Abstract
Assimilable organic carbon (AOC) is commonly used to measure the growth potential of microorganisms in water, but has not yet been investigated for measuring microbial growth potential in soils. In this study, a simple, rapid, and non-growth based assay to determine AOC in soil was developed using a naturally occurring luminous strain Vibrio harveyi BB721 to determine the fraction of low molecular weight organic carbon in soil water extract. Calibration of the assay was achieved by measuring the luminescence intensity of starved V. harveyi BB721 cells in the late exponential phase with a concentration range from 0 to 800 µg l−1 glucose (equivalent to 0–16.0 mg glucose C kg−1 soil) with the detection limit of 10 µg l−1 equivalent to 0.20 mg glucose C kg−1 soil. Results showed that bioluminescence was proportional to the concentration of glucose added to soil. The luminescence intensity of the cells was highly pH dependent and the optimal pH was about 7.0. The average AOC concentration in 32 soils tested was 2.9±2.2 mg glucose C kg−1. Our data showed that AOC levels in soil water extracts were significantly correlated (P<0.05) with microbial biomass determined as microbial biomass carbon, indicating that the AOC concentrations determined by the method developed might be a good indicator of soil microbial biomass. Our findings provide a new approach that may be used to determine AOC in environmental samples using a non-growth bioluminescence based assay. Understanding the levels of AOC in soil water extract provides new insights into our ability to estimate the most available carbon pool to bacteria in soil that may be easily assimilated into cells for many metabolic processes and suggest possible the links between AOC, microbial regrowth potential, and microbial biomass in soils.
Highlights
Availability of carbon has been assumed to be the primary determinant factor for bacterial growth in both water and soil, there are reports of limitation by other nutrients, e.g. nitrogen and phosphorus [1]
biodegradable dissolved organic carbon (BDOC) is the fraction of the dissolved organic carbon (DOC) which can be metabolized by bacteria within a few days to a few months
The assimilable organic carbon (AOC) concentrations in water samples were calculated by relating the equivalent yields of bacteria that were produced on defined substrates to generate a standard curve
Summary
Availability of carbon has been assumed to be the primary determinant factor for bacterial growth in both water and soil, there are reports of limitation by other nutrients, e.g. nitrogen and phosphorus [1]. The most common methods to estimate carbon availability are measurements of biodegradable dissolved organic carbon (BDOC), biological oxygen demand (BOD), and assimilable organic carbon (AOC). BDOC is the fraction of the dissolved organic carbon (DOC) which can be metabolized by bacteria within a few days to a few months. BDOC measures the change in dissolved organic carbon (DOC) in a water sample after incubation with microorganisms for a period of time. AOC is the most bioavailable fraction of BDOC since it can be readily assimilated by microorganisms for growth [3,4]. Carbon is generally fractionated into pools with different degrees of recalcitrance, primarily to determine carbon turnover rates under different environmental conditions and soil management [5]
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