Microbial linkages in the N and S cycles: Experimental evidence from lakes and wetlands

Background/Question/Methods

Human activities have doubled the reactive nitrogen (N) globally due largely to agricultural activities.  Similarly, industrialization has altered the sulfur (S) cycle through fossil fuel combustion.  These anthropogenic perturbations have caused environmental problems such as coastal eutrophication and acid deposition.  Though human activities have affected both elemental cycles, scientists study the cycles separately.  Furthermore, the freshwater S cycle has not received the same research attention as the marine S cycle.  The N cycle, however, has been studied in depth across ecosystems.  For example, we know that much of the N (mostly as nitrate, NO₃^-) that enters watersheds is lost before reaching the oceans; however, there is still considerable uncertainty about the location and magnitude of NO₃^- sinks.  A key problem in ecosystem ecology relates to how and where this N is removed. 

Research on NO₃^- removal processes has emphasized assimilation or denitrification (conversion to N₂).  The increasing application of tracer techniques (e.g., stable isotopes) has yielded a growing body of evidence for alternative processes of NO₃^- transformation, including dissimilatory reduction of nitrate to ammonium (DNRA) and chemoautotrophic denitrification via sulfur oxidation.  In this seminar, I will address the following questions: 1) Is there evidence for these alternative N cycling pathways in freshwaters? 2) Who is responsible for the coupled elemental cycling? and 3) What controls the relative importance of multiple, simultaneously occurring NO₃^- removal pathways?  I will address these questions through experiments that combined approaches from ecosystem stable isotope tracer additions, molecular environmental microbiology, and laboratory biogeochemical assays conducted in wetland sediments and the hypolimnion of a hypereutrophic lake. 

Results/Conclusions          

These studies were inspired by the observation that direct injection of NO₃^- into anoxic sediments resulted in the production of sulfate (SO₄^2-).  Experiments across 26 freshwater ecosystems suggested that SO₄^2- production could account for 15-30% of the overall NO₃^- removed.  A similar process is known to occur in marine ecosystems, and further work sought to isolate the bacterium responsible for this coupled N-S cycling.  Using a combination of enrichment techniques and environmental clone libraries, we determined that Sulfurimonas denitrificans conducts some of the coupled N-S cycling in freshwaters.  S. denitrificans populations are driven by suflide (H₂S) concentration.  Furthermore, H₂S and carbon both regulate N cycling processes such as NO₃^- removal, denitrification and DNRA.  Understanding how multiple coupled elemental cycles interact may lead us to reanalyze N and S cycling in freshwater ecosystems, especially those facing anthropogenic perturbation of both cycles.