Which Populations of Aquatic Bacteria Should We Enumerate?

Abstract

Bacteria in aquatic systems comprise at least four distinct populations. The planktonic population floates in the water column with the particle-associated population, while the epilithic and epiphytic populations are attached to submerged surfaces, and the sediment population occupies the water-filled spaces of the sediment. Samples taken from the water column include the planktonic and particle-associated bacteria, and core samples include the sediment population, but special techniques are required to sample the epilithic and epiphytic organisms. In all bacterial populations, except the planktonic, the natural adhesion of the cells to surfaces and to each other produces adherent microcolonies whose cohesion presents special problems in bacterial enumeration. Since we have described freshwater systems in which epilithic bacterial populations are numerically dominant, and since large sediment populations are found in most aquatic systems, it is imperative that bacterial enumeration studies recognize each of these distinct populations. The authors examine the methods available for the enumeration of the bacteria in each distinct population and suggest a holistic approach to bacterial enumeration that can be applied to any aquatic system.