box b in (a). c Magnified image of box c in (a). d The image of mineral crystals in the pit. e Magnified image of the mineral crystals of box e in (d). f Bryozoans-induced bio-geochemical degradation pit on the glaze cross-section. g Magnified image of box g in (f). h Magnified image of iron-rich and iron-manganese spherical particles of box h in (f). i Magnified image of iron-rich and iron-manganese spherical particles of box i in (f). j the image of round and flaky calcareous algae.

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Fig. 7: XRD patterns of bio-geochemical degradation regions on the glaze surface induced by three different biofouling.
figure 7

a XRD pattern of the bio-geochemical degradation region caused by barnacles. b XRD pattern of the bio-geochemical degradation region caused by red coral. c XRD pattern of the bio-geochemical degradation region caused by bryozoans. d XRD measurement areas.

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The analysis indicates that barnacles (Ba) adhering to bio-geochemical degradation areas primarily consist of organic matter, including carbon (C), nitrogen (N), oxygen (O), and sulfur (S), along with significant deposits of pyrite (FeS2). In regions where red coral (RCo) attaches, hydroxyapatite (Ca5(PO4)3(OH)) forms, along with notable bio-accumulation of element I. The bio-geochemical degradation areas associated with Rhizocaulus (Rh) display characteristics typical of organic film adhesion, made up of C, N, and O. Bryozoans (Br) contribute to the most severe bio-geochemical degradation, resulting in deep pits and generating an abundance of iron-manganese oxide mineral deposits, which include goethite (α-FeO(OH)), pyrolusite (MnO2), and a smaller amount of pyrite (FeS2).

Organic composition analysis of bio-geochemical degradation features on glaze surfaces

Blue fluorescence was observed in the bio-geochemical degradation areas of the celadon glaze surface when subjected to 365 nm UV irradiation. This phenomenon, particularly pronounced in the cross-section of the glaze-matrix. According to relevant literature, the blue fluorescence may be associated with microbial metabolites or the proteins in EPS components17. This offers valuable insights into the interaction between biofouling and the glaze. As depicted in Fig. 8, the characteristics of the fluorescence signals are notable: the fluorescence areas closely correspond with the spatial distribution of biofouling, a pronounced fluorescence reaction may evident in the bio-geochemical degradation regions of the glaze (Fig. 8b, c), and filament-like fluorescence can also be detected in these areas of bio-geochemical degradation (Fig. 8a).

Fig. 8: Under 365 nm UV irradiation, fluorescence on the surface and glaze-matrix cross-section of celadon.
figure 8

a Arrows indicate fluorescence response of the biodeposits on the surface of the glaze. b Arrows indicate fluorescence response of the biodeposits at the glaze-matrix cross-section.

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To perform a detailed analysis of the molecular composition and structural characteristics in the blue fluorescence region, we employed attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). This method was used to characterize the organic bio-geochemical degradation products that developed on the surface of the celadon glaze due to four types of biofouling: barnacles, red coral, Rhizocaulus, and bryozoans, as depicted in Fig. 9.

Fig. 9: ATR-FTIR spectra of four types of biofouling on the surface of the celadon glaze and corresponding corroded areas.
figure 9

a ATR-FTIR spectra of four types of biofoulings on the surface of the celadon glaze. b ATR-FTIR spectra of corresponding corroded areas on the glaze surface.

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According to relevant literature, the N-H stretching vibration peak of the amide A band18 is 3538–3328 cm−1. The O-H stretching vibration peak is between 3290–3257 cm−1, while the C-H stretching vibration peak of methylene (-CH2) occurs at 2922–2917 cm−1. The C-H stretching vibration peak for methyl (-CH3)19 is 2852–2849 cm−1. The C=O stretching vibration peak of the amide I band appears between 1686–1619 cm−1. The peaks at 1558–1538 cm−1 correspond to out-of-phase N-H bending and C-N stretching vibrations of the amide II band, indicating the presence of protein18. C-H bending vibration peaks of (-CH2) are noted at 1466 and 1457 cm−1, while the symmetric stretching vibration peaks of the carbonate ν3 band are between 1418–1402 cm−1. The N-H bending and C-N stretching peaks for the amide III band20 at 1350 cm−1 and 1348 cm−1. The peaks at 1107–1003 cm−1 include C-O, C-C, C-O-C, P-O-C, and other functional groups associated with extracellular polymer substances (EPS) and characteristic for Si–O stretching vibrations21,22. The symmetric stretching vibration peaks of phosphate are found at 938 cm−1 and 941 cm−1, while the out-of-plane bending vibration peaks of the carbonate ν2 band23 lie between 872–870 cm−1. The Si-O symmetric stretching vibration peak of silicate is in the range of 796–772 cm−1, the tensile vibration of S-S and Fe-S bonds is at 685–602 cm−1, and the stretching vibration peak of metal oxides is at 532–531 cm−1. Lastly, the stretching vibration peak of sulfide (S2−) occurs within 455–420 cm−1, and the tensile vibration peak of Fe(II)-S24 is between 416–407 cm−1 (Table 2).

