Microbial community composition across a coastal hydrological system affected by submarine groundwater discharge (SGD).

Dini Adyasari,Christiane Hassenrück,Natasha Dimova,Daniel Montiel,John M Senko

doi:10.1371/journal.pone.0235235

Abstract

Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH4+-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO43- were the major predictor of community shift within subsurface water samples. Higher microbial diversity was found in coastal pore water in comparison to surface water samples. Gammaproteobacteria, Bacteroidia, and Oxyphotobacteria dominated the bacterial community. Among the archaea, methanogens were prevalent in the peat-dominated SGD site, while the sandy SGD site was characterized by a higher proportion of ammonia-oxidizing archaea. Cyanobium PCC-6307 and unclassified Thermodesulfovibrionia were identified as dominant taxa strongly associated with trends in environmental parameters in surface and subsurface samples, respectively. Microbial communities found in the groundwater and peat layer consisted of taxa known for denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This finding suggested that microbial communities might also play a significant role in mediating nitrogen transformation in the SGD flow path and in affecting the chemical composition of SGD discharging to the water column. Given the ecological importance of microorganisms, further studies at higher taxonomic and functional resolution are needed to accurately predict chemical biotransformation processes along the coastal hydrological continuum, which influence water quality and environmental condition in Mobile Bay.

Highlights

Submarine groundwater discharge (SGD) is defined as groundwater flow across the landocean interface to the coastal water
In a subterranean environment inhabited by both sulfate-reducing bacteria (SRB) and methanogenic archaea, their abundance is controlled by available substrate concentration, organic loading [57, 58], and ion concentration [59], which were not measured in this study
Microbial community composition across the surface hydrological continuum was primarily governed by salinity, while DON and PO43- concentrations were the strongest predictors of community shift within the subsurface ecosystems

Summary

Introduction

Submarine groundwater discharge (SGD) is defined as groundwater flow across the landocean interface to the coastal water. Subterranean estuaries (STEs), where SGD flows through before discharging to the coastal water, are active mixing zones and biogeochemical cycling hot spots [19] They are subjected to both seasonal water table fluctuation and increased nutrient input from the groundwater coming from the land side, as well as changes in oxygen saturation, quick redox switches and organic matter inputs from tidal fluctuation and sea level fluctuation from the marine side [20,21,22]. Microbial communities are found to play a significant role in mediating nitrogen cycling in the STEs, such as nitrification and denitrification, and subsequently alter the chemical composition of SGD discharging to the overlying water [26,27,28]. In comparison with surface estuaries, the understanding of the subsurface microbial community and their response to biotic and abiotic reactions is still limited

Methods

Results

Discussion

Conclusion