Microorganisms that adapt to moderate and high salt environments use a variety of solutes, organic and inorganic, to counter external osmotic pressure. The organic solutes can be zwitterionic, noncharged, or anionic (along with an inorganic cation such as K+). The range of solutes, their diverse biosynthetic pathways, and physical properties of the solutes that effect molecular stability are reviewed.

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Organic compatible solutes of halotolerant and halophilic microorganisms

The osmoadaptation of most micro-organisms involves the accumulation of K+ ions and one or more of a restricted range of low molecular mass organic solutes, collectively termed ‘compatible solutes’. These solutes are accumulated to high intracellular concentrations, in order to balance the osmotic pressure of the growth medium and maintain cell turgor pressure, which provides the driving force for cell extension growth. In this review, I discuss the alternative roles which compatible solutes may also play as intracellular reserves of carbon, energy and nitrogen, and as more general stress metabolites involved in protection of cells against other environmental stresses including heat, desiccation and freezing. Thus, the evolutionary selection for the accumulation of a specific compatible solute may not depend solely upon its function during osmoadaptation, but also upon the secondary benefits its accumulation provides, such as increased tolerance of other environmental stresses prevalent in the organism’s niche or even anti-herbivory or dispersal functions in the case of dimethylsulfoniopropionate (DMSP). In the second part of the review, I discuss the ecological consequences of the release of compatible solutes to the environment, where they can provide sources of compatible solutes, carbon, nitrogen and energy for other members of the micro-flora. Finally, at the global scale the metabolism of specific compatible solutes (betaines and DMSP) in brackish water, marine and hypersaline environments may influence global climate, due to the production of the trace gases, methane and dimethylsulfide (DMS) and in the case of DMS, also couple the marine and terrestrial sulfur cycles.

Ecological significance of compatible solute accumulation by micro‐organisms: from single cells to global climate

Publisher Summary This chapter focuses on the osmoadaptation in bacteria. Bacterial ability to osmoadapt seems to be the norm rather than the exception. Under the right environmental circumstances (external osmolytes or precursors thereof), many bacteria, so far believed to be salt-sensitive, show considerable osmotolerance. Mechanisms for survival and growth in a high-osmolarity environment aim at maintaining osmotic equilibrium across the membrane. Compatible solutes used and produced by halo/osmophilic bacteria are useful tools to study and understand water solvent interactions, and to resolve principles of enzyme stabilization. They are bound to offer a range of technological applications as universal stress protectants. Among anaerobic halophilic eubacteria, attempts to find organic osmolytes have been unsuccessful; instead,high cytoplasmic amounts of potassium and sodium chloride were detected and proteins displayed an excess of acidic amino acids. As all enzymes investigated also showed maximal activity at high salt concentrations, there seems to be sufficient evidence that this eubacterial physiological group is adapted to an ionic cytoplasm. The salt-in-cytoplasm type of osmoadaptation is therefore apparently not confined to archaebacterial representatives, although NaCl rather than KCl seems to be the dominant cytoplasmic solute in these eubacteria. The best-studied model systems of osmotic adaptation are E. coli and S. typhimurium, which are discussed.

Osmoadaptation in Bacteria

Salt stress is primarily osmotic stress, and halophilic/halotolerant microorganisms have evolved two basic mechanisms of osmoadaplation: the KCI-type and the compatible-solute type, the latter representing a very flexible mode of adaptation making use of distinct stabilizing properties of compatible solutes. A comprehensive survey, using HPLC and NMR methods, has revealed the full diversity of euhacterial compatible solutes found in nature. With the exception of proline (a proteinogenic amino acid) they are characterized as amino acid derivatives of the following types: betaines, ectoines, N-acetylated diamino acids and N-derivatized carboxamides of glutamine. From our present knowledge of hiosynthetic pathways it appears that, apart from glycine betaine, all nitrogen-containing compatible solutes originate from two major pathways (the aspartate branch and the glutamate branch). Uptake of compatible solutes from the growth medium (environment) seems to have preference over de novo synthesis. Therefore in the natural ecosystem the solutes of primary producers (mainly glycine betaine), which are readily excreted upon dilution stress, certainly play an important role as a ‘preferred’ solute source for heterolrophic organisms, and as a ‘vital’ source for organisms unable to synthesize their own compatible solutes.

Microbial behaviour in salt‐stressed ecosystems

The accumulation of compatible solutes is a prerequisite for the adaptation of microorganisms to osmotic stress imposed by salt or organic solutes Two types of strategies exist to cope with high external solute concentrations; one strategy is found in the extremely halophilic Archaea of the family Halobacteriaceae and the Bacteria of the order Haloanaerobiales involving the accumulation of inorganic ions The other strategy of osmoadaptation involves the accumulation of specific organic solutes and is found in the vast majority of microorganisms The organic osmolytes range from sugars, polyols, amino acids and their respective derivatives, ectoines and betaines The diversity of these organic solutes has increased in the past few years as more organisms, especially thermophilic and hyperthermophilic Bacteria and Archaea, have been examined The term compatible solute can also be applied to solutes that protect macromolecules and cells against stresses such as high temperature, desiccation and freezing The mechanisms by whihh compatible solutes protect enzymes, cell components and cells are still a long way from being thoroughly elucidated, but there is a growing interest in the utilization of these solutes to protect macromolecules and cells from heating, freezing and desiccation

An overview of the role and diversity of compatible solutes in Bacteria and Archaea

A novel cyclic amino acid was detected in and subsequently isolated from extremely halophilic species of the bacterial genus Ectothiorhodospira. The structure of this new compound was elucidated by a combination of nuclear magnetic resonance (NMR) techniques and mass spectrometry. 1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid is only accumulated within the cytoplasm under certain growth conditions and seems to serve an osmoregulatory function. There is no previous reference to this molecule in the chemical literature and we, therefore, propose to use the trivial name 'ectoine', due to its discovery in members of the bacterial genus Ectothiorhodospira. (formula: see text).

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1985.tb08903.x

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid. A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira.

Pocket Porphyrins with Intramolecular Ligands



The synthesis of phorphyrins 12 with a pocket and an intramolecularly attached pyridine ring as a ligand is described. The key step involves porphyrin cyclization of 1,3,5-benzene-tricarboxylic tris[2-(2-formylphenoxy)ethyl] ester (10), building up the pocket, and 3-[3-(4-alkyl-2-formylphenoxy)propyl]pyridine (5), introducing the pyridine ligand, with four equivalents of pyrrole (11). Preparation procedures for the starting materials are described; NMR data of the target porphyrins 12 and their assignments are tabulated.

Heinz‐Peter Pfeiffer

Papers

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid. A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira.

Taschenporphyrine mit intramolekularen Liganden

Chirale Lattenzaun-Porphyrine