Cellular and molecular aspects of metal sequestration and toxicity in earthworms

Metals accumulated within the tissues of earthworms are not homogeneously d istributed. Rather, they are sequestered predominantly within the posterior alimentary fraction. The clear organotropism of non-essential metals is re inforced by the subcellular compartmentation of metals according to their k nown ligand-binding affinities: in earthworm chloragocytes and intestinal e pithelial cells, in situ X-ray mapping in high-pressure frozen thin cryosec tions revealed that Pb, Zn, Ca and P were co-distributed in a discrete subc ellular compartment, whilst Cd and S were co-distributed in another. Tissue homogenization and fractionation confirmed these differences. Cd is seques tered by cysteine-rich metallothionein isoforms; one isoform may be a true Cd-detoxification protein because it bears two putative glycosylation sites , and may thus deliver Cd to the lysosomal system for cytological isolation . Metal accumulation imposes metabolic costs that can reduce somatic growth and/or reproduction in earthworms: the sequestered bioinactive metal fract ion "diverts" energy into enhanced metallothionein synthesis and associated processes; the bioavailable metal fraction(s) may cause direct toxicologic al damage by competitive inhibition of metalloenzymes acid metal-dependent transcription factors, and indirect damage if repair and compensatory stres s responses are invoked. Molecular responses to metal-exposure stress are, in principle, linked to whole organism life-cycle parameters which, in turn , are ecologically relevant.