COMMUNITY ASSEMBLY, ALTERNATE COMMUNITY STATES, AND THE INTRODUCTION SUCCESS OF A BIOREMEDIATIVE SOIL MICROORGANISM

摘要

A promising application of biotechnology is the field deployment of non-indigenous bioremediating microorganisms for amelioration of environmental contaminants. Successful colonization and persistence of the introduced organism is an implicit assumption of such an approach. Research in the assembly of ecological communities, however, has shown differences in the history of species invasions can lead to alternate community states with different vulnerabilities to invasion. Such findings could, therefore, have ramifications for the implementation of bioremediation processes where field introductions are employed. Here, in a soil microcosm setting, we tested the following hypotheses: 1) permuting the introduction sequence of competing microbial strains can produce alternate community states, and 2) the capacity of a genetically-engineered bioremediative soil microorganism - Pseudomonas fluorescens HK44 - to successfully colonize a competitive soil environment can be dependent on the invasion history associated with the recipient community. Alternate soil microcosm communities were created by the sequential introduction into a chemically contaminated soil media of three competing microbial strains (Mycobacterium Pyr-1, Sphingomonas A8AN3 and Rhodococcus Sm-1) under three alternative introduction sequences. HK44 was then introduced into each of these assembled communities. All introductions occurred at four-week intervals and the microcosms were allowed to equilibrate for five months following the introduction of HK44. Cell abundance of HK44 and other resident populations were monitored during the post-introduction phase using traditional dilution plate methods. ANOVA and multivariate analyses of community membership data taken at the conclusion of the experiment revealed alternate community states arising from each observed sequence. Significantly, results showed that HK44 only successfully colonized communities formed under two of the three sequence treatments observed. It was entirely absent in the third treatment for the duration of the experiment. In the two sequences where it was successful, a difference in the relative cell abundance of nearly three orders of magnitude was exhibited 14 days following their introduction. By the conclusion of the experiment 17 weeks later, however, both had converged to an approximate density of 4.0 x 106 cells per gram soil. Our results show that assembly processes can function in soil ecosystems to produce alternate community states. As some of these communities can reach a state that is resistant to invasion, some knowledge of community history parameters for targeted sites could play an integral role in effective bioremediation planning and implementation. Finally, we suggest the effects arising from permutations in the sequence of invasion could be directly harnessed in the remediation process. Here, instead of the introduction of a single remediating strain, several introductions involving a host of organisms could be employed. The sequence and timing of these introductions could then be manipulated to optimize the efficacy of the remediation process.