5:15 PM - 7:15 PM
[AHW27-P16] Assessment of Terrestrial Influence on Coral Reef Abundance through Microbial Community Analysis and Exchangeable Phosphate in Seawater for Sediments in Sekisei Lagoon
Keywords:Coral reefs ecosystem, Terrestrial loads, Metagenomic analysis
Sekisei Lagoon, one of the largest coral reef areas in Japan, extends between Ishigaki Island and Iriomote Island. In recent years, in addition to rising seawater temperatures, the excessive supply of nutrients and organic matter from terrestrial sources has posed significant threats to this ecosystem. In particular, terrestrial phosphate has been suggested to adsorb onto and accumulate in calcareous sediments, potentially harming corals. Our laboratory defines phosphate adsorbed and accumulated in calcareous sediments as "Exchangeable Phosphate in Seawater (EPS)." In collaboration with the Ministry of the Environment's Coral Community Monitoring Project, we have established 31 fixed survey sites in Sekisei Lagoon for continuous monitoring. Previous studies have revealed significant correlations between EPS and coral density decline or increased bleaching, but the causal relationship remains unclear. Furthermore, not only phosphorus but also organic matter accumulates in sediments, likely influencing microbial community structures. In coral reef ecosystems, microorganisms play a crucial role in organic matter decomposition, nutrient cycling and interact with corals, making it essential to understand these microbial changes. This study aims to analyze sediment microbial communities using 16S rRNA amplicon sequencing to elucidate the causal relationship between EPS and coral decline and to estimate the origins of accumulated organic matter.
Sediment samples from the 31 fixed sites (S1-S31) in Sekisei Lagoon were collected in September 2024 as part of the Ministry of the Environment's Coral Community Monitoring Project. The samples were frozen for preservation until DNA extraction. DNA was extracted from approximately 500 mg of sediment using the FastDNA Spin Kit for Soil (MP Biomedicals). The V4-V5 region of the 16S rRNA gene was amplified by PCR, and amplicon libraries were sequenced using the MiSeq next-generation sequencing platform (Illumina). The obtained sequence data were analyzed using SilvaNGS to identify microbial taxa, and microbial community compositions were determined for each site. The microbial community structure across the 31 sites was then examined in relation to EPS levels, coral density, coral health, and algal cover.
Cluster analysis of the microbial communities at the 31 sites classified them into two distinct clusters based on EPS levels: the High-EPS Cluster (0.86±0.20 µg/g) and the Low-EPS Cluster (0.52±0.22 µg/g). In the High-EPS Cluster, several cyanobacterial species were identified as indicator taxa, suggesting that EPS accumulation promotes the activity of nitrogen- and CO2-fixing microorganisms, leading to increased organic matter accumulation in sediments. Additionally, an increase in the abundance of methanogenic archaea and sulfate-reducing bacteria was observed, indicating that EPS accumulation creates a reductive sediment environment that may affect the ecological balance of coral reef ecosystems. In contrast, the Low-EPS Cluster was characterized by indicator species such as ammonia-oxidizing archaea and aerobic nitrifying bacteria, which contribute to nitrogen cycling in oligotrophic environments. Further analyses are currently conducted to examine the relationships between microbial community structure, coral cover, bleaching, and algal cover. These analyses are expected to provide insights into the impact of EPS on coral reef ecosystems and help clarify its causal relationship with coral decline.
Sediment samples from the 31 fixed sites (S1-S31) in Sekisei Lagoon were collected in September 2024 as part of the Ministry of the Environment's Coral Community Monitoring Project. The samples were frozen for preservation until DNA extraction. DNA was extracted from approximately 500 mg of sediment using the FastDNA Spin Kit for Soil (MP Biomedicals). The V4-V5 region of the 16S rRNA gene was amplified by PCR, and amplicon libraries were sequenced using the MiSeq next-generation sequencing platform (Illumina). The obtained sequence data were analyzed using SilvaNGS to identify microbial taxa, and microbial community compositions were determined for each site. The microbial community structure across the 31 sites was then examined in relation to EPS levels, coral density, coral health, and algal cover.
Cluster analysis of the microbial communities at the 31 sites classified them into two distinct clusters based on EPS levels: the High-EPS Cluster (0.86±0.20 µg/g) and the Low-EPS Cluster (0.52±0.22 µg/g). In the High-EPS Cluster, several cyanobacterial species were identified as indicator taxa, suggesting that EPS accumulation promotes the activity of nitrogen- and CO2-fixing microorganisms, leading to increased organic matter accumulation in sediments. Additionally, an increase in the abundance of methanogenic archaea and sulfate-reducing bacteria was observed, indicating that EPS accumulation creates a reductive sediment environment that may affect the ecological balance of coral reef ecosystems. In contrast, the Low-EPS Cluster was characterized by indicator species such as ammonia-oxidizing archaea and aerobic nitrifying bacteria, which contribute to nitrogen cycling in oligotrophic environments. Further analyses are currently conducted to examine the relationships between microbial community structure, coral cover, bleaching, and algal cover. These analyses are expected to provide insights into the impact of EPS on coral reef ecosystems and help clarify its causal relationship with coral decline.