|Liu, YJ; Zaprasis, A; Liu, SJ; Drake, HL; Horn, MA: The earthworm Aporrectodea caliginosa stimulates abundance and activity of phenoxyalkanoic acid herbicide degraders, ISME Journal, 5, 473-485 (2011)|
2-Methyl-4-chlorophenoxyacetic acid (MCPA) is a widely used phenoxyalkanoic acid (PAA) herbicide. Earthworms represent the dominant macrofauna and enhance microbial activities in many soils. Thus, the effect of the model earthworm Aporrectodea caliginosa (Oligochaeta, Lumbricidae) on microbial MCPA degradation was assessed in soil columns with agricultural soil. MCPA degradation was quicker in soil with than without earthworms. Quantitative PCR was inhibition-corrected per nucleic acid extract and indicated that copy numbers of tfdA-like and cadA genes (both encoding oxygenases initiating aerobic PAA degradation) in soil with earthworms were up to 3 and 4 times higher than without earthworms, respectively. tfdA-like and 16S rRNA gene transcript copy numbers in soil with earthworms were 2 and 6 times higher than without earthworms, respectively. Most probable numbers (MPNs) of MCPA-degraders approximated 4x105 gdw-1 in soil prior to incubation and in soil treated without earthworms, whereas MPNs of earthworm-treated soils were approximately 150 x higher. The aerobic capacity of soil to degrade MCPA was higher for earthworm treated than not earthworm-treated soil. Burrow walls and 0 to 5 cm depth bulk soil displayed higher capacities to degrade MCPA than did soil from 5-10 cm depth bulk soil, expression of tfdA-like genes in burrow walls was 5 times higher than in bulk soil, and MCPA degraders were abundant in burrow walls (MPNs of 5x107 gdw-1). The collective data indicate that earthworms stimulate abundance and activity of MCPA degraders endogenous to soil via their burrowing activities and might thus be advantageous for enhancing PAA degradation in soil.
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