Entomopathogenic nematodes and their natural enemies in a soil pollution gradient

Background/Question/Methods

Mining activities pollute the environment with by-products that cause unpredictable impacts in surrounding areas. Lead, zinc and iron have been mined at the Cartagena–La Unión mine in southeastern Spain for more than 2500 years. The mine closed in 1991 but high concentrations of metals and waste residues remain in this area. In order to characterize the effects of pollution gradients emanating from the mine on subterranean communities, we measured entomopathogenic nematodes (EPNs, 6 species), nematophagous fungi (NF, 7 species), bacterial ectoparasites of EPNs (2 species) and a group of free-living nematodes that sometimes compete with EPNs (FLNs) using real-time qPCR. Target organisms and more than 30 soil physical-chemical properties were measured in soil samples (n=193) from six areas within a pollution gradient: Mined area-no vegetation (M-NV); Mined area–sparce vegetation (M-SV); Mine adjacent–sparce vegetation (MA-SV); Mine adjacent–heavy vegetation (MA-HV), Mine distant–sparse vegetation (MD-SV); Mine distant–agriculture (MD-Ag). Total nematodes were also determined by microscopy. The impacts of mining activity on the composition of these communities were estimated by univariate (correlations, Chi², SPSS) and multivariate (Redundacy Analysis, RDA, CANOCO) analyses.

Results/Conclusions

PCR targeted organisms were detected in 69% of the samples. All nematode species were detected. The FLNs were more abundant (P<0.001) in non-mined, disturbed soils (MA-SV, MD-SV and MD-Ag), whereas the EPN Heterorhabditis indica also occupied all habitats, but with greatest abundance in M-SV. A second EPN, Steinernema feltiae, was detected throughout the mining gradient in both disturbed and undisturbed soils (M-SV, MA-HV and MD-SV). The remaining 3 EPN species occurred in single samples from M-SV and MA-HV.  Five NF species, but no ectoparasitic bacteria were detected. Paecilomyces lilacinus and Catenaria sp. were common in all habitats, and Catenaria exhibited similar spatial patterns as the FLNs (P=0.065). Mined areas were significantly depauperate of nematodes, with soils characterized by low organic matter and pH, and high heavy metals contents. Redundacy Analysis explained 78.3% of species-environment relationships. Significant variables were pH, SOM, K,  Carbonates, Mn, Zn, Mg and Pb. The first factor described the mining gradient, with total nematodes associated with plant nutrient elements and disassociated with heavy metals. The second factor associated FLNs and disassociated Catenaria sp. with Ca and P. Whereas FLNs responded strongly to the pollution gradient, EPNs appear to have adapted to any condition that might support arthropod hosts.