Abstract
Subterranean estuaries (STEs) have been recognized as important ecosystems for the exchange of materials between the land and sea, but the microbial players of biogeochemical processes have not been well examined. In this study, we investigated the bacterial and archaeal communities within 10 cm depth intervals of a permeable sediment core (100 cm in length) collected from a STE located at Gloucester Point (GP-STE), VA, United States. High throughput sequencing of 16S rRNA genes and subsequent bioinformatics analyses were conducted to examine the composition, diversity, and potential functions of the sediment communities. The community composition varied significantly from the surface to a depth of 100 cm with up to 13,000 operational taxonomic units (OTUs) based on 97% sequence identities. More than 95% of the sequences consisted of bacterial OTUs, while the relative abundances of archaea, dominated by Crenarchaea, gradually increased with sediment core depth. Along the redox gradients of GP-STE, differential distribution of ammonia- and methane-oxidizing, denitrifying, and sulfate reducing bacteria was observed as well as methanogenic archaea based on predicted microbial functions. The aerobic-anaerobic transition zone (AATZ) had the highest diversity and abundance of microorganisms, matching with the predicted functional diversity. This indicates the AATZ as a hotspot of biogeochemical processes of STEs. The physical and geochemical gradients in different depths have attributed to vertical stratification of microbial community composition and function in the GP-STE.
Highlights
At the land-sea margin, coastal permeable sediments form the interface between the freshwaters of the coastal unconfined aquifers and seawater-derived saline pore water
We found sharp gradients of salinity, dissolved oxygen (DO), and Oxidation-Reduction Potential (ORP) in the groundwater sampled from different depths of the sampling site in the GP-subterranean estuary (STE)
The spatial distribution of salinity indicated that seawater can be transported to the STE via two different ways: vertical permeation from the surface, and horizontal transportation from seawater terminal
Summary
At the land-sea margin, coastal permeable sediments form the interface between the freshwaters of the coastal unconfined aquifers and seawater-derived saline pore water. Recent studies have demonstrated that the permeable sediments are highly active in biogeochemical reactions due to special physiochemical properties such as steep redox, salinity and oxygen gradients, longer residence times, and stronger particle–water interactions (Moore, 1999; Santos et al, 2008; Gonneea and Charette, 2014; O’Connor et al, 2015; Seidel et al, 2015; Reckhardt et al, 2017). This mixing zone, home to a variety of important biogeochemical reactions, is referred to as a subterranean estuary (STE) (Moore, 1999). STEs have been impacted by increased nutrient input from both fresh water and sea water, and changes of physicochemical conditions due to anthropogenic activities (Talbot et al, 2003; Slomp and Van Cappellen, 2004; Moore et al, 2008)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
