Bioimaging of metals in environmental toxicological studies: Linking localization and functionality

One of the major challenges in metal ecotoxicology is to identify the sites and forms of metal bindings which are subsequently linked with the metal functions such as bioavailability, trophic transfer, bioaccumulation, and toxicity. Over the past decades, there had been numerous efforts to tease out the metal speciation and binding strategies with further identification of their biological significance. Bioimaging by directly visualizing the metal distribution in different biological systems (cells, tissues or whole individuals) has emerged as a new tool in metal ecotoxicological studies. This review examines the various modern technologies for nanoscale (subcellular) or macroscale (tissues or individuals) imaging of metals in aquatic organisms, including autoradiography, mass spectrometry (laser ablation inductively coupled plasma mass spectrometry, nano-secondary ion mass spectrometry), X-ray fluorescence, and fluorescent bioprobes. Some of the techniques are more advanced and sophisticated than the others, and each of them has its own advantages and disadvantages. Currently, there are very few studies which attempted to link the metal imaging with functionality. With the advent of modern sophisticated technology, it is clear that bioimaging will become an important tool in metal toxicological studies and reveal many mechanisms underlying metal transport in biological systems with their potential implications for metal toxicity. Other technology needs to be further supplemented for metals, especially by coupling with speciation analysis. One exciting potential is to study the cellular (or organismic) responses to different metals by combining different bioimaging technologies. While bioimaging is mainly qualitative in nature, quantitative assessment should be further explored.