Table 2 ATR-FTIR peaks identification for four biofouling species and associated bio-geochemical degradation areas on celadon glaze
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The organic structure of the celadon glaze surface comprises four biofouling layers, each exhibiting distinctive peaks associated with protein content. These peaks include N-H stretching vibrations at 3538–3328 cm−1, the amide I band at 1686–1619 cm−1, and the amide II band at 1558–1538 cm−1. Additionally, red coral, Rhizocaulus, and bryozoans demonstrate lipid characteristics at 2922–2849 cm−1, suggesting biofilms are enriched with lipid components. Notably, the correlation peak for extracellular polymeric substances (EPS) and silicon is especially pronounced in red coral and Rhizocaulus, appearing in the range of 1107–1003 cm−1. Furthermore, carbonate peaks at 1418–1402 cm−1 and silicate peaks at 796–772 cm−1 are predominantly present in bryozoans and red coral, while a phosphate peak at 938–941 cm−1 is identified in bio-geochemical degradation products (Fig. 9a).

The black areas caused by barnacles and bryozoans showed typical protein degradation characteristics, showing that the amide band I (1644 cm−1) and band II (1538 cm−1) vibration peaks remained but weakened, accompanied by sulfide deposition (413 cm−1). Ca5(PO4)3(OH) deposition dominated the yellow region formed by red corals (941 cm−1) while retaining protein characteristic peaks (amide I and II bands), and the yellow chromogenic substances were identified as carotenoids. This bio-geochemical degradation characteristic was closely related to the metabolites and mineralization ability of red corals25. The brown zone caused by Rhizocaulus shows a complex characteristic of protein peak (1653 cm−1) and sulfide enrichment (442 cm−1) (Fig. 9b).

In summary, the primary constituents of bio-gel related to biofouling comprise proteins, lipids, and EPS. However, the distribution and concentration of functional groups vary among different species. Characterization of products within the relevant bio-geochemical degradation area indicates that degradation of proteins and deposition of inorganic salts have occurred.

Analysis of the composition of microbial communities in underwater burial environments of the celadon

Celadon samples have been immersed in the sludge of the Haitan Strait for an extended period, and the microbial flora present in this sludge has a significant impact on the bio-geochemical degradation processes affecting sedimentary organisms on the glaze surface of the celadon. This study focused on microbial detection of the celadon samples based on several scientific considerations. Firstly, the celadon samples have been buried for hundreds of years, with their bio-geochemical degradation primarily influenced by silt and microorganisms found in the buried waters. As a reservoir of microbial metabolites, silt offers a more comprehensive reflection of the functional characteristics of bacteria over long periods. Secondly, changes in environmental conditions following the excavation of the celadon samples have altered the microbial species at the interface between biological sediments and the celadon. Consequently, this study primarily involved the collection of celadon coating sludge, with frozen preservation for microbial species identification, to explore and elucidate the interactions between microorganisms and the glaze.

High-throughput sequencing results reveal the bacterial community structure in Haitan Strait mud at the phylum level (Fig. 10a). Proteobacteria were dominant, with a relative abundance of 35%. Crenarchaeota (14%), Desulfobacterota (10%), and Actinobacteria (9%) were also abundant. Acidobacteriota and Bacteroidota each accounted for 4%. Other phyla, such as NB1-j, Chloroflexi, Gemmatimonadota, and Fusobacteriota, made up 5% of the community, indicating a low-abundance long-tail distribution.

