Effects of soil organic matter content in Cd toxicity in Eisenia
fetida: Implications for soil health assessment based on the
use of biomarkers and standard toxicity tests
A Irizar, MP Rodriguez, I Marigómez, M Soto*
CBET Research Group, Dept. Zoology & Animal Cell Biology, University of the Basque
Country (UPV/EHU). E-48080. Bilbao, Basque Country
(*) manu.soto@ehu.es
The organic matter (OM) content is a very variable parameter in real soils, and can
affect the mobility and bioavailability of soil pollutants. Although, this parameter is fixed
(10%) in standard soils for toxicity testing of chemical compounds using Eisenia fetida
earthworms as sentinel organisms (acute toxicity test –ATT- and reproduction test –
RT- OECD 1984, 2004), it can become a confounding factor in the assessment of real
soil health. E. fetida has been successfully employed for real soil health assessment
using a series of biomarkers in target cells. One of the most widely used biomarkers is
the neutral red retention capacity measured in coelomocytes (immune cells). Thus, in
order to obtain feasible conclusions in soil health assessment it is of vital importance to
determine the influence of variable OM content on the coelomocyte responses and on
the standard toxicity tests (ATT and RT).
To achieve this aim, Eisenia fetida earthworms were exposed to standard soils with
different OM content (6, 10 and 145 OM) and artificially contaminated with 5, 25, 125
and 625 mg Cd/kg soil. Coelomocytes were extruded from earthworms after 3 d of Cd
exposure and NRR time was measured spectrophotometrically at 540nm. Growth and
mortality were measured after 14 d of exposure. Dead earthworms were counted for
the calculations of the LC50 values and living animals were weighted. For the
reproduction test, 10 individuals were exposed for 28 d to contaminated OECD soils
and removed, and cocoons were maintained for another 28 days in soils. Then, the
number of cocoons and juveniles was quantified and the EC50 toxicity values were
calculated.
Cd toxicity was strongly affected by variable proportions of OM in soils as observed at
different levels of biological organization. The increase of OM in soil increased Cd
accumulation in earthworm tissues at increasing metal concentrations. However, the
toxicity produced by this Cd accumulation was lower when compared with the same
exposure doses at low OM as indicated by a less reduced NR retention capacity of
coelomocytes, lower mortality rate and weight loss. On the other hand, reproduction
was more affected by Cd in soils with higher OM content. It seems that a lower energy
investment in reproduction occurred due to higher OM availability causing a higher
sensitivity of cocoons and juveniles. In conclusion, the OM content is a crucial
parameter that affects the estimation of the toxicity of chemical compounds in soils. OM
should be monitored together with biomarker measurements to obtain realistic
conclusions on the health status of real soils.
Acknowledgements: Funded by the Basque Gov: ETORTEK programme (BERRILUR,
IE09-242), and Grant to Consolidated Res Groups (GIC07/26-IT-393-07), and by the
Univ. Basque Country (UFI 11/37). AI is recipient of a pre-doctoral fellowship from the
Basque Government.