MARINE ECOLOGY PROGRESS SERIES Published September 25 Mar Ecol Prog Ser Metal bioavailability assessment in 'mussel-watch' programmes by automated image analysis of autometallographical black silver deposits (BSD) in digestive cell lysosomes Manu Soto*, Ionan Marigomez Biologia Zelularra Atala, Zoologi eta Animali Zelulen Dinamika Saila, Zientzi Fakultatea, Euskal Herriko Unibertsitatea, 644 P.K., E-48080 Bilbo, Basque Country. Spain ABSTRACT: The extent of autometallographical black silver deposits (BSD) was quantified by auto- mated image analysis in digestive lysosomes of the digestive gland of mussels collected from 2 estuaries of the coast of Biscay for 1 yr. Additionally, metal composition of digestive lysosomes was characterised by X-ray microprobe analysis. Zn levels were the most variable among the metals analysed by AAS (Cd, Cr, Cu, Ni, Zn, Pb, Fe). A logarithmic regression model explained the changes in volunle density (VD) of BSD by changes in the Znkhell-wt index recorded in mussels from different sites of the coast. Similar regression models were obtained between the VD of BSD and metal/shell-wt ~ndicesresulting from pooling the 7 metals (pmol metal g-' dry shell weight). In conclusion, metal bioavailability can be estimated by analysing the VD of BSD in the digestive lysosomes of sent~nel mussels. This index may allow workers to discard chemical analysis of biological samples when low traces of metals are present in the tissues, since the method proposed herein provides a quick and cost- effective alternative to routine chemical analyses in biomonitoring programmes. Only when values of VD of BSD reach the plateau of the function would a more accurate chemical analysis be required. This approach does not require special facilities or specialised technicians and would reduce the time and cost of routine metal determination in water pollution monitoring programmes. KEY WORDS: Field validation . Autometallography . Digestive lysosomes . Automated image analysis AAS . Metal content . Correlation analysis . X-ray microanalysis . Metal pollution n~onitoring INTRODUCTION have criticised the reliability of these estimates be- cause a great number of natural variables other than A variety of technical approaches have been applied changes in metal bioavailability may affect metal con- in pollution biomonitoring programmes in order to centrations in the soft body of molluscs (Fischer 1984, estimate the bioavailable fraction of metals. Many of 1988, Marig6mez & Ireland 1990, Mangomez et al. these techniques deal with the chemical analysis of 1990, Theede & Soria 1990, Soto et al. 1995, 1996~). marine molluscs by AAS (atomic absorption spectro- The application of an index (metal/shell-weight index, photometry) and provide data on the metal concentra- i.e. metal content in soft tissues/dry shell weight of tion in sentinel molluscs. Previous works have reported mussels) to exclude the effect of those variables and to on many variables (biotic and abiotic) affecting the reflect more accurately the bioavailable fraction of estimation of metal bioavailability in terms of metal metals in the environment represents an attractive concentrations in soft tissues of sentinel mussels alternative (Fischer 1984, 1986, 1988, Marigomez & (Phillips 1976, 1980, Lobe1 & Marshal1 1988, Widdows Ireland 1989, Marigomez et al. 1990, Regoli & Orlando & Donkin 1992, Rainbow 1993).However, some authors 1993, Soto et al. 1995).However, this index (calculated for each metal: pg metal in soft dry tissue weight/g dry shell weight; and/or computed as normalised data for all the metals: pm01 total metal in soft dry tissue O inter-Research 1997 Resale of full article not permitted 142 Mar Ecol Prog Ser 156: 141-150, 1997 weight/g dry shell weight) still is affected by certain before this index can be applied m 'mussel-watch' bio- variables to some extent. monitoring programmes. For this purpose, the auto- On the other hand, some histochemical analyses may mated quantification of BSD by means of automated allow an accurate determination of the metal levels in image analysis is also needed to avoid subjectivity in biological samples and may reduce the influence of semi-quantification procedures. undesired natural variations (Marigomez et al. 1995, In the present study, the extent of BSD in digestive Soto & Marigomez 1995, Soto et al. 1996a, b). Auto- lysosomes was quantified by automated image analy- metallograpby is a method based on the autoinduced sis in a field study and correlated with the metal con- silver amplification of metal ions in biological sections. tent of sentinel mussels. The aim of this work was to Tissue sections are covered by a photographic emul- validate the use of the index as an alternat~veanalysis sion and rinsed in a photographic developer, and after- to complement chemical analyses for the assessment wards, black silver deposits (BSD) reveal the presence of the bioavailable fraction of metals in 'mussel-watch' of metal ions in the tissue (Zn, Cd, Cr, Ni, Cu, Au, Ag, biomonitoring programmes. Pb, Fe, Hg, etc., except Ca; Danscher 1981, 1984, 