Saltwater Intrusion Modifies Microbial Community Structure and Decreases Denitrification in Tidal Freshwater Marshes

Abstract

Environmental changes can alter the interactions between biotic and abiotic ecosystem components in tidal wetlands and therefore impact important ecosystem functions. The objective of this study was to determine how saltwater intrusion affects wetland nutrient biogeochemistry, with a specific focus on the soil microbial communities and physicochemical parameters that control nitrate removal. Our work took place in a tidal freshwater marsh in South Carolina, USA, where a 3.5-year saltwater intrusion experiment increased porewater salinities from freshwater to oligohaline levels. We measured rates of denitrification, soil oxygen demand, and dissimilatory nitrate reduction to ammonium (DNRA) and used molecular genetic techniques to assess the abundance and community structure of soil microbes. In soils exposed to elevated salinities, rates of denitrification were reduced by about 70% due to changes in the soil physicochemical environment (higher salinity, higher carbon:nitrogen ratio) and shifts in the community composition of denitrifiers. Saltwater intrusion also affected the microbial community responsible for DNRA, increasing the abundance of genes associated with this process and shifting microbial community composition. Though rates of DNRA were below detection, the microbial community response may be a precursor to increased rates of DNRA with continued saltwater intrusion. Overall, saltwater intrusion reduces the ability of tidal freshwater marshes to convert reactive nitrogen to dinitrogen gas and therefore negatively affects their water quality functions. Continued study of the interrelationships between biotic communities, the abiotic environment, and biogeochemical transformations will lead to a better understanding of how the progressive replacement of tidal freshwater marshes with brackish analogues will affect the overall functioning of the coastal landscape.

Salinity controls soil microbial community structure and function in coastal estuarine wetlands

“…It was also reported by Neubauer et al . (2019) that salinity changes caused by saltwater intrusion suppressed the denitrification rates (convert reactive nitrogen to dinitrogen gas) in freshwater marshes in South Carolina, USA, meanwhile, the microbial community responsible for denitrification processes was also altered. It should be noted that the hzo genes encode for a key function in the anammox process, and a high diversity of anammox bacteria was in fact reported for marine environments (Li et al ., 2010).…”

Microbial and nutrient dynamics in mangrove, reef, and seagrass waters over tidal and diurnal time scales

“…The dynamics of microbial communities over short temporal scales (hours to days) are less studied. Studies from estuarine and coastal environments showed that tidal mixing and salinity are major drivers of microbial community structure (Lu et al 2015, Chen et al 2019, Neubauer et al 2019). In the case of open ocean environments with more stable physical and chemical features, observed changes in microbial communities may be due to biological interactions.…”

Simulation of Density and Flow Dynamics in a Lagoon Aquifer Environment and Implications for Nutrient Delivery From Land to Sea

“…Nutrient mobilization, cycling and transport within and across coastal aquifers are controlled by salinity (Tobias et al, 2001;Charette and Sholkovitz, 2002;Baldwin et al, 2006;Weston et al, 2006Weston et al, , 2011Jun et al, 2013;Neubauer et al, 2019), redox conditions (electron acceptor and donor availability and fluxes) (Reddy and DeLaune, 2008;Tobias and Neubauer, 2019;Moore and Joye, 2021) and residence times (Hopkinson and Giblin, 2008;Steinmuller and Chambers, 2018;Neubauer et al, 2019). In order to understand nutrient dynamics in near coastal aquifers thoroughly, an understanding of the temporal and spatial subsurface flow dynamics are a prerequisite.…”