ALUMINUM DYNAMICS ON GILLS OF ATLANTIC SALMON FRY IN THE PRESENCE OF CITRATE AND EFFECTS ON INTEGRITY OF GILL STRUCTURES

The effect of citrate as organic acid anions (A(-)) on the accumulatio n of aluminum (Al) on gills of Atlantic salmon (Salmo salar) fry was e xamined in acidic media. Treatments with (Al-A(-)) = +10, 0, and -10 m u eq/l ([Al] = 0, 10, 20, 30, 40, 50, 60 mu eq/l and [citric acid] = 0 , 10, 20, 50 mu eq/l) were used. Depuration of gills loaded with Al (1 0 mu eq/l) was also examined in the presence of citric acid (0, 10, 20 , 40 mu eq/l). Elimination of Al from gills was a much more rapid proc ess (< 48 h) than net accumulation (still increasing after 15 d). The presence of citrate reduced net Al accumulation on gills during uptake but it did not accelerate elimination of Al during the rapid phase of depuration. The effect of citrate on uptake was probably a result of the competition between A(-) and negatively charged binding sites on t he gill surface for inorganic Al, whereas short-term depuration of Al was considered to be primarily a physical phenomenon related to the ef ficiency of the sloughing process. The severity of gill lesions observ ed by scanning and transmission electron microscopy and supported by m orphometrics was related to [Al] in solutions. Slight reductions in gi ll lesions occurred in the presence of citrate but protection of gills was not complete, indicating a greater bioavailability of Al than pre dicted. During depuration, the recovery of gill tissues from Al effect s was slightly enhanced by increasing [citric acid]. Electron-dense pa rticles on the gill epithelial surface and within chloride cell vesicl es in fish exposed to high [Al] and after Al loading were possibly rel ated to Al deposits. These particles were absent from the lamellar sur faces of gills after only 5 d depuration in Al-free media, indicating that removal of particles from the gill surface was rapid and effectiv e. X-ray microanalysis revealed the presence of Al on the gills of fis h exposed to Al and its absence after depuration. Probing on the gill surface detected small amounts of Al on epithelial cells and in chlori de cell crypts, but most of the Al was in deposits or secretions on th e lamellar surface, adding further support to the premise that the maj ority of Al accumulated on gills over a short time period can be readi ly sloughed off.