Linking Carbon, Nitrogen, and Sulfur dynamics in a Spartina Alterniflora Saltmarsh: Characterizing the Sources of Organic Matter Decomposed by Sediment Microbes

Amanda Spivak, Marine Chemistry & Geochemistry



Saltmarsh wetlands buffer coastal bays and estuaries from landscape inputs of inorganic nutrients. Inorganic nitrogen that is delivered to saltmarshes, from leaky septic systems or over-fertilized lawns and agricultural fields, can be converted into plant biomass, buried in the sediments, or transformed into N2 gas and emitted into the atmosphere, among other fates. By removing inorganic nitrogen at the land-sea interface, wetlands help mitigate the symptoms of coastal eutrophication, such as hypoxia and harmful algal blooms. However, the efficiency of nitrogen removal by saltmarsh wetlands may be affected by chronic nutrient pollution. Therefore, it is important to understand how sediment microbial processes that control nitrogen transformations in wetlands will respond to long-term nutrient stress.

Through the proposed research, I plan to determine how nutrient loading affects the sources of organic carbon decomposed by sediment bacteria in a Spartina alterniflora saltmarsh over an annual cycle. In addition, I will conduct stable isotope labeling experiments to determine whether bacterial reliance on carbon exuded from S. alterniflora roots differs in wetlands with high vs. low levels of nitrogen loading and plant life cycle stage. I will focus on carbon sources supporting sulfate reducing bacteria, because the sulfides produced by these bacteria are used by sulfur-oxidizing bacteria to reduce nitrate and fix carbon. Consequently, a change in the availability of labile carbon compounds to sulfate reducing bacteria may have cascading effects on sulfide production and the ability of sulfur-oxidizing bacteria to reduce nitrate. The factors that control whether sulfur-oxidizing bacteria transform nitrate into ammonium (which is bioavailable) or N2 (which is lost to the atmosphere), are the subject of a recently funded NSF grant to S. Sievert (WHOI), Z. Cardon (MBL), and A. Giblin (MBL). Thus, the research described in this COI proposal is complementary to Sievert, Cardon, and Giblin’s study and provides a unique opportunity for cross-department and cross-institution collaboration.

Results from this COI proposal will describe whether the carbon sources used by sediment bacteria are similar in S. alterniflora marshes with different levels of nitrogen loading. My findings will also demonstrate whether seasonal changes in S. alterniflora life cycle affect the availability of labile carbon to sediment bacteria. Determining how above-ground plant and below-ground microbe interactions are affected by nutrient loading is important in understanding how wetland carbon dynamics will change under different eutrophication scenarios. Combined results from the proposed COI study and Sievert, Cardon, and Giblin’s project will provide a mechanistic description of the factors controlling nitrate reduction by sulfur-oxidizing bacteria and detailed information about how sediment microbes control linked carbon, nitrogen, and sulfur cycles in S. alterniflora wetlands.