Mapping of heavy metal ion sorption to cell–extracellular polymeric substance–mineral aggregates by using metal-selective fluorescent probes and confocal laser scanning microscopy

Biofilms, organic matter, iron/aluminum oxides, and clay minerals bind toxic heavy metal ions and control their fate and bio-availability in the environment. The spatial relationship of metal ions to biomacromolecules such as extracellular polymeric sub-stances (EPS) in biofilms with microbial cells and biogenic minerals is complex and occurs at the micro- and submicrometer scale. Here, we review the application of highly selective and sensitive metal fluorescent probes for confocal laser scanning mi-croscopy (CLSM) that were originally developed for use in life sciences and propose their suitability as a powerful tool for map-ping heavy metals in environmental biofilms and cell-EPS-mineral aggregates (CEMAs). The benefit of using metal fluorescent dyes in combination with CLSM imaging over other techniques such as electronmicroscopy is that environmental samples can be analyzed in their natural hydrated state, avoiding artifacts such as aggregation from drying that is necessary for analytical electronmicroscopy. In this minireview, we present data for a group of sensitive fluorescent probes highly specific for Fe3, Cu2, Zn2, and Hg2, illustrating the potential of their application in environmental science. We evaluate their application in combination with other fluorescent probes that label constituents of CEMAs such as DNA or polysaccharides and provide selec-tion guidelines for potential combinations of fluorescent probes. Correlation analysis of spatially resolved heavy metal distribu-tions with EPS and biogenic minerals in their natural, hydrated state will further our understanding of the behavior of metals in environmental systems since it allows for identifying bonding sites in complex, heterogeneous systems. Biofilms are the dominant form of microbial life on Earth (1),and the organic material that is present in biofilms signifi