Fine-scale measurement of diffusivity in a microbial mat with nuclear magnetic resonance imaging

Noninvasive H-1-nuclear magnetic resonance (NMR) imaging was used to invest igate the diffusive properties of microbial mats in two dimensions. Pulsed field gradient NMR was used to acquire images of the H2O diffusion coeffici ent, D-s,D- and multiecho imaging NMR was used to obtain images of the wate r density in two structurally different microbial mats sampled from Solar L ake (Egypt). We found a pronounced lateral and vertical variability of both water density and water diffusion coefficient, correlated with the laminat ed and heterogeneous distribution of microbial cells and exopolymers within the mats. The average water density varied from 0.5 to 0.9, whereas the av erage water diffusion coefficient ranged from 0.4 to 0.9 relative to the va lues obtained in the stagnant water above the mat samples. The apparent wat er diffusivities estimated from NMR imaging compared well to apparent O-2 d iffusivities measured with a diffusivity microsensor. Analysis of measured O-2 concentration profiles with a diffusion-reaction model showed that both the magnitude of calculated rates and the depth distribution of calculated O-2 consumption/production zones changed when the observed variations of d iffusivity were taken into account. With NMR imaging, diffusivity can be de termined at high spatial resolution, which can resolve inherent lateral and vertical heterogeneities found in most natural benthic systems.