[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-05] Ecological Roles of Fecal Microbiome of Sea Cucumber in Coral Reef Ecosystem

*Kohei Hamamoto¹, Miyuki Nishijima², Ayumi Maeda³, Kodai Gibu², Akira Iguchi² (1.Center for Marine Environmental Studies, Ehime University, 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 3.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Sea cucumber, Microbiome, Metabarcoding, Coral reefs

Sea cucumbers are invertebrates widely distributed in coastal sea floor all over the world. Due to their increasing economic value in recent years, they are being overexploited and hence being threatened in many areas including coral reefs. Among the many species, those inhabiting tropical and subtropical coral reefs are mostly deposit feeders, consuming microalgae, bacteria, and detritus through ingestion of surrounding sediments. Recently, it has been reported that sea cucumbers may suppress coral diseases and reduce coral colony mortality through their feeding behavior, drawing attention to their ecological importance. However, the mechanisms by which sea cucumbers suppress coral diseases remain largely unknown. In this study, we compared the microbial composition in 299 samples of sea cucumber feces, nearby sediments, and distant sediments collected from seven sites around Okinawa Island, including seagrass meadow, sandy bottoms, and rocky shore with living scleractinian corals. The results showed that the bacterial community composition in sea cucumber feces was significantly different from that in surrounding sediments, with some bacteria being enriched compared to the surrounding sediments. Furthermore, when estimating the composition of enzyme genes coded by the bacterial community, it was found that the potential for producing chitinase, which corals release as a biophylaxis when detecting fungal pathogens, was higher in sea cucumber feces than in surrounding sediments. Additionally, the potential for degrading biodegradable plastics such as polyhydroxybutyrate (PHB) was also higher in sea cucumber feces. Future challenges include conducting empirical experiments to evaluate the actual enzyme activity in feces for chitin and PHB, which were indicated to have production potential by the bacterial community in sea cucumber feces.

Next, a preliminary survey was conducted to investigate the microbial communities associated with different species of sea cucumbers living in different environments (sandy bottoms, gravel zones, and rocky shore with living scleractinian corals) at the same site. Feces were collected from 14 sea cucumbers, including Stichopus chloronotus, Stichopus herrmanni, Actinopyga miliaris, Holothuria atra, Bohadschia argus, Pearsonothuria graeffei, at the coast of Motobu Town, northwestern Okinawa Island. Metabarcoding analysis of the 16S region was performed on the obtained samples, revealing that sea cucumbers in different environments had distinctly different bacterial communities in their feces. Notably, Pearsonothuria graeffei, which lives directly above reef-building corals or on rocky surface, had a clearly different bacterial community composition. While Proteobacteria and Cyanobacteria dominated in feces from sandy bottoms and gravel zones, Bacteroidota and Fusobacteria dominated in the rocky shore. Interestingly, the diversity of bacterial communities in feces was significantly lower in the rocky shore compared to the other two environments. Analysis of enzyme gene composition showed that most enzymes were shared across all three environments, but some enzymes were found only in specific environments. These results suggest that different species of sea cucumbers at the same site have different bacterial communities in their feces and likely play different ecological roles.