Fertilization Results in a Dramatic Decrease in Diversity of Active Denitrifying Bacteria

Salt marsh sediments are sites where some of the highest rates of denitrification have ever been reported and similarly, they have among the highest reported diversity of key genes in the denitrification pathway. Although salt marsh sediments are hot spots for denitrification, they are also habitats under threat from both rising seas and increased anthropogenic nutrients. Previous work indicated that increased nutrient supply to marsh creeks resulted in increased rates of denitrification, which, when coupled with decreased belowground plant biomass, resulted in a collapse of the marsh creek bank and a restructuring of marsh geomorphology. The genetic response of marsh sediment denitrifying communities to increase in nitrogen, however, has been modest. Recent data from our laboratory suggests that the disconnect between the minor change in denitrifier community composition and the denitrification rates may be due to high rates of microbial dormancy observed in salt marsh sediments. Our results, via analysis of coupled 16S rRNA and the 16S rRNA gene demonstrate that fertilization results in a loss of active microbial diversity. Here we report, for the first time, the role that fertilization plays in structuring the active portion of the denitrifying bacterial community. We sequenced nirS DNA and mRNA transcripts from sediments underlying tall Spartina alterniflora from four salt marsh creeks in the Plum Island salt marsh complex. Two of the four marshes have received artificial nutrient enrichment for up to 10 years. Our results indicate that fertilization reduces the genetic diversity of denitrifiers, favoring only a small subset of those bacteria capable of this critically important process