The Problem of Excess and/or Limitation of the Habitat Conditions: Do Natural Assemblages Exist?

Abstract

The structure of the communities present in both Lake Cisó and the microbial mats at the Ebro Delta, in Catalonia, Spain, as well as their changes with time and depth, were studied following a variety of approaches including taxonomic and physiological analyses, trophic state, population dynamics and structure of the food web, and competition among bacteria. Habitats with either an excess or a limitation of some natural condition harbour different kinds of microbial assemblages, which can be regarded as a means for self-regulation of the participating populations. Assemblages of phototrophic bacteria (cyanobacteria and purple and green sulphur bacteria) are among the most stable and balanced microbial communities. The population structure of a phototrophic community in Lake Cisó is determined by light penetration and by the abundance of sulphide. These two environmental factors are rarely found together in present day aquatic systems, except in habitats such as the metalimnion of some lakes (as in the case of Lake Cisó) and shallow water marine sediments (microbial mats). Simultaneous presence of green and purple sulphur bacteria has been observed in many occasions in karstic lakes. However, few studies have analysed which environmental factors are important in determining whether purple or green sulphur bacteria should be more abundant at a given time of the year. Light quantity and light quality have been implicated in the selection of purple or green sulphur bacteria in different lakes. This phenomenon has been studied in detail in Lake Cisó, where both groups of bacteria are present at different times of the year, thus providing a natural experiment where changes in environmental conditions change the relative abundance of the green and purple bacteria. Major factors determining the abundance of these two groups of bacteria are: solar radiation, temperature, accumulated rainfall, and the vertical distribution of both oxygen and sulphide.