Microbial Insights into Freshwater Tidal Wetland Redox Biogeochemistry

One way that aquatic-terrestrial interfaces such as wetlands experience climate extremes is as shifts in the magnitude and dynamics of water flow. This directly impacts biogeochemical processes along environmental gradients such as redox conditions. Greenhouse gas fluxes, e.g., carbon dioxide (CO₂) and methane (CH₄), that derive from microbially-catalyzed reactions are directly linked to water saturation and electron acceptor and donor availability. The dependence of such fluxes on geochemistry has been directly linked to microbial feedback, thereby requiring an increased knowledge of microbial functions mediating those fluxes. Therefore, we conducted research to seek predictable patterns in the functional actuality (expressed genes) and metabolism (expression of extracellular metabolites including gaseous products) of a wetland soil microbial community over time and across varying water and oxygen levels.

We tested the impact of varying redox conditions on the microbiome in soils sampled (5-15 cm depth) from a tidal freshwater wetland in the lower Columbia River, WA, USA. Soil microcosms were incubated under one of 3 treatments aimed at varying redox through saturation or through headspace oxygen: (i) aerobic-drying (ii) anoxic-saturation, and (iii) fluctuating-moisture. Microcosms were destructively sampled for analyses after 15, 35, and 45 days of incubation. Headspace CO₂ and CH₄ were measured throughout the incubation using a Cavity Ring-Down Spectrometer; water-extractable metabolites were measured by 1D-¹H- Nuclear Magnetic Resonance, and metatranscriptomes were obtained by sequencing the messenger Ribosomal Nucleic Acid on an Illumina HiSeq 2500 platform (2x250 cycle runs) and were annotated for membership to TIGRFAM protein families.

We focus on functions including fermentation, respiration and methane production since ultimately, greenhouse gas fluxes from the terrestrial subsurface need to be predicted under conditions of climate change. We expect that the combination of metatranscriptomic and metabolomic data will reveal key microbiologic features needed for effective modeling of biogeochemical fluxes from hydrodynamic ecosystems.