The kinetics and elemental composition of cellular units that mediate production and respiration are the basis for the metabolic and stoichiometric theories of ecological organization. This theoretical framework extends to the activities of microbial enzymes released into the environment (ecoenzymes) that mediate the release of assimilable substrate from detrital organic matter. In this paper, we analyze the stoichiometry of ecoenzymatic activities in the surface sediments of lotic ecosystems and compare those results to the stoichiometry observed in terrestrial soils. We relate these ecoenzymatic ratios to energy and nutrient availability in the environment as well as microbial elemental content and growth efficiency. The data, collected by US Environmental Protection Agency, include the potential activities of 11 enzymes for 2,200 samples collected across the US, along with analyses of sediment C, N and P content. On average, ecoenzymatic activities in stream sediments are 2-5 times greater per gC than those of terrestrial soils. Ecoenzymatic ratios of C, N and P acquisition activities support elemental analyses showing that microbial metabolism is more likely to be C-limited than N or P-limited compared to terrestrial soils. Ratios of hydrolytic to oxidative activities indicate that sediment organic matter is more labile than soil organic matter and N acquisition is less dependent on humic oxidation. The mean activity ratios of glycosidases and aminopeptidases reflect the environmental abundance of their respective substrates. For both freshwater sediments and terrestrial soils, the mean C:nutrient ratio of microbial biomass normalized to growth efficiency approximates the mean ecoenzymatic C:nutrient activity ratios normalized to environmental C:nutrient abundance. This relationship defines a condition for biogeochemical equilibrium consistent with stoichiometric and metabolic theory.
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Acknowledgments The US EPA Office of Water sponsored the National Rivers and Streams Assessment. AR Olsen supervised the creation of the survey design. LR Seifert and AA May contributed greatly to the ecoenzyme analyses. We are indebted to the numerous, state, federal, and contractor field crews who collected the NRSA data used in this project. The views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the US Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. J.J.F.S. was supported by the National Science Foundation (DBI-0630558). R.L.S. was supported by NSF EaGER (DEB-0946288) and Ecosystem Studies programs (DEB-0918718). Source data for terrestrial soils were contributed by Sinsabaugh RL, Lauber CL, Weintraub MN, Ahmed B, Allison SD, Crenshaw C, Contosta AR, Cusack D, Frey S, Gallo ME, Gartner TB, Hobbie SE, Holland K, Keeler BL, Powers JS, Stursova M, Takacs-Vesbach C, Waldrop M, Wallenstein M, Zak DR, Zeglin LH.
- Ecological stoichiometry
- Extracellular enzyme activity
- Freshwater sediment
- Microbial activity
- Threshold element ratio