Abstract of PhD Thesis

Microbial Ecology of Synthetic Pyrethroid Degradation in Soil

Mairin Cannon, University College Dublin (2008)

Synthetic pyrethroids are a group of compounds derived from the natural insecticide pyrethrin, found in the flower heads of certain Chrysanthemum species. They are widely used for the control of animal ectoparasites such as sheep scab due to their limited toxicity towards mammals. Current practice for the disposal of synthetic pyrethroid- based sheep dips in Ireland is dilution with water prior to spreading on land, where it is assumed degradation, mediated by microbial processes will remove the compound. The aim of this study was to isolate an effective synthetic pyrethroid-degrading bacterium and examine its potential for degradation of used sheep dip prior to land-spreading.  The response of soil microbial communities to the addition of synthetic pyrethroids was also assessed. 

Nine synthetic pyrethroid degrading bacteria were isolated from enrichment cultures. One of these, a Gram negative bacterium, identified as Pseudomonas putida IMD 512, degraded a range of synthetic pyrethroids at significantly higher concentrations (200 mg l-1) than any bacterium reported to date. Although this organism effectively removed 75% cypermethrin from a medium at 30 oC within 28 days, it did not increase the rate or extent of cypermethrin removal from a non-sterile used sheep dip. Cypermethrin degradation by the indigenous microbial population in the used sheep dip could however be stimulated by aeration.

A microcosm-based experiment was used to investigate the effect of cypermethrin on microbial community structure in a silt-loam soil. Cypermethrin removal was shown to be microbially mediated and approximately 50% cypermethrin was removed from soil amended with 100 mg kg-1 after 56 days, with most degradation occurring within the first three days. No significant cypermethrin removal was observed in soil amended with 1000 mg kg-1 cypermethrin. Cypermethrin did not appear to adversely affect soil microbial activity, nor did it inhibit the growth of some common soil bacteria examined.

Molecular analysis of microcosm soils revealed that although cypermethrin addition influenced bacterial and fungal community structures, the response of both communities largely depended upon cypermethrin concentration and time. In particular, fungal community composition was strongly influenced by cypermethrin concentration in addition to the length of time after application. Although high concentrations of cypermethrin showed no appreciable effects on bacterial community structure, a large proportion of less abundant bacterial ribotypes were negatively affected by high concentrations (1000 mg kg-1) of the pyrethroid. Clones generated from soil community DNA consisted largely of gamma proteobacteria.