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[AOS17-09] Transition of microbial communities in surface sediments during whale-fall decomposition
Keywords:Whale fall, Microbial community, 16S rRNA gene
Large vertebrate carcasses, such as whale falls, provide substantial organic matter to the deep-sea floor and are utilized by many marine organisms in various ways. Organic matter from these carcasses likely diffuses into surrounding sediments, potentially affecting local environments and ecosystems. However, the extent and mechanisms of this diffusion are not well understood. Moreover, research on microbial communities, the primary agents of organic matter decomposition, remains scarce. The effects of large vertebrate carcass decomposition on sediment microbial communities remain poorly documented.
In this study, we artificially submerged a whale carcass on a shallow seafloor and investigated the associated changes in sediment organic matter content, stable carbon isotopic composition, and microbial community over time during its decomposition. The defleshed whale skeleton was placed at a depth of approximately 15 m off the coast of Kanazawa University’s Noto Marine Laboratory, Noto Peninsula, Ishikawa Prefecture, Japan, in August 2023. Over three months, sediment samples were periodically collected at fixed intervals within a 1 m radius of the carcass. Total organic carbon (TOC) content and stable carbon isotope ratios (δ¹³C vs. VPDB) of sedimentary organic matter were measured, and microbial community composition was analyzed using 16S rRNA gene sequencing.
The elevated TOC value of the sediments just below the whale carcass (exceeding 1.5 wt%) persisted for up to three months. The δ¹³C values of organic matter in the sediments began decreasing one month after placement, reaching -22.5‰, which was lower than background levels. Both TOC and δ¹³C variations were mainly confined within a 50 cm radius of the carcass.
Similarly, microbial community changes were most pronounced within 50 cm of the carcass. Sulfate-reducing bacteria (SRB) showed a spatial gradient, with relative abundance decreasing with distance from the carcass. At 1 m from the carcass, SRB abundance remained below 20% throughout the experiment, whereas at 10 cm, it reached nearly 50%, approximately twice the background level. In contrast, sulfur-oxidizing bacteria (SOB), including genus Sulfurimonas and Sulfurovum, showed a more pronounced temporal response, with their abundance increasing sharply after one month. However, SOB remained at low overall abundance throughout the experiment, suggesting that the sediment near the carcass remained in a reduced state even after three months.
In this study, we artificially submerged a whale carcass on a shallow seafloor and investigated the associated changes in sediment organic matter content, stable carbon isotopic composition, and microbial community over time during its decomposition. The defleshed whale skeleton was placed at a depth of approximately 15 m off the coast of Kanazawa University’s Noto Marine Laboratory, Noto Peninsula, Ishikawa Prefecture, Japan, in August 2023. Over three months, sediment samples were periodically collected at fixed intervals within a 1 m radius of the carcass. Total organic carbon (TOC) content and stable carbon isotope ratios (δ¹³C vs. VPDB) of sedimentary organic matter were measured, and microbial community composition was analyzed using 16S rRNA gene sequencing.
The elevated TOC value of the sediments just below the whale carcass (exceeding 1.5 wt%) persisted for up to three months. The δ¹³C values of organic matter in the sediments began decreasing one month after placement, reaching -22.5‰, which was lower than background levels. Both TOC and δ¹³C variations were mainly confined within a 50 cm radius of the carcass.
Similarly, microbial community changes were most pronounced within 50 cm of the carcass. Sulfate-reducing bacteria (SRB) showed a spatial gradient, with relative abundance decreasing with distance from the carcass. At 1 m from the carcass, SRB abundance remained below 20% throughout the experiment, whereas at 10 cm, it reached nearly 50%, approximately twice the background level. In contrast, sulfur-oxidizing bacteria (SOB), including genus Sulfurimonas and Sulfurovum, showed a more pronounced temporal response, with their abundance increasing sharply after one month. However, SOB remained at low overall abundance throughout the experiment, suggesting that the sediment near the carcass remained in a reduced state even after three months.