Beyenal Haluk, Lewandowski Zbigniew
Center for Biofilm Engineering and Department of Civil Engineering, Montana State University, Bozeman, MT 59717, USA.
Biotechnol Prog. 2002 Jan-Feb;18(1):55-61. doi: 10.1021/bp010129s.
It appears that biofilms arrange their internal structure according to the flow velocity at which they are grown, which affects the internal mass transfer rate and microbial activity. In biofilms grown at various flow velocities we determined the vertical profiles of the local relative effective diffusivity (termed D(l)) at several locations within each biofilm. From these profiles we calculated the surface-averaged relative effective diffusivity (termed D(sa)) at various distances from the bottom and plotted it against these distances. The D(sa) decreased linearly toward the bottom, forming well-defined profiles that were different for each biofilm. The gradients of these profiles were multiplied by the diffusivity of oxygen, zeta = D(w) dD(sa)/dz, and plotted versus the flow velocity at which each biofilm was grown. The gradients were low at flow velocities below 10 cm/s, reached a maximum at a flow velocity of 10 cm/s, and decreased again at flow velocities exceeding 10 cm/s. The existence of a maximum indicates a possibility that two opposing forces were affecting the slope of the profiles. To explain these observations we hypothesized that biofilms, depending on the flow velocity at which they are grown, arrange their internal architecture to control (1) the nutrient transport rate and (2) the mechanical pliability needed to resist the shear stress of the water flowing past them. It appears that biofilms attempt to satisfy the second goal first, to increase their mechanical strength, and that they do so at the expense of the nutrient transfer rate to deeper layers. This strength increase is associated with an increase in biofilm density, which slows down the internal mass transport rate. Biofilms grown at low flow velocities exhibit low density and high effective diffusivity but cannot resist higher shear stress, whereas biofilms grown at higher flow velocities are denser and can resist higher shear stress but have a lower effective diffusivity.
生物膜似乎会根据其生长时的流速来安排内部结构，这会影响内部传质速率和微生物活性。在不同流速下生长的生物膜中，我们测定了每个生物膜内几个位置处的局部相对有效扩散率（称为D(l)）的垂直分布。根据这些分布，我们计算了距底部不同距离处的表面平均相对有效扩散率（称为D(sa)），并将其与这些距离作图。D(sa)朝着底部呈线性下降，形成了每个生物膜都不同的明确分布。这些分布的梯度乘以氧气的扩散率，ζ = D(w) dD(sa)/dz，并与每个生物膜生长时的流速作图。在流速低于10 cm/s时梯度较低，在10 cm/s的流速时达到最大值，而在流速超过10 cm/s时又再次下降。最大值的存在表明可能有两种相反的力在影响分布的斜率。为了解释这些观察结果，我们假设生物膜根据其生长时的流速来安排内部结构，以控制(1)营养物质传输速率和(2)抵抗流过它们的水的剪切应力所需的机械柔韧性。似乎生物膜首先试图满足第二个目标，即增加其机械强度，并且这样做是以牺牲营养物质向更深层的传输速率为代价的。这种强度的增加与生物膜密度的增加有关，这会减慢内部传质速率。在低流速下生长的生物膜密度低且有效扩散率高，但不能抵抗更高的剪切应力，而在高流速下生长的生物膜密度更大，能够抵抗更高的剪切应力，但有效扩散率较低。
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