Relationship between phylogenetic distance and growth rates of soil bacterial isolates in culture

Background/Question/Methods:

The vast majority of carbon fixed in terrestrial environments enters the soil microbial food-web. Despite widespread recognition of the effects that both the quantity and quality (carbon:nutrient ratios) of these inputs have on rates of decomposition and nutrient cycling, very little is known about how these factors influence microbial community structure and functioning. Recent efforts to apply life history strategy theory, specifically r-K selection, to microbial ecology draw on observational patterns of microbial community composition across environments and the differential response of taxa to addition of specific carbon substrates in the field. These studies suggest phyla-level patterns in life history strategy of soil, but are unable to directly compare the growth responses of individual taxa independent of the effects of interactions within the community. To better understand the potential application of life history strategy theory to soil bacteria, over 1300 cultures were isolated from soil inocula be collected from replicate monoculture plots of each of the 16 plant species grown in the BioCON long-term experiment under ambient C and N conditions. Cultures were isolated on a 100X dilution of soil extract agar containing cycloheximide to prevent fungal growth and allow development of slow growing bacterial cultures. Day of colony appearance was marked and up to 8 colonies appearing on each day for each plant species were further streaked for isolation. Easily culturable organisms growing on dilute soil extract agar are not likely to represent the end members of life history strategies; rather, these strategies exist on a continuum making differences found in this study conservative relative to the full range of differences likely occurring in soil communities.

Results/Conclusions:

Colony appearance began on day 3 and ended by day 23. Isolates represent at least 5 bacterial phyla. Growth rates of isolates were measured on a 10X of nutrient broth and growth was measured as a change in absorbance at 580, 600, and 620 nm wavelengths over a 16 hour incubation period. Growth curves were fit to the data using a newly-developed automated curve-fitting process (presented and discussed here). Growth rates varied over several orders of magnitude with a distribution skewed towards slower growing organisms. Growth rates of bacterial isolates will be compared by the plant species plots from which these isolates orginated as well as comparison of the relationship between 16S-based phylogenetic distance and growth rates for a subset of 200 isolates from Sanger sequencing data of the 16S ribosomal RNA gene.