Impact of oil on concentrations of microbial genes related to carbon and nitrogen cycling in Bay Jimmy, LA, sediments

Diverse microorganisms play important roles in biogeochemical cycles in estuaries. In order to identify the potential impacts of oil contamination on microbial groups that control critical biogeochemical cycles in estuarine sediments, the concentrations of genes involved in nitrogen and carbon cycling were compared in oiled and unoiled areas of Bay Jimmy, LA. Total bacterial numbers were higher by a factor of two in unoiled than in oiled surface sediments (0-2 cm sections), regardless of the degree of oiling (high vs low). The numbers converged in 2-4 cm and deeper sections, however, suggesting impacts of oil may be limited to the surface sections. Similar trends were observed with concentrations of nifH, characteristic of nitrogen fixation. More significant impacts were observed with genes representing sulfate reduction (dsrB) and nitrate reduction (nirS and nirK), which showed significant decreases to the 4-6 cm depths. The impact of oil on concentrations of genes characteristic of methanogens (mcrA) increased with depth, possibility due to availability of electron donors. These trends suggest that the impacts of oil on sediment microbial communities may be complex. Further work is required to determine the mechanisms behind these trends. Exposure to oil resulted in decrease species richness and diversity of sulfate reducers while increasing those of denitrifiers (even though total numbers declined), indicating that oil impacts on microbial communities are complex. In conclusion, oil spill influenced the population sizes and assemblage structures of prokaryotes in complex ways.