Environ. Sci. Technol. 2008, 42, 5375–5376
Response to Comment on “Avoidance of Aluminum Toxicity in Freshwater Snails Involves Intracellular SiliconsAluminum Biointeraction” We thank you for the opportunity to respond to Dr. Exley’s correspondence (1) on our paper (2) in terms of: (i) his concerns over the definition of “hydroxyaluminosilicate”; and (ii) his alternative explanation for our findings. (i) We have used hydroxyaluminosilicate (HAS) as a term to indicate that Al is coordinated to OH groups as well as O-Si groups, including Al bound to the surface of polysilicate in aqueous environments, and with evidence of some imogolite-like configuration. In a recent paper (3), Nakanishi and Wada (the latter of whom is credited with encouraging wider spread use of the term “HAS” from 1980 (4), although this definition has been in use since at least the 1960s (5– 8)) stated: “In all these studies, the structure, stability and reactivity of HAS ions have not been well elucidated. Some consensual results are that they are cationic, their Si/Al ratios are variable depending on those of the starting solutions (Wada & Wada, 1980), and the configuration of one silicon in HAS ions is essentially the same as that in imogolite” (3). We support this view, as opposed to a more narrow interpretation of HAS, based on its preparation route rather than its structure, as supported by Dr. Exley. (ii) The uptake and concentration of Alsand indeed other toxic metal ionssinto lysosomes is well documented, not only in L. stagnalis digestive gland but in a number of other invertebrates (e.g., ref (9)) as well as in vertebrates (10, 11). Hence, Dr. Exley’s suggestion that no such mechanism exists for this is implausible. Transient Alinduced toxicity, gauged from animal behavior, is observed but then, at least for a period, normalizes even with continued Al exposure. This normalization of behavior coincides with recruitment of Si to the same lysosomal compartments that contain Al, as previously reported by us (12). The ex vivo Al/Si interaction that Dr. Exley favors (and indeed has shown to be an important factor in limiting the bioavailability of Al) can be discounted in our experiments on three counts. First, only traces of Si (unavoidable background) were present during Al exposuresso the quantity of HAS, if formed at all in such an ultradilute (sic) silicate solution, would be very low compared to non-HAS aluminum. Second, there is no change in the chemistry of the water, and therefore exposure to the snail, during the period of behavioral normalization. Third, by undertaking carefully controlled serial exposures to Al and Si we were able to avoid Al/Si interactions in the aqueous environment, over and above baseline. Thus we concluded, beyond reasonable doubt, on the occurrence of Al/Si interactions in vivo, at least in L. stagnalis. We believe these arguments are clearly elucidated in our paper. Next we attempted to characterize the Al/Si species. Dr. Exley appears to confuse the granule (tertiary lysosome/residual body) shown in our Figure 4A for a silica particle and he proposes his hypothesis of “HAS adsorbed onto a dietary phytolith”. However, our mapping clearly shows that Al and Si are colocalized within the lysosome, in a poorly crystalline phase, down to the few nm scale (orders of magnitude smaller than phytoliths) with Si but not Al also found in isolation at the same nm scale (Figure 4B and C). The electron energy loss spec10.1021/es801212h CCC: $40.75
Published on Web 06/18/2008
2008 American Chemical Society
trometry (EELS) edge for Al is at least consistent with a proto-imogolite material (Al EELS edges are sensitive to changes in Al coordination (13)). Furthermore, if this were a silica phytolith with adsorbed HAS then the mapping techniques employed would have revealed a Si-rich core with an Al-rich halo as the concentration of Al relative to Si would increase toward the edges of the projected image (the microscopy being a transmission technique). We have never seen this, or phytoliths without Al, in treated or control animals. The work required to quantify digestive gland granule (lysosomal) changes with respect to Si and Al concentrations over time is considerable. So far it appears that Al concentrates in these granules and is then followed by recruitment of Si (not the reverse as Dr. Exley states) and it is likely that maturation of an Al/Si species proceeds at the same time as that of the lysosome itself. Our paper provides an analytical microscopy snapshot at day 15 of a mature digestive gland granule. Although not stated we hope it was implicit that this represented a typical such granule and was not a “one-off”. Our data, we believe, point to in situ HAS formation; further work is required to characterize this occurrence over time and to unravel the mechanisms involved. Currently we favor Al-induced signaling for the recruitment of Si from elsewhere in the snail to Al-rich lysosomes leading to HAS formation through [Si]-dependent polymerization of silicic acid, interaction with Al and further maturationsthe latter driven chemically and biologically. We acknowledge Dr. Exley’s committed views on the role of silicic acid in biology. We retain open minds including, conceivably, that our observations will be explained by in situ binding of monomeric silicic acid to polyhydroxy Al, but we have no data yet for this and have heard nothing to divert us from our hypothesis.
