The survival strategy of the soil microbial biomass

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This proposal is based on recent investigations into the trigger molecule response of the soil microbial biomass (defined as the mass of all soil micro-organisms of less than a volume of about 5000 cubic micrometer). Our work on trigger molecules results directly from a long period of research by Professors D S Jenkinson FRS, D S Powlson and P C Brookes at Rothamsted on the soil microbial biomass. The picture that has emerged is that the soil microbial biomass is a relatively huge population, often exceeding 10 tonnes fresh weight in UK arable soils (equivalent to about 100 sheep per hectare), with, overall, a very slow metabolic rate. This is a result of substrate being, usually, very scarce in soil relative to the size of the biomass. For example the biomass in an arable soil at Rothamsted contains about 500 kg C ha yet only about 1.2 t C ha enters this soil annually as roots, root exudates, stubble etc. Unsurprisingly, therefore the biomass has many characteristics typical of a resting or dormant population, eg. slow respiration rate and slow cell division rate. Yet, paradoxically, the biomass also exhibits other characteristics typical of a population undergoing exponential growth in vitro. Thus, it has an ATP concentration and adenylate energy charge (AEC) typical of those found in micro-organisms growing exponentially in vitro. These features are, we believe, evolutionary responses to the fact that while substrate may be scarce in soil it is never an impossibility. An alternative survival strategy, based upon resting spores may enable other, more speculative, organisms which invest energy to maintain high ATP and AEC, to take more rapid advantage of fresh substrate when it, occasionally, becomes available. Our work on trigger molecules has revealed that the biomass responds to micrograms concentrations of fresh substrate by evolving more C as carbon dioxide than was contained in the original substrate (possibly the microbiological equivalent of the Aitkins diet?). We hypothesise that, on sensing these low concentrations of substrate, the biomass becomes activated to a higher energy state, synthesising the suite of enzymes required for substrate degradation and new cellular growth and reproduction. We now wish to investigate this further by determining: 1. which substances act as trigger molecules? 2. What is the relationship between soil properties and the trigger response. 3. What is the source of the extra C ie. endocellular reserves or labile soil organic matter? 4. which specific microbial groups are stimulated by different trigger molecules? this is a collaborative project between Professors Phil Brookes (Rothamsted Research), Tony O¿Donnell (University of Newcastle) and Richard Evershed (University of Bristol). We will use a number of soil microbiological and biochemical methods eg. microbial biomass, soil enzymic techniques, ATP and adenylate charge, stable isotope techniques, gas chromatography coupled with mass spectrometry, and molecular biology. Joint with BB/C518222/1 and BB/C518749/1