Thullner Martin, Kampara Makeba, Richnow Hans H, Harms Hauke, Wick Lukas Y
Departments of Environmental Microbiology and Isotope Biogeochemistry, UFZ - Helmholtz Centre for Environmental Research, Leipzig, Germany.
Environ Sci Technol. 2008 Sep 1;42(17):6544-51. doi: 10.1021/es702782c.
The microbial degradation of organic substrates often exhibits a fractionation of stable isotopes which leads to an enrichment of the heavier isotope in the remaining substrate. The use of this effect to quantify the amount of biodegraded substrate in contaminated aquifers requires that the isotope fractionation factor is constant in time and space. In many natural and engineered systems the bioavailable concentration at the location of the enzymes differs from the average bulk concentration of the substrate. When enzymatically driven substrate degradation is coupled to a preceding transport step controlling the bioavailability of the substrate, the observed isotope fractionation becomes a function of the bulk substrate concentration. The sensitivity of the observed isotope fractionation factor toward such substrate concentration changes depends on the ratio of bulk substrate concentration and Michaelis-Menten constant and on the ratio between the specific affinity of the microorganisms toward the substrate and the first order rate constant of the bioavailability limiting transport process. Highest sensitivities toward substrate concentration were found for combinations of high substrate concentration with low substrate bioavailability (i.e., high ratios of substrate concentration and Michaelis-Menten constant, and high ratios of specific affinity and transport rate constant). As a consequence, changes in concentration and isotopic composition of a bioavailability limited substrate in batch experiments should not exhibit a linear relation in a Rayleigh plot, and the slope of the Rayleigh plot should show a decreasing trend with concentration decrease. When using isotope fractionation to quantify biodegradation along groundwater flow paths, changes in observed isotope fractionation might occur while contaminant concentration decreases along a flow path.
有机底物的微生物降解常常表现出稳定同位素分馏现象，这会导致剩余底物中较重同位素的富集。利用这种效应来量化受污染含水层中生物降解底物的量，要求同位素分馏因子在时间和空间上保持恒定。在许多自然和工程系统中，酶作用位置处的生物可利用浓度与底物的平均总体浓度不同。当酶促驱动的底物降解与控制底物生物可利用性的前期传输步骤相结合时，观察到的同位素分馏就成为总体底物浓度的函数。观察到的同位素分馏因子对这种底物浓度变化的敏感性取决于总体底物浓度与米氏常数的比值，以及微生物对底物的比亲和力与生物可利用性限制传输过程的一级速率常数之间的比值。在高底物浓度与低底物生物可利用性的组合情况下（即底物浓度与米氏常数的高比值，以及比亲和力与传输速率常数的高比值），发现对底物浓度的敏感性最高。因此，在分批实验中，生物可利用性受限底物的浓度和同位素组成变化在瑞利图中不应呈现线性关系，并且瑞利图的斜率应随浓度降低呈现下降趋势。当利用同位素分馏来量化沿地下水流路径的生物降解时，在污染物浓度沿水流路径降低的同时，观察到的同位素分馏可能会发生变化。
