Dynamic hydrologic and biogeochemical processes drive microbially enhanced iron and sulfur cycling within the intertidal mixing zone of a beach aquifer

Abstract

Intertidal aquifers host a reactive zone comprised of Fe mineral‐coated sands where fresh and saline groundwaters mix. This zone may significantly influence the export of C, N, P, Fe, and other metals in submarine groundwater discharge (SGD). Toward determining the roles of microbes in Fe and S mineralization, and the interplay of microbiology with geochemistry and physical hydrology, we conducted a biogeochemical study of pore waters at Cape Shores, Delaware. Here, fresh groundwater provides Fe(II), which precipitates as FeIIIOOH predominantly through microbial Fe(II) oxidation. Candidate division OP3 was the dominant microbial group associated with Fe(II)‐ and Fe(III)‐rich regions of the aquifer, suggesting that this uncharacterized phylum may be involved in Fe(II) oxidation. Saline water brings O2, sulfate, and organic C into the intertidal mixing zone. Microbial reduction of sulfate produces sulfide that is transported to the Fe‐mineralized zone leading to the transformation of FeOOH to Fe(II) sulfides. Microbial populations are structured by the availability of chemical species supplied along groundwater flow paths. Seasonal changes in the relative supply of fresh and saline groundwater affect solute fluxes, and therefore, microbial controls on the location and composition of the Fe‐mineralized zone. Ultimately, the composition, extent, and dynamics of the Fe‐mineralized zone will affect the sequestration, affinity, and residence time of solutes bound for export to coastal oceans through SGD.

Coexistence of Microaerophilic, Nitrate-Reducing, and Phototrophic Fe(II) Oxidizers and Fe(III) Reducers in Coastal Marine Sediment

“…It is known that microaerophilic Fe(II) oxidizers can reach numbers of up to 10 7 cells/ml in iron-rich marine environments, such as Fe(II)-containing hydrothermal vents (36,37). While Gallionella, Sideroxydans, and Leptothrix seem to be restricted to limnic and terrestrial habitats, the genus Mariprofundus may be restricted to marine habitats (38)(39)(40)(41)(42). Information about the presence of photoferrotrophs in marine sediments is scarce, and there are only two isolates, Rhodovulum iodosum and Rhodovulum robiginosum, which were both isolated from a tidal mud flat in the German Wadden Sea (43,44).…”

The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low-water line

“…3). Thus, the USP at Spiekeroog beach may extend down to greater depths than previously shown in other studies using salinity measurements and/or hydrological modeling (Charette and Sholkovitz, 2006;McAllister et al, 2015;Santos et al, 2008;Ullman et al, 2003). The majority of these studies were, however, carried out in microtidal environments, which may explain why the USP does not reach depths exceeding 5 m.…”

“…This will lead to a refined groundwater flow model, to a better understanding of the key processes in biogeochemistry, of the role of organisms for OM degradation, and of the fluxes at the sediment-water interface. To date, most studies on hydrological and biogeochemical processes in subterranean estuaries of beach systems were carried out in microtidal and/or low-energy environments such as semi-protected bay situations (Charette and Sholkovitz, 2006;McAllister et al, 2015;Santos et al, 2008;Ullman et al, 2003). Therefore, a maximum sampling depth of about 5 m was sufficient to resolve the USP at these study sites.…”

Metagenomic potential for and diversity of N‐cycle driving microorganisms in the Bothnian Sea sediment

“…However, complete gene sets encoding the full denitrification pathway could not be detected. Another recent study found strong correlations between the occurrence of candidate division OP3 16S rRNA genes and the content of oxidized iron minerals at the discharge zone of an intertidal aquifer, speculating on the possible involvement of some members of this group in iron cycling (McAllister et al, ). The iron‐rich sediments of the Bothnian Sea might thus provide a suitable habitat for members of this group.…”