1991, Hacker et al. 1988, Danscher & Montagnese 1994, Soto et al. in press b). MATERIALS AND METHODS To our knowledge, few studies applying autometal- lography have been carried out to demonstrate metal- Experimental design. The Abra Estuary (Fig. 1; Bay lic presence in cell compartments of ~llolluscs(Hernel- of Blscay, 43" 19' to 43'23' N. 03O05' to 03" 00' W) was raad & Herwig 1988, Herwig et al. 1989, Soto et al. used as a natural experimental basin; it has well- 1996a, b). However, semi-quantitative estimates of the known differences (spatial and seasonal) in the levels extent of BSD in digestive cell lysosomes and basal of metallic pollutants (Azkona et al. 1984, Swindlehurst lamina of molluscs experimentally exposed to metals & Johnston 1991, Soto et al. 1995). In accordance with are significantly correlated with metal concentration in previous work dealing with estimation of metal bio- the digestive gland measured by conventional methods availability by means of atomic absorption spectropho- using AAS (Soto & Marigomez in press, Soto et al. in tometry (Soto et al. 1995) 3 sampling sites were chosen: press a).Therefore, the semi-quantitative evaluation of Zierbena (showing a high bioavailability for iron) BSD in digestive cell lysosomes of molluscs is proposed on the western side, Arrigunaga (with intermediate as an index for routine screening of metal pollution. levels of metal bioavailability) on the eastern side, and Soto & Marigomez (in press) concluded that BSD Plentzia (43" 26' N, 2" 55' W), located in a small unpol- extent is a low cost index that accurately reflects the luted estuary with low levels of metals. Additionally, total concentration of metals (measured by AAS) in mussels known to show high bioavailability for Zn the digestive gland of mussels, and concomitantly, the (cone. t SD = 501.5 * 194.2 pg Zn g-' dry weight; Soto fraction of bioavailable metals for molluscs in sea- 1995) were also collected in Medakoz (43"24'N, water However, field validation studies are required 2" 93'W) near the Abra Estuary in winter 1989. RRICLNACA Fig. 1. Location of the Abra and Urdaibai estuaries on the Biscayan coast Soto & Marigomez: AnalysiIS of lysosomal BSD in mussels 143 The second estuary selected for this study was the manually adjusted in the first measurement of a given Estuary of Urdaibai (Fig. 1; Biscayan coast; 43'20'N section to correct for slight differences in staining 3"W), which was declared a 'Natural Reserve of the intensity between different sections. Further measure- Biosphere' (UNESCO) in 1984. Metal bioavailabilities ments used the same threshold. Five measurements previously reported in this estuary did not change with were made in each section of 10 mussels per experi- season and there were no differences between sites. mental group in order to calculate the volume density. Only discrete events of increased metal bioavailability The computer uses the stereoscopic images to gener- have been recorded (Soto et al. 1996~).Two sampling ate the VD according to Lowe et al. (1981): VD = sites were chosen: Mundaka (on the western side) and VB/Vc), where VD = volume density, VB = volume of Laga (outside of the estuary to the east). black silver deposits (pm", and VC = volume of diges- Mussels Mytilus galloprovincialis Lmk, from the tive cell cytoplasm (pm3). minimum low tide level and measuring 3.5 to 4.5 cm Chemical analysis. Fifty mussels per site and month in maximum shell length (Marigomez et al. 1992) were were immediately transferred to the laboratory in collected in winter (January and February 1992),sum- plastic bottles within portable iceboxes. Then, mussels mer (July and August 1992) and autumn (September were distributed in plastic tanks in a thern~ostatically and October 1992) in the Abra and Urdaibai estuaries. controlled (13 to 15°C) continuous-water-flow system, Autometallography (Danscher 1984). Ten mussels with active-charcoal and glass-wool filtered natural per site and month were dissected in situ, rinsed in seawater, for 48 h in the absence of food in order to Bouin's fixative (Martoja & Martoja-Pierson 1970) and eliminate the gut contents prior to metal analysis. After immediately transferred to the laboratory in portable dissection, soft tissues were rinsed in distilled water iceboxes. Tissue samples were fixed at 4OC for 36 h, de- and dried at 120°C for 48 h until constant weight was hydrated in alcohols, cleared in methylbenzoate, rinsed reached. Fifty mussels per sampling site were grouped in benzene and embedded in paraffin. Histological sec- in pools of 5 mussels each (giving 10 replicates per tions (? pm) were cut in a Leitz 1512 microtome. sample), digested in concentrated nitric acid, diluted Paraffin sections were dewaxed in xylene, hydrated with 0.1 M nitric acid and analysed by atomic absorp- in ethanol-water mixtures and left at 40°C until com- tion spectrophotometry (Perkin Elmer 2280 spectro- pletely dry. Afterwards, tissue sections were covered photometer) with simultaneous background correction with a uniform thin layer of photographic emulsion and a sensitivity of 0.3 mg 1-l.