Nitrogen and phosphorus cycling in the digestive system and shell biofilm of the eastern oyster Crassostrea virginica

摘要

The shell and digestive system of bivalves provide microhabitats for an array of microbes that can mediate biogeochemical cycles. Here, we investigated nitrogen (N) and phosphorus (P) fluxes in these microhabitats in the eastern oyster Crassostrea virginica. From the anoxic oyster digestive system, we measured significant production of dinitrogen gas (N2-N; mean ?± SE: 0.59 ?± 0.20 ?μmol ind.-1 h-1) and nitrous oxide (N2O; 0.001 ?± 0.0004 ?μmol ind.-1 h-1), indicative of denitrification. The oxic shell biofilm released N2O (0.0003 ?± 0.0001 ?μmol ind.-1 h-1), as well as ammonium (NH4+; 1.26 ?± 0.20 ?μmol ind.-1 h-1) and nitrite (NO2-; 0.05 ?± 0.01 ?μmol ind.-1 h-1), but not N2-N, suggesting a combination of nitrification and heterotrophic activity. The biofilm released more dissolved inorganic P than the digestive system, although the rate of release from whole oysters was closer to the rate from the digestive system alone. N remineralized by oysters is released almost exclusively as NH4+, at a ratio of 18.4:1 with P, i.e. relatively close to the Redfield ratio (16:1). In an ecological context, this study supports the growing literature on the ability of oysters themselves to engage in denitrification activity and at rates potentially exceeding rates of sediment denitrification. The denitrification in the digestive system appears to proceed to completion and has a very small N2O cost (1%). Restoring oyster populations may therefore be an important method for N reduction in coastal systems.