Literature Cited (1) Exley, C. Comment on “Avoidance of aluminum toxicity in freshwater snails involves intracellular siliconsaluminum biointeraction”. Environ. Sci. Technol. 2008, 42 (14), 5374. (2) White, K. N.; Ejim, A. I.; Walton, R. C.; Brown, A. P.; Jugdaohsingh, R.; Powell, J. J.; McCrohan, C. R. Avoidance of aluminum toxicity in freshwater snails involves intracellular siliconsaluminum biointeraction. Environ. Sci. Technol. 2008, 42 (6), 2189–2194. (3) Nakanishi, R.; Wada, S.-I. Reactivity with phosphate and phytotoxicity of hydroxyaluminosilicate ions synthesized by instantaneous mixing of aluminum chloride and sodium orthosilicate solutions. Soil Sci. Plant Nutr. 2007, 53, 545– 550. (4) Wada, S.-I.; Wada, K. Formation, composition and structure of hydroxy-aluminosilicate ions. J. Soil Sci. 1980, 31, 457– 467. (5) Tadzhiev, F. Kh.; Korobchenko, G. S. Strength of the bond of sorbed cations with large particles of certain minerals. Tr. Tashkentsk. Politeckhn. Inst. 1963, 22, 150–157. (From Zhurnal Khimii (1965) Abstr. 8B1039). (6) Porotnikova, T. P.; Derevyankin, V. A.; Kuznetsov, S. I.; Tsvetkova, M. P.; Chirkov, A. K. Composition of aluminiumsilicon-oxygen complexes in aluminate solutions studied by the Raman effect method. Zh. Prikl. Khim. 1973, 46, 639–641. (7) Fan, C. W.; Markuszewski, R.; Wheelock, T. D. Behavior of quartz, kaolinite, and pyrite during alkaline leaching of coal. ACS Symp. Ser. 1986, 301, 462–472. (8) Kennedy, J. A.; Powell, H. K. J. Colorimetric determination of aluminium(III) with Chrome Azurol S and the reactivity of hydrolyzed aluminium species. Anal. Chim. Acta 1986, 184, 329–333. (9) Marigomez, I.; Soto, M.; Cajaraville, M. P.; Angulo, E.; Giamberini, L. Cellular and subcellular distribution of metals in molluscs. Microsc. Res. Tech. 2002, 56, 358–392. VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5375
(10) Galle, P.; Berry, J-P.; Duckett, S. Electron microprobe ultrastructural localization of aluminium in rat brain. Acta Neuropathol. 1980, 49, 245–247. (11) Kametani, K. Detection of aluminium by energy dispersive X-ray microanalysis at high accelerating voltages with semithin sections of biological sample. J. Electron Micros. 2002, 51, 265–274. (12) Desouky, M.; Jugdaohsingh, R.; McCrohan, C. R.; White, K. N.; Powell, J. J. Aluminium-dependent regulation of intracellular silicon in the aquatic invertebrate Lymnaea stagnalis. Proc. Natl. Acad. Sci., U.S.A. 2002, 99, 3394–3399. (13) Garvie, L. A. J.; Craven, A. J.; Brydson, R. Use of electronenergy loss near-edge fine structure in the study of minerals. Am. Mineral. 1994, 79, 411–425.
5376
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 14, 2008
Catherine R. McCrohan, Keith N. White, Rachel C. Walton, Andrew P. Brown, Ravin Jugdaohsingh, and Jonathan J. Powell Faculty of Life Sciences, University of Manchester, U.K., Institute for Materials Research, University of Leeds, U.K., and MRC Human Nutrition Research, Elsie Widdowson Laboratory, Cambridge, U.K. * Corresponding author e-mail:
[email protected].
ES801212H