Fig. 10: Microbial community composition at the phylum and class level in sediments associated with celadon artifacts from the Haitian strait.
figure 10

a Microbial community composition at the phylum level. b Microbial community composition at the class level.

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To provide greater detail regarding the functional structure of the community, the bacterial composition was further analyzed at the class level, as shown in Fig. 10b. Gammaproteobacteria represented the dominant class with a relative abundance of 25%, while Nitrososphaeria was the next most abundant at 13%. Alphaproteobacteria (11%), Acidimicrobiia (9%), Bacteroidia (4%), Desulfobulbia (4%), and Desulfuromonadia (3%) were also present in notable proportions. Additional classes, including Desulfobacteria, Thermoanaerobaculia, and Syntrophobacteria, each contributed 2%.

Based on previous studies, Proteobacteria are widely involved in sulfide formation, with several of their classes (e.g., Gammaproteobacteria and Alphaproteobacteria) playing indirect roles in the transformation of low-valent sulfides through the oxidation of sulfides or intermediate sulfur compounds26. Desulfobacterota, a key group of sulfate-reducing bacteria, exhibited a relative abundance of 10%. Further identification at the class level revealed the presence of Desulfobulbia and Desulfuromonadia, which, together with Proteobacteria, suggest the likely presence of an active sulfur cycle in this environment.

Discussion

The attachment process of marine organisms to the surface of the celadon glaze follows a distinct pattern of ecological succession that requires long-term observation for validation. Based on the results of morphological and compositional analysis from this study, the following stages can be hypothesized:

The biofouling process on marine surfaces can be divided into two critical phases. The first is the microfouling stage, primarily involving the attachment of bacteria and microalgae. The second is the macrofouling stage, encompassing the organisms observed in this study, such as barnacles, red coral, Rhizocaulus, and bryozoans27,28. This study highlights a strong correlation between the biofilm formation process and the organic residues found on the celadon glaze surface. SEM-EDS analysis reveals a significant enrichment of C, N, O, and S elements at all four biological attachment sites on the surface of the glaze, indicating the presence of an organic surface coating. Additionally, ATR-FTIR spectroscopy identifies characteristic peaks associated with proteins (ranging from 1686 cm−1 to 1538 cm−1) and extracellular polymeric substances (~1003 cm−1), suggesting that these organic components likely originate from marine biofouling residues. Based on the combined analytical data and supporting literature, the formation mechanism can be explained as follows:

In the initial stage (Fig. 11), dissolved organic matter (e.g., polysaccharides and proteins) in seawater forms a conditioning film on the glaze surface through physicochemical interactions, altering the surface’s physical and chemical properties. This provides a critical interfacial condition for subsequent microbial attachment29,30. As the conditioning film develops, bacteria and microalgae adhere to the surface of the glaze. These microbes grow and proliferate by secreting extracellular polymeric substances (EPS), forming a biofilm. With the maturation of the biofilm, the microorganisms establish strong bonds with the glaze surface through covalent bonds, hydrogen bonds, and hydrophobic interactions, leading to the thickening and stabilization of the biofilm31. This stable microbial community not only modifies the local micro-environment but also releases chemical signaling molecules that attract macro-organism such as barnacles, red coral, Rhizocaulus, and bryozoans larvae and spores to settle and grow32. The protein- and polysaccharide-rich organic film detected under all four types of macroscopic bio-residues in this study (Figs. 3–6; Table 1) constitutes definitive evidence of this biofilm formation process and lays the foundation for the subsequent development of differentiated bio-geochemical degradation microenvironments.

Fig. 11
figure 11

Schematic representation of biofouling succession stages.

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Macrofouling organisms (Barnacles, Red coral, Rhizocaulus, and Bryozoans) continuously secrete extracellular polymeric substances (EPS) to form biofilms, providing a substrate for microbial colonization. The distinct micro-environments created by different macro-organisms during growth attract specific microbial communities from surrounding sediments, whose biomineralization products and metabolic activities collectively corrode the glaze.

Microbial analysis of sediment from the Haitian Strait shows that the mud is mainly inhabited by Proteobacteria, Desulfobacterota, and Actinobacteria, among others. Literature and bio-geochemical degradation product analyses from this study indicate that certain functional groups within Proteobacteria may be involved in the bio-geochemical degradation on the surface of the celadon glaze.

