Introduction

X-ray microanalysis (XRMA) is a young science, but in the past 20 years it has produced major advances in the material and biological sciences. This is particularly the case for the study of the uptake of pollutant heavy metals by animals and plants.

Before the arrival of XRMA it had been established that marine animals were capable of taking-up metals in large amounts and concentrating them in particular tissues. These effects were investigated by atomic absorption spectrophotometry and to a limited extent by histochemistry. However, structural resolution was limited and most specimen preparation procedures either removed the metals from the tissues or produced some degree of translocation. These restrictions did not assist investigation into the cytology of metal metabolism. This situation changed dramatically with the arrival of XRMA which coupled the structural resolution of the electron microscope with the element identification of the energy dispersive X-ray detector. Early applications to marine organisms were described by Nott & Parkes (1975) and Walker et al. (1975a,b).

The metals that are found in tissues can be grouped biologically into those essential for metabolism and those which are non-essential. Most metals are toxic when they occur in excess amounts. Chemically the metals can be grouped into the hard acid type (for example Na, Mg, K and Ca) which bind electrostatically with a preference for O > N > S donating ligands, and the soft acid type (for example Cu, Cd, Hg and Ag) which bind covalently with a preference for S > N > O.