The physiological mechanisms utilized by soil bacteria for acclimation to s
udden increases in soil water potential are poorly understood. In this stud
y, 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 isolat
es were cultured at a low solute water potential (-3.0 MPa) and subjected t
o rapid water potential increases of 0.5 to 2.0 MPa. The small amount of pr
otein 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 env
ironment 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 d
ilution had no effect on culturability, and the amount of solutes released
was small (<10% of the intracellular solute pool). The Pseudomonas spp. rel
eased a maximum of 22 to 26% of their amino acid pool and 54 to 60% of thei
r low molecular weight neutral sugar pool. The amounts of amino acids and l
ow molecular weight carbohydrates released and the reduction in culturabili
ty was, in general, proportional to the magnitude of dilution. Pseudomonas
fluorescens tolerated a 0.5 MPa water potential increase, but water potenti
al shocks of greater magnitude resulted in a large reduction in culturabili
ty and an increase in the amount of solutes released. These results suggest
that a potential source of mineralizable C following the wetting of dry so
ils is the release of organic compatible solutes from the microbial communi
ty.