Shotgun metagenomic analysis on the microbial communities in the Ryukyu limestone aquifer of the southern part of Okinawa-jima (Nov.2021-Jan.2022)

While microbial biogeochemical activities such as those involving denitrification and sulfate reduction have been considered to play important roles in the material cycling in various aquatic ecosystems, our current understanding of the microbial community in the groundwater ecosystem is strikingly insufficient. In order to assess the groundwater from three location in the Ryukyu limestone aquifer of the Okinawa-jima locating in the southernmost region of Japan, we performed shotgun metagenomic analysis on their microbial communities at three sites and screened the functional genes involved in the nitrogen metabolism. The shotgun analysis showed that bacteria accounted for 90-98 % in the microbial communities including archea, eukaryotes and viruses for all the three sampling sites. The bacterial communities involving the nitrogen metabolism shifted by month in each site, indicating that this metabolism was accomplished not with specific bacteria, but rather with the integration of the bacterial community. Interestingly, site 3 contained much higher denitrification genes, narG/narH/nxrB, narG/narZ/nxrA, napA, nirK, norB and nosZ, than the other two sites. This site was thought to have undergone the denitrification reaction in association with high amounts of dissolved organic carbon (DOC). While site 2 was characterized by a high nitrate-nitrogen content with a low amount of DOC, this site provided a moderate amount of denitrification genes. Site 1 showed markedly low amounts of all nitrogen metabolism genes. Taken together, the nitrogen metabolism in the Ryukyu limestone aquifer was found to change depending on environmental factors.