Microorganisms and Interfaces

Microorganisms are, by definition, the smallest of all living organisms, yet they play a dominant role in primary productivity in aquatic systems, in decomposition of waste organic materials, and in biogeochemical transformations of mineral elements in both terrestrial and aquatic systems (Alexander 1971). All microorganisms are dependent upon inorganic nutrients and, in the case of heterotrophs, organic nutrients for satisfactory growth and metabolism. Some natural microbial habitats are characterized by a relatively constant input of nutrients, which sustain a high level of microbial activity; but many habitats receive only spasmodic or chronically low nutrient inputs, resulting in relatively low microbial activity. Such habitats include many soils, the open oceans, and oligotrophic lakes and streams. The majority of microorganisms require an aqueous environment for growth. Where the aqueous phase is nutrient-deficient, substantial microbial activity usually is encountered only at interfaces in the system. Interfaces are boundaries between any two phases in a heterogeneous system and possessing physicochemical properties that differ from those of either phase. Natural microbial habitats are heterogeneous systems containing a wide range of solid-liquid, gas-liquid, liquid-liquid, and, occasionally, solid-gas interfaces. Such interfaces serve as sites of nutrient accumulation in the form of lipids, macromolecules, other organic molecules, and inorganic nutrients. Microorganisms tend to accumulate at solidliquid interfaces (Stark et al. 1938, ZoBell and Anderson 1936) and gas-liquid interfaces (Parker and Barsom 1970). They achieve substantial growth at these interfaces even in highly oligotrophic conditions. The interfaces can be regarded as sites of escape from the nutrient-deficient aqueous phase; thus, all interfaces exert a potential influence on microbial distribution, growth, metabolism, and succession in such habitats. The solid phases found in nature include large surfaces, such as rocks, ship hulls, plants, and animals, as well as small surfaces, such as clay particles, plankton, and biological debris suspended in the aqueous phase. The major gas phase is air at the surface of large water bodies, but this phase also may consist of H2, CO2, CH4, and H2S in anaerobic sediments and soils, as well as in gas bubbles originating from sediments. The most obvious liquid-liquid interface is that encountered following an oil spill in oceans and estuaries, where the oil phase overlies the aqueous phase. Although interfaces provide an advantageous situation with respect to the growth of microorganisms, there are instances where microbial activity at such sites is of nuisance value to man. Microbial growth on the inside of pipelines and on ship surfaces, for example, increases surface roughness and thus creates greater fluid frictional resistance; microbial growth on heat transfer surfaces has an insulating effect that alters the heat transfer characteristics of the system (Characklis 1980). A study of microorganisms and interfaces, then, must consider the modes of transport of the organisms to interfaces and the physicochemical factors involved in interactions between the microorganisms and the interfaces. Recent research has concentrated mainly on the behavior of bacteria at interfaces, because bacteria colonize interfaces very rapidly. The smallest of all living organisms (most range from 0.5 to 2.0 /tm in length), bacteria possess a net negative surface charge and tend to behave as colloidal particles in an aqueous phase. Critical studies of both the biological and colloidal properties of bacteria (Marshall 1976) have substantially expanded the information on bacteria at interfaces. The surface charge properties of bacteria may be altered by lowering the pH of the aqueous phase or by adding polyvalent cations (Santoro and Stotzky 1967), as well as by adsorption of organic materials to the bacterial surface (Neihof and Loeb 1974).