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The net primary production of the biosphere is consumed largely by microorganisms, whose metabolism creates the trophic base for detrital foodwebs, drives element cycles, and mediates atmospheric composition. Biogeochemical constraints on microbial catabolism, relative to primary production, create reserves of detrital organic carbon in soils and sediments that exceed the carbon content of the atmosphere and biomass. The production of organic matter is an intracellular process that generates thousands of compounds from a small number of precursors drawn from intermediary metabolism. Osmotrophs generate growth substrates from the products of biosynthesis and diagenesis by enzyme-catalyzed reactions that occur largely outside cells. These enzymes, which we define as ecoenzymes, enter the environment by secretion and lysis. Enzyme expression is regulated by environmental signals, but once released from the cell, ecoenzymatic activity is determined by environmental interactions, represented as a kinetic cascade, that lead to multiphasic kinetics and large spatiotemporal variation. At the ecosystem level, these interactions can be viewed as an energy landscape that directs the availability and flow of resources. Ecoenzymatic activity and microbial metabolism are integrated on the basis of resource demand relative to environmental availability. Macroecological studies show that the most widely measured ecoenzymatic activities have a similar stoichiometry for all microbial communities. Ecoenzymatic stoichiometry connects the elemental stoichiometry of microbial biomass and detrital organic matter to microbial nutrient assimilation and growth. We present a model that combines the kinetics of enzyme activity and community growth under conditions of multiple resource limitation with elements of metabolic and ecological stoichiometry theory. This biogeochemical equilibrium model provides a framework for comparative studies of microbial community metabolism, the principal driver of biogeochemical cycles.
Modeling studies suggest that production of extracellular enzymes has first priority on cell
metabolism above the level of maintenance respiration (Schimel & Weintraub 2003, Moorhead
& Sinsabaugh 2006, Moorhead et al. 2012, Wang et al. 2012a).
Approximately 1–4% of the production of heterotrophic microbial communities is used to make
enzymes for secretion into the environment; many more are released through lysis (Maire et al.
2012).
For osmotrophic prokaryotes and fungi, the C sources are low–molecular
mass compounds, with individual taxa limited to a small number (∼1–20) of growth substrates.
These growth substrates are generated from the myriad products of biosynthesis and diagenesis
by enzyme-catalyzed reactions that occur largely outside cells (Burns 1978, Chrost 1991, Burns &
Dick 2002, Shukla & Varma 2011, Trasar-Cepeda et al. 2011, Dick 2012).
Cellulose production accounts for about half of terrestrial NPP
(Ericksson et al. 1990). Hemicelluloses, polymers of xylose, mannose, galactose, arabinose, and
glucose, compose 20–30% of the mass of plant cell walls (Ericksson et al. 1990).
Lignin, the most recalcitrant component of plant litter, accounts for
about one-quarter of terrestrial NPP.
Lignin degradation is a
rate-limiting process in litter decomposition and a lignocellulose index (ratio of lignin to lignin
plus cellulose) of 0.7 is considered the limit of decomposition, i.e., the transition between plant
litter and soil organic matter (Berg & McClaugherty 2010).
Sinsabaugh et al. (1992, 1993) showed that differences in decomposition rate of birch sticks
placed at six sites were directly related to cellulolytic enzyme activities and inversely related to the
activities of enzymes involved in P and N acquisition. These studies provided the empirical basis
for a model that linked EEA to mass-loss rates using ratios of C-, N-, and P-acquiring EEAs as
indicators of microbial resource allocation (Sinsabaugh & Moorhead 1994).
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