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Microbial carbon metabolism along a salinity gradient: coupling biogeochemistry to microbial community composition.
Givler, Kim*,1, Neubauer, Scott1, Sikaroodi, Masoumeh2, Emerson, David3, Gillevet, Patrick2, Megonigal, Patrick1, 1 Smithsonian Environmental Research Center, Edgewater, MD2 George Mason University, Manassas, VA3 American Type Culture Collections, Manassas, VA
ABSTRACT- The dominant pathways of anaerobic carbon metabolism follow well-known trends across a salinity gradient. Methanogenesis dominates where salinity is low, but is suppressed by sulfate reduction as salinity increases. In mineral soils, Fe(III) reduction is expected to dominate both methanogenesis and sulfate reduction. Our goal was to determine whether changes in microbial community composition reflect these well-understood shifts in biogeochemical activity. Microbial communities were studied by Most Probable Number (MPN) culture techniques and Amplicon Length Heterogeneity (ALH) fingerprinting, and standard biogeochemical techniques were used to quantify methanogenesis, Fe(III) reduction, and sulfate reduction. Duplicate cores were taken at a series of eight tidal wetland areas ranging from fresh to mesohaline along the Patuxent River estuary, Maryland. Sulfate reduction rates increased three orders of magnitude from the least saline to the most saline site (0.00031 mgSO4 cm-3d-1 at 0.4‰ to 0.12 mgSO4 cm-3d-1 at 15‰), with an associated two-fold decrease in methane production (0.0032 umolCH4 g-1 hour-1 at 0.4‰ to 0.44 umolCH4 g-1 hour-1 at 15.2‰). However, these trends were not reflected in the MPN cell counts, which showed no obvious variations in sulfate-reducing bacterial communities. MPN counts for Fe-reducing bacteria were below detection in most of the freshwater marsh samples, although iron reduction rates (13 umol Fe/g soil/d) were among the highest observed at any of the sites. Large variations in biogeochemical activity were not reflected in the community composition of culturable microorganisms. We anticipate that the more sensitive ALH fingerprinting technique in progress will be able to detect changes in microbial biological diversity.
Key words: iron reduction, sulfate reduction, methanogenesis, microbial community composition