Periodic change in coastal microbial community structure associated with submarine groundwater discharge and tidal fluctuation

Coastal areas where submarine groundwater discharge (SGD) occurs are active mixing zones with characteristic biogeochemical and ecological functions. In this study, we investigated the microbial community associated with the changes in groundwater discharge flux at a coastal beach site on Jeju Island, South Korea. We performed water chemistry analyses, 16S rRNA gene-based pyrosequencing, and microbial community statistical analyses on coastal water samples systematically collected as functions of tidal stage and distance from the groundwater discharge point. We also carried out groundwater level monitoring and numerical simulation of the coastal aquifer to reproduce tidally induced variations in the SGD rates of the study site. Pyrosequencing and statistical analyses revealed a periodic shift in the microbial communities in the coastal waters as functions of tidal stage and SGD rates. Interestingly, the community structures in the samples collected at flood and ebb tide were markedly different, despite the similarities in their water chemistry. Groundwater simulation and canonical correspondence analyses suggest that groundwater discharging at higher velocities at ebb tide can detach and transport subsurface bacteria from the aquifer to the coastal water body, resulting in an increase in facultative anaerobes in the ebb tide samples. In addition, release of the sand-attached bacteria as a result of particle resuspension and flushing of shallow subsurface bacteria in the intertidal zone could contribute to shaping the relative abundance of the coastal microbial community. We conclude that SGD rate is an important factor influencing the dynamics of the bacterial community structure at the coastal zone of the study site.