Diffusion processes and boundary layers in microbial mats

Abstract

On a bacterial scale, the interface between a microbial mat and the overflowing water is a remarkable environment governed by low Reynolds number hydrodynamics, by diffusional solute transport, and by exposure to extreme chemical fluctuations and gradients. The thin surface layer of benthic phototrophic mats, in which all photosynthesis and most of the respiration of the mat community takes place, is generally about a mm thick. Yet, these mats have a productivity and organic turn-over of a similar magnitude as planktonic ecosystems. Since the euphoric zone of mats is typically 10³–10⁵-fold thinner than that of the water column, the microbial activity per unit volume is correspondingly 10³–10⁵-fold higher. As a consequence, there is a dynamic balance between the rapid production and consumption of oxygen, the concentration of which may fluctuate between > 1 atm. partial pressure and total depletion within minutes during shifting light conditions. The microorganisms living at the mat surface are physiologically adapted to these chemical extremes. Many also have a highly developed motility and tactic response to the environmental factors. The responses are often simple for the individual cells, but together they may lead to complex behavioral patterns of the whole populations. This paper will review some of the microbiologically important properties of the interface and how some bacteria are adapted to this environment.