Release of intracellular solutes by four soil bacteria exposed to dilution stress

The physiological mechanisms utilized by soil bacteria for acclimation to sudden increases in soil water potential are poorly understood. In this study, we examined the physiological responses of soil isolates of Pseudomonas chlororaphis, P. fluorescens, Bacillus pumulis, and Streptomyces griseus to a sudden increase in solution water potential (dilution). Bacterial isolates were cultured at a low solute water potential (-3.0 MPa) and subjected to rapid water potential increases of 0.5 to 2.0 MPa. The small amount of protein and DIVA released bg a 2.0 MPa dilution suggests that water potential increases up to 2.0 MPa did not cause significant cell lysis. In response to dilution, intracellular solutes were released into the extracellular environment rather than polymerized into osmotically less-active compounds or catabolized to CO2. In general, the Gram-positive isolates B. pumulis and S. griseus were more tolerant to dilution than the Pseudomonas spp., since dilution had no effect on culturability, and the amount of solutes released was small (<10% of the intracellular solute pool). The Pseudomonas spp. released a maximum of 22 to 26% of their amino acid pool and 54 to 60% of their low molecular weight neutral sugar pool. The amounts of amino acids and low molecular weight carbohydrates released and the reduction in culturability was, in general, proportional to the magnitude of dilution. Pseudomonas fluorescens tolerated a 0.5 MPa water potential increase, but water potential shocks of greater magnitude resulted in a large reduction in culturability and an increase in the amount of solutes released. These results suggest that a potential source of mineralizable C following the wetting of dry soils is the release of organic compatible solutes from the microbial community.