Release of Intracellular Solutes by Four Soil Bacteria Exposed to Dilution Stress

Abstract

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 DNA released by 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.

Steam treatment of surface soil: how does it affect water-soluble organic matter, C mineralization, and bacterial community composition?

“…We can also assume that the decrease in cultivability was either due to cell lyses or reflected a physiological stress. Bacteria surviving the steaming process have been exposed to osmotic stress and could release intracellular solutes (Halverson et al 2000), which is consistent with the observed increase in carbohydrates in the soil solution. Consequently, the dead bodies and/or osmolytes would substantiate the following flush in CO 2 release.…”

Rainfall-induced carbon dioxide pulses result from sequential resuscitation of phylogenetically clustered microbial groups

“…The fast response observed in our study could be a result of history; soil microorganisms from Mediterranean climates may be adapted to respond rapidly to favorable water conditions to compete for resources and survive in this water pulse-driven ecosystem. Survival of extreme water potential fluctuations, in general, may be more characteristic of soil bacteria than microorganisms from other environments that experience less water potential fluctuation (42).…”

Seasonal variation in carbon dioxide exchange over a Mediterranean annual grassland in California

“…The next generation of soil respiration models will need to assess these pulses and the rate they decrease as the upper soil dries, in addition to considering the basal effects of soil temperature and deep soil moisture. To do so we need better information on the physiological mechanisms that describe how soil microbes respond to sudden increase in moisture (Halverson et al, 2000).…”