Chlor-alkali plant contamination of Aussa River sediments induced a large Hg-resistant bacterial community
Introduction
The sediments of Marano lagoon are polluted by mercury (Hg), not only by the Isonzo River that is impacted by former Hg mining in Idrija, Slovenia, but also by the Aussa River, a short spring river, receiving outflow of an old chlor-alkali plant (CAP). The CAP began operation in 1949 and it has been estimated that a total of 186,000 kg of Hg (with a maximum of about 20 kg day−1) was discharged into the Aussa–Corno River system that enters the lagoon (Piani et al., 2005). The CAP employed the “Solvay Method” that used metallic Hg to separate sodium and chlorine from brine. The industry not only discharged Hg(0) into the river, but also excess brines, ammonia and limestone. In addition, at the same industrial site, cellulose was produced from cane (Arundo donax sp), which enriched sediments with organic carbon (Acquavita et al., 2012). In 1984, a modern wastewater treatment system was installed and a significant reduction of Hg in the environment was observed. However, diffuse and periodic cases of similar contamination still occur in several areas (Maserti and Ferrara, 1992; Lodenius, 1998; Biester et al., 2000; Hissler and Probst, 2006).
Covelli et al. (2009) reported that Hg in sediments in this region is mainly in its elemental form, as was expected since it was discharged into this form by the CAP, although a minor fraction was also chelated by organic matter. Therefore, Hg contamination of sediments might induce a selection of Hg-resistant bacterial populations which attenuate Hg toxicity. During the last five decades, scientific aspects of Hg resistance in bacteria, from genes to environmental applications, have been investigated (Barkay et al., 2003) even in pristine environments such as in polar regions (Møeller et al., 2010). Hg-resistant bacteria possess the mer operon that may be located on a plasmid, transposon, or chromosome. Its genetic components include the gene sequence merR, merT, merP, merC, merD, merA, and merB (Nascimento and Chartone-Souza, 2003). The gene merA codes for the enzyme mercuric reductase (MerA), which is responsible for Hg2+ reduction to Hg(0) and its subsequent volatilization and removal. The passive efflux of Hg(0) from bacterial cells is considered a beneficial adaptation and in the past it was suggested as one of biotechnological process to reduce Hg contamination (Barkay et al., 2003). In this way, Hg-resistant bacteria can survive at high Hg concentration in the environment while contributing to the detoxification of the surrounding habitat.
To better understand Hg toxicity and bioavailability in the Aussa River and lagoon system, the surficial layer of sediments (0–1 cm) was investigated in terms of certain anions and labile organic molecules such as proteins (PRT), carbohydrates (CHO) and lipids (LIP), and their carbon equivalent sum, biopolymeric carbon (BPC). Furthermore, these biochemical molecules are synthesized and/or utilized by microbes under either aerobic or anaerobic conditions in the presence of various nutrients, which are adsorbed to sediment particles and stimulate bacterial growth. Particular attention was dedicated to isolation and identification of strains harboring merA genes by using molecular and chemical tests to determine Hg2+ reduction activity in vivo.
Section snippets
Sampling
Sediment samples were collected in October 2008. Sampling was performed by boat at four stations (A2, A3, A4 and A6) along the main axis of the Aussa River (Fig. 1). Measurements of temperature, pH, and Eh were conducted in situ with conventional tools. Surface sediments (0–1 cm) were sampled using a Kajak light gravity corer. One aliquot of sediments (100 g) was freeze-dried and used for analytical determination of biopolymeric carbon, anions, and for Hg analyses. A second aliquot (50 g) was
Sediment characterization
The first centimeter of sediments was characterized by determining in situ pH, Eh and temperature for each station, and they were mainly anoxic at neutral pH. In addition, biopolymeric carbon, Hg-T, MeHg and anions were also analyzed in freeze-dried samples (Table 1). During sampling in October, the sediment temperature was from 19 to 21 °C due to a mild autumn.
In general, the three biochemical classes, i.e. PRT, CHO and LIP varied, especially at stations A2 and A3, before and after Banduzzi's
Conclusions
Processing and seepage wastewaters containing elemental Hg(0), which were historically discharged by a CAP into the Aussa River system and therefore into the Marano Lagoon, have been significantly reduced since 1984 due to the construction of wastewater treatment facilities (Piani et al., 2005). Lately CAP activity was suspended and some Hg variations in the first cm of sediment were already observed in this study in respect to the last survey (Covelli et al., 2009). In station A3 at the
Acknowledgments
The authors are very grateful for the fieldwork and sampling operations carried out by Stefano Caressa. The “MIRACLE” (“Mercury Interdisciplinary Research for Appropriate Clam farming in Lagoon Environment”) Project was supported financially by the Commissario Delegato for the Marano and Grado Lagoon in 2008–2009 (coordinator: S. Covelli).
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