Bioconcentration and Intracellular Storage of Hexachlorobenzene in Charophytes and Their Potential Role in Monitoring and Remediation Actions Susanne C. Schneider † and Luca Nizzetto* ,†,‡ † Norwegian Institute for Water Research, Gaustadalle ́ en 21, NO-0349, Oslo, Norway ‡ Research Centre for Toxic Compounds in the Environment (RECETOX), Kamenice 126/3, CZ-62500 Brno, Czech Republic * S Supporting Information ABSTRACT: It has been hypothesized that highly hydrophobic substances (LogK OW > 5) including many persistent organic pollutants cannot overtake protective tissues and diffuse inside the body of plants due to steric hindrance or very slow diffusion. We investigated the bioaccumulation of hexachlorobenzene (HCB, LogK OW = 5.5) in a benthic charophycean macro-alga: Chara rudis. Chara species are a group of common freshwater algae with a complex body structure encompassing a protective layer of cortex cells surrounding large internode cells. The charophyte cell wall has many features in common with that of higher plants; therefore, they are useful models to investigate bioaccumulation mechanisms in general. We found that HCB diffused through the cortex and reached the cytoplam of internode cells. More than 90% of the HCB mass found in the organism was in the cortex and 10% in the internode cell cytoplasm. The cortex reached a pseudoequilibrium partitioning with water, and the bioconcentration factor was in the same range as that of lower aquatic organisms such as phytoplankton. Charophytes are therefore very efficient accumulators of hydrophobic compounds. Based on the structural and ecological features of charophytes, we speculated on their possible use as biomonitors and bioremediation tools. ■ INTRODUCTION Persistent organic pollutants (POPs) are a class of ubiquitous bioaccumulative chemical contaminants representing a global concern for human and environmental health. POPs are generally characterized by LogK OW values > 5, 1 conferring them affinity for the biotic phase. 2 In addition, binding to the organic matrix (particularly in primary producers in water) influences their environmental behavior by controlling multi- media distribution 3 and therefore their overall fate. It has been hypothesized that steric hindrance or kinetic limitation may inhibit permeation and diffusion of highly hydrophobic substances through surface structures of auto- trophs (such as leaf cuticles, cell walls, or membranes). This may be the reason why root intake and translocation of compounds with logK OW > 5 were not observed in terrestrial vascular plants 4,5 (though some exceptions exist). 4,6 Such a mechanism would imply that the internal tissues (and their intracellular environment) of a multicellular autotroph may not be directly exposed to hydrophobic toxicants, therefore preventing the insurgence of possible acute toxicity responses mediated by intracellular receptors. The steric hindrance hypothesis was recently challenged. Cellular intake of phenanthrene (a compound with LogK OW of about 4.5) 1 was qualitatively documented using fluorescence microscopy techniques. 5,7,8 Phenanthrene from an artificially contaminated atmosphere was shown to di ffuse inside epidermis and mesophyll cells in terrestrial plant leaves. In aquatic organisms, intracellular transport of hydrophobic substances (namely PCBs) has been shown to occur only for a unicellular phytoplankton species (namely, Chlamydomonas reinhardtii). This relatively simple organism lacks a protective surface tissue. For Chlamydomonas reinhardtii, bioconcentration dynamics may be controlled by diffusion of chemicals through the cell membrane followed by a slow accumulation in subcellular structures such as the thylakoids. 9-11 In contrast to unicellular phytoplankton species, higher aquatic plants and some macroscopic algae have a more complex multicellular structure with protective surface tissues. Charophytes are benthic algae and represent an intermediate evolutionary step between unicellular algae and higher plants of which they are close ancestors. 12 They are therefore a good study model to investigate chemical uptake mechanisms in organized multicellular autotrophs. Within the charophytes, Received: June 4, 2012 Revised: September 27, 2012 Accepted: October 12, 2012 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es302185g | Environ. Sci. Technol. XXXX, XXX, XXX-XXX