Sulfate-reducing bacteria (SRB) are primarily in the classes of Desulfobulbia and Desulfuromonadia within the phylum of Desulfobacterota33. Manganese-oxidizing bacteria (MOB) are prevalent in the class of Alphaproteobacteria and Gammaproteobacteria within the phylum of Proteobacteria34. Iron-oxidizing and iron-reducing bacteria (IOB) are mainly found in Gammaproteobacteria35.

Microbial extracellular polymeric substances (EPS) create biofilms on the surface of the glaze, which facilitates the attachment of macrofouling organisms and drives bio-geochemical degradation by modulating the micro-environment. Functional groups, such as carboxyl and hydroxyl, in EPS, can chelate metal ions like Fe3+ and Ca2+, promoting their enrichment. Under anaerobic conditions, SRB generated H2S and organic acids, lowering local pH, accelerating silicate network dissolution and alkali metal ion leaching (Na+, K+, Ca2+)36. This biochemical coupling exhibits species-dependent variations.

Barnacles produce hydrophobic proteins and polysaccharides that create an internal anoxic micro-environment, ideal for sulfate-reducing bacteria (SRB). These bacteria metabolize and release hydrogen sulfide (H2S) as a by-product, which then reacts with dissolved ferrous iron (Fe2+) from the glaze layer to form iron sulfide. The cell membranes of SRB serve as an ion matrix for the precipitation of iron sulfide and act as nucleation sites for crystal growth. Subsequently, the iron sulfide reacts with the H2S released by the SRB to yield pyrite (FeS2)37, which deposits on the celadon glaze surface (Fig. 12a).

Fig. 12: Schematic representation of synergistic bio-geochemical degradation process induced by elements and microorganisms in marine environments.
figure 12

a Bio-geochemical degradation process on the surface of the glaze affected by barnacle attachment. b Bio-geochemical degradation process on the surface of the glaze affected by red coral attachment. c Bio-geochemical degradation process on the surface of the glaze affected by Rhizocaulus attachment. d Bio-geochemical degradation process starting on the surface of the glaze affected by bryozoan attachment.

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Red corals sustain elevated oxygen concentrations within their dendritic porous skeletons, where sulfides are not produced. They actively absorb calcium ions (Ca2+) from seawater via specialized physiological mechanisms and obtain phosphate ions (PO43−) through their metabolism and the role of symbiotic microorganisms. The acidic proteins secreted from the coral’s surface serve as organic templates. These proteins possess carboxylic acid groups (-COO−) that specifically bind to calcium ions, thereby establishing an ordered interface for apatite nucleation. Within the weakly alkaline micro-environment created by the coral’s mucus, amorphous calcium phosphate (ACP) is initially deposited and is then gradually converted into hydroxyapatite (Ca5(PO4)3(OH)), which exhibits higher crystallinity38 (Fig. 12b).

Rhizocaulus attachment structures create physical barriers that enrich diatoms and foster SRB and iron-oxidizing bacterial growth. The extracellular polymeric substances (EPS) secreted by Rhizocaulus are abundant in proteins and polysaccharides. Their functional groups facilitate the adsorption of sodium (Na+) and chloride (Cl−) ions from seawater through ion exchange. Moreover, the Rhizocaulus characteristics of EPS reduce water diffusion39, encourage localized evaporation, and lead to the supersaturated precipitation of salts. Consequently, NaCl crystals are more likely to accumulate on the surface of the glaze compared to other organisms (Fig. 12c).

The honeycomb-like structure of bryozoans is formed by a network of collagen and polyphenols. In seawater, Mn2+ is oxidized to Mn4+ by manganese-oxidizing bacteria. As the local environment changes, Mn4+ interacts with surrounding substances40, eventually leading to the formation of a MnO2 mineralized layer37 on the surface of the glaze, while the shallow transition zone serves as a source of energy for iron-oxidizing bacteria due to oxygen diffusion. This environment promotes the metabolism of goethite (α-FeO(OH))38. In deep anoxic zones, sulfate-reducing bacteria facilitate the precipitation of FeS241. This cycle generates electrochemically active heterostructures that enhance electron transfer, resulting in corrosion. Additionally, the adhesives and acidic secretions produced by marine organisms exert corrosive effects on silicate artifacts. The acidic compounds generated through biological metabolism contain hydroxyl groups that can be complexed with iron and calcium ions, leading to pitting and other forms of degradation42 (Fig. 12d).

