Description: To understand mineral transport pathways for shell secretion and to assess differences in cellular activity during mineralization, we imaged with TEM and FE-SEM ultrastructural characteristics of outer mantle epithelium (OME) cells. Imaging was carried out on Magellania venosa shells embedded/etched, chemically fixed/decalcified and high-pressure frozen/freeze-substituted samples from the commissure, central shell portions and from puncta. Imaging results are complemented with morphometric evaluations of volume fractions of membrane-bound organelles. At the commissure the OME consists of several layers of cells. These cells form oblique extensions that, incross-section, are round below the primary layer and flat underneath fibres. At the commissure the OME is multi-cell layered, in central shell regions it is single-cell layered. When actively secreting shell carbonate extrapallial space is lacking, because OME cells are in direct contact with the calcite of the forming fibres. Upon termination of secretion, OME cells attach via apical hemidesmosomes to extracellular matrix membranes that line the proximal surface of fibres. At the commissure volume fractions for vesicles, mitochondria and lysosomes are higher relative to single-cell layered regions, whereas for endoplasmic-reticulum and Golgi apparatus there is no difference. FE-SEM, TEM imaging reveals the lack of extrapallial space between OME cells and developing fibres. In addition, there is no indication for an amorphous precursor within fibres when these are in active secretion mode. Accordingly, our results do not support transport of minerals by vesicles from cells to sites of miner-alization, rather by transfer of carbonate ions via transport mechanisms associated with OME cell membranes.

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Description: Chemical contaminants are released from mining, domestic and industrial effluents into an aquatic environment. Sediments (n = 10) were collected with an Eckman grab at ten stations in the Densu Estuary for ecological risk assessment. The spatial distribution of organic characteristics and ecological risk of metals—zinc, lead, copper, mercury, iron, and manganese in sediment—were analyzed using standard methods. The organic parameters occurred in the ranges, as follows: % C, 0.76 to 2.05, % TN, 0.06 to 0.015; % TP, 0.44 to 1.38; and C/N, 12.31 to 34.81. The ranges of metal concentrations (mg/kg) were as follows: Fe, 201.10 to 720.90; Mn, 40.10 to 152.70; Zn, 7.3 to 158.3; Pb, 1.9 to 84.7; Cu 3.4 to 23.0; and Hg, 0.01 to 0.05. The mean concentration of metals in the sediment were Fe 〉 Mn 〉 Pb 〉 Zn 〉 Cu 〉 Hg. The highest mean concentration of Fe suggested redox conditions in the Densu Estuary. There is a low contamination factor (CF) for five metals (Zn, Hg, Fe, Cu, and Mn) (CF 〈 1) to high contamination of Pb (3 〈 CF ≤ 6). The average Pb concentration was above local and geological backgrounds, suggesting an anthropogenic source of pollution from industrial and domestic effluents and agrochemicals. The sediment was extremely enriched by Pb (EF 〉 50) with a positive index of geoaccumulation (0 〈 Igeo ≤ 2) than other metals. There is considerable to a very high degree of contamination (DC) (3 ≤ DC ≥ 6) of metals in the sediment of Densu Estuary. The potential ecological risk index (≤ 40 PERI 〈 80) suggested a very low to moderate ecological risk of metal pollution. The study provides baseline knowledge on geochemical contamination in tropical estuarine systems for the development of effective chemical control strategies towards sustainable management of coastal waters.