Abstract
The intracellular accumulation of small organic solutes was described in the marine bacterium Rhodopirellula baltica, which belongs to the globally distributed phylum Planctomycetes whose members exhibit an intriguing lifestyle and cell morphology. Sucrose, α-glutamate, trehalose and mannosylglucosylglycerate (MGG) are the main solutes involved in the osmoadaptation of R. baltica. The ratio and total intracellular organic solutes varied significantly in response to an increase in salinity, temperature and nitrogen content. R. baltica displayed an initial response to both osmotic and thermal stresses that includes α-glutamate accumulation. This trend was followed by a rather unique and complex osmoadaptation mechanism characterized by a dual response to sub-optimal and supra-optimal salinities. A reduction in the salinity to sub-optimal conditions led primarily to the accumulation of trehalose. In contrast, R. baltica responded to salt stress mostly by increasing the intracellular levels of sucrose. The switch between the accumulation of trehalose and sucrose was by far the most significant effect caused by increasing the salt levels of the medium. Additionally, MGG accumulation was found to be salt- as well as nitrogen-dependent. MGG accumulation was regulated by nitrogen levels replacing α-glutamate as a K+ counterion in nitrogen-poor environments. This is the first report of the accumulation of compatible solutes in the phylum Planctomycetes and of the MGG accumulation in a mesophilic organism.
Highlights
A large number of microorganisms rely exclusively on the accumulation of low-molecular-weight organic compounds, designated compatible solutes, for osmoadaptation, indicating that this is a very successful strategy [1]
The total solute pool under salt stress depended on the phase of growth and was higher in cells harvested during the stationary phase than in cells harvested during the exponential phase (Fig. 1)
The total solute pool under salt stress depended on the phase of growth and it was higher in cells harvested during the stationary phase than in cells harvested during the exponential phase
Summary
A large number of microorganisms rely exclusively on the accumulation of low-molecular-weight organic compounds, designated compatible solutes, for osmoadaptation, indicating that this is a very successful strategy [1]. Compatible solutes can be uptaken from the environment or synthesized de novo to fulfill the specific requirements of the organism. These compatible solutes, named osmolytes, were associated with the need to reach an osmotic equilibrium, but are viewed as very versatile protectors of cell components against a number of stress agents [2]. The accumulation of a-glutamate, a universal negatively charged compatible solute, reaches a physiological plateau prior to the activation of the osmoadaptive phenomena to higher salt stress [3,4]
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