Microbial life at high salt concentrations: phylogenetic and metabolic diversity

At the symposium on 'General & Applied Aspects of Halophilic Microorganisms', held in Alicante, Spain in September 1989, Hans Trüper presented a lecture entitled: "Halophily, taxonomy, phylogeny and nomenclature". The talk summarized different aspects of the taxonomy of halophilic microorganisms, the phylogenetic position of the different types of halophiles within the microbial world, and the physiology of the diverse groups of extreme and moderate halophiles. An attempt was made to detect consistent patterns and correlations between the phylogeny of halophilic microorganisms and the ways they cope with the high salt concentrations in the environment in which they live. A summary of this talk was published three years later in the proceedings volume of the Alicante meeting [1], and there Prof. Trüper and his coworkers presented a critical analysis of the data. They noted a few regular patterns that link the physiological properties of the halophilic organisms with their phylogenetic position. Four such general observations were presented:(1) "All eubacteria that gain energy from photosynthesis or oxygen-respiration and are capable of haloadaptation, are able to accumulate and/or produce compatible solutes."(2) "Archaeobacteria and anaerobic fermenting eubacteria are incapable of synthesizing organic compatible solutes."(3) "Most eubacteria that can grow in non-complex media are capable of ectoine biosynthesis."(4) "Extreme halophily in non-fermenting eubacteria is usually accompanied by glycine betaine biosynthesis. The complete de novo synthesis of betaine from carbon dioxide or simple organic carbon compounds has only been proven for cyanobacteria, Ectothiorhodospira species and Actinopolyspora halophila."In the eighteen years that have passed since the Alicante meeting our understanding of the salt-loving microbial world has greatly increased. A large number of new halophilic organisms have been discovered and described, including organisms phylogenetically unaf