Tidal cycling dynamics of nitrate reductase in a tidal freshwater wetland

Background/Question/Methods

Tidal freshwater wetlands (TFW) experience freshwater tides, creating unique redoximorphic soil characteristics and transporting nutrients into the system. One process of particular interest in TFW soils is denitrification, which helps remove reactive nitrogen from the wetland and thus ameliorate eutrophication of downstream systems. The nitrite reductase gene (nir) is an essential functional gene in the denitrification pathway; it catalyzes the production of NO, committing the nitrogen species to a gaseous end product. Research suggests that the two versions of this gene, nirS and nirK, are mutually exclusive in microorganisms, but the ecology of the two groups is poorly understood. This study quantifies the expression (cDNA) and abundance (DNA) of nirK and nirS using quantitative PCR (qPCR), and compares the results to environmental variables to identify possible drivers of the distribution and expression of the two groups over a tidal cycle. Measured soil properties include redox, pH, organic matter content, saturation, texture, and pore water chemistry. Research was conducted in a TFW at the VCU Rice Rivers Center (James River, Virginia) in a 3x3 m² plot that was sampled (five replicates) every two hours over a twelve hour time period during a spring tide.

Results/Conclusions

Redox potential ranged from 222 mV to -140 mV, decreasing as soil saturation increased from low to high tide. The site was inundated with 40 cm of water at high tide. Pore water had greater concentrations of nitrate at high tide compared to low tide. There was no change in the soil organic matter content over a tidal cycle, and the soil was characterized as a sand/loam/clay mixture. Gene expression (cDNA copy number) responded to tidal cycling, whereas little variation in abundance (DNA copy number) was observed. Overall, abundance and expression levels were higher for the nirS gene than the nirK gene. These findings agree with previous research that suggests nirS dominates over nirK in soil habitats that are waterlogged. Microbial transformations are paramount in the biogeochemical cycling of nitrogen. However, microbial communities often have their ecological interactions overlooked in favor of studies of their physiological rates. Understanding how microbial communities respond to environmental changes, such as tidal cycling, is imperative for designing accurate biogeochemical models and can inform wetland management and restoration practices.