Bacterial mediation in the utilization of carbon and nitrogen from detrital complexes by Crassostrea virginica

Abstract

Unattached, cellulolytic bacteria isolated from a salt marsh were cultured on [15N]ammonium sulfate and [14C]glucose and fed to the American oyster Crassostrea virginica. Oysters were able to digest and assimilate bacterial C with an assimilation efficiency of 52.5%. We estimate that free‐living bacteria may be capable of supplying up to ∼9.5% of the total C requirements of oysters in their natural habitat. Cellulolytic bacteria were also cultured on 14C‐labeled refractory Spartina alterniflora particles as the sole C source and [15N]ammonium sulfate as a source of N. These labeled bacteria, together with the S. alterniflora, were fed to the oysters. The refractory C from these detrital complexes was assimilated by the oysters with an efficiency of 10.3%. It was (P= 0.0007) greater than the assimilation efficiency of 2.7% measured in a previous study for oysters feeding on the refractory S. alterniflora substrate alone. This result provides direct experimental evidence that cellulolytic bacteria in the environment can contribute to the transfer of C from refractory detritus to an ecologically important suspension‐feeding macroinvertebrate. We calculated that C. virginica, when fed detrital complexes, assimilated bacterial N with an efficiency of 57.2% but that assimilation of total N present in the detrital complexes was only 3.4%. We speculate that this low assimilation efficiency was due to most (94%) of the N being in the form of condensation products such as humic geopolymers and extracellular polymeric substances secreted by bacteria which could not be digested and absorbed by the oysters. Calculations show that detrital complexes in the natural environment may provide a significant contribution to an oyster’s C demand. The magnitude of this contribution can increase from 1.3 to 60% as both absolute bacterial abundance and proportion of bacteria attached to detrital particles increase, raising the oysters’ efficiency of filtration for these substrates.