The mechanisms that govern differential micro-environments produce a spatial gradient of bio-geochemical degradation products. This gradient extends from strongly reducing sulfides at the bio-organism to the interface of the glaze, to mixed valence oxides in the transition zone, and culminates in fully oxidized products at the surface. This mineral phase differentiation not only reflects the temporal and spatial dynamics of biological metabolism but also underscores the intricate interactions between microbial communities and environmental factors that influence the bio-geochemical degradation process.

In summary, attachment behaviors of marine organisms on the surface of the celadon glaze start the formation of diverse bio-geochemical degradation products. These reflect both organism-driven mineralization and biogeochemical processes involving microbes. A joint mechanism of organic template-guided mineralization and microbial metabolic synergy drives the bio-geochemical degradation of celadon glaze. However, current analysis relies mainly on correlation data from this study and inferences from existing models. Further controlled experiments are needed to validate specific biogeochemical pathways, microbial gene expression, and organic-inorganic interactions.

In conclusion, the bio-geochemical degradation mechanisms of four types of biofouling—barnacles, red corals, Rhizocaulus, and bryozoa—attached to the surface of the glaze of Yue kiln celadon (FL-3) recovered from the Five Dynasties shipwreck at Fenliuweiyu have been systematically analyzed. This study elucidates hypotheses of the scientific mechanisms behind the bio-geochemical degradation characteristics of ceramic relics caused by different marine organisms.

(1) Differences in the composition of bio-geochemical degradation products: the bio-geochemical degradation products resulting from barnacles, red corals, and Rhizocaulus primarily consist of organic matter (C, N, O, S, etc.). Notably, the black bio-geochemical degradation products associated with barnacle attachment include pyrite (FeS2) coated with organic films. The yellow bio-geochemical degradation area induced by red coral attachment is identified as hydroxyapatite (Ca5(PO4)3(OH)). Furthermore, Rhizocaulus generates a flaky organic film on the surface of the celadon glaze, along with sieve-like diatom remains. The bio-geochemical degradation products of these three biological deposits exhibit weak binding forces with the glaze, allowing for relatively easy physical removal through mechanical scraping. Conversely, the bio-geochemical degradation products resulting from bryozoan adhesion comprise complex structures of organic matter enveloped in ferromanganese minerals (including goethite α-FeO(OH), and pyrolusite MnO2), accompanied by a small proportion of pyrite (FeS2). The depth of bio-geochemical degradation pits in this case is significantly greater than that observed in other biological action areas. This organic-inorganic hybrid structure not only enhances the mechanical stability of the bio-geochemical degradation products but also considerably complicates their removal due to their deep embedding within the glaze layer. The findings of this study offer a crucial reference for the cleaning and conservation of marine effluent ceramics.

(2) Hypothesis of bio-deposition and bio-geochemical degradation process: a mechanism characterized as “organic template-guided mineralization-microbial synergy” is proposed. Initially, microorganisms adhere to form a biofilm, subsequently attracting the larvae or spores of macro-organisms that adhere to and proliferate on the celadon surface. During the growth of the aforementioned macro-organisms, distinct micro-environments are created within the glaze, fostering the growth and metabolism of various microorganisms present in the sediment. Extracellular polymeric substances (EPS) secreted by these microorganisms facilitate mineral nucleation by chelating metal ions (Fe2+, Ca2+) and modulating the micro-environment. Furthermore, mineral deposits are formed through interactions involving the sulfur cycle and oxidation-reduction coupling reactions. Sulfate-reducing bacteria (SRB) produce H2S and Fe2+, leading to the formation of FeS2 in anaerobic zones, while Fe/Mn oxidizing bacteria catalyze the oxidation of Fe2+/Mn2+ in oxygen-rich environments, resulting in the generation of heterostructures that expedite electron transfer and promote deep corrosion, particularly in areas affected by bryozoan attachment.