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Title: Assimilable organic carbon (AOC) in soil water extracts using Vibrio Harveyi BB721 and its implication for microbial biomass

Author
item MA, JINCAI - University Of California
item Ibekwe, Abasiofiok - Mark
item WANG, HAIZHEN - Zhejiang University
item XU, JIANMING - Zhejiang University
item LEDDY, MENU - Orange County Water District
item YANG, CHING-HONG - University Of Wisconsin
item CROWLEY, DAVID - University Of California

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2012
Publication Date: 5/4/2012
Citation: Ma, J., Ibekwe, A.M., Wang, H., Xu, J., Leddy, M., Yang, C., Crowley, D.E. 2012. Assimilable organic carbon (AOC) in soil water extracts using Vibrio Harveyi BB721 and its implication for microbial biomass. PLoS One. 7(5): e28519. DOI: 10.1371/journal.pone.0028519.

Interpretive Summary: Carbon availability is the primary determinant of microbial growth potential in both water and soil, but must be empirically defined for different carbon pools that vary in their availability. Assimilable organic carbon is the most active fraction of biodegradable dissolved organic carbon since it can be readily used by microorganisms for growth. In this study, we developed a simple method for determining the amount of assimilable organic carbon available to bacteria in soil using a naturally occurring marine bacterium with the ability to produce light. Our results showed that assimilable organic carbon could be used as a routine test for monitoring soil and sediment quality and health. Since the concentrations of labile carbon vary for different organic materials, methods to assay the assimilable organic carbon in soils provide a means to assess the ability of organic amendments to promote microbial growth and to track changes in growth potential over time. Assimilable organic carbon from the soils studied significantly correlated with total soil organic carbon, total nitrogen, and humic acid concentrations and thereby providing new understandings into our ability to estimate the most available carbon pool to bacteria in soil that can be easily used for growth. This research will directly benefit water quality managers, soil scientists, farmers, conservation biologists, and the public.

Technical 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 15 soils tested was 5.4 ± 1.2 mg glucose C kg-1. Our data showed that AOC levels in soil water extracts were significantly correlated (P < 0.01) with microbial biomass determined as total phospholipid fatty acids and 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.