Introduction The mechanism of Hg resistance in bacteria is based on the reduction of ionic Hg to volatile elemental Hg(0), catalysed by an inducible enzyme, mercuric reductase. This enzyme has been cloned and sequenced (Robinson & Tuovinen 1984, Summer 1986, Foster 1987, Silver & Walderhaug 1992) and is responsible for environmental Hg detoxification. Hg volatilization has also been observed in the yeasts Cryptococcus spp., Candida albicans and Saccharomyces cerevisiae (Brunker & Bott 1974, Yannai et al. 1991) and in the algae Chlamydomonas sp. and Chlorella pyrenoidosa (Ben-Bassat & Mayer 1975, 1977). Ben-Bassat & Mayer (1977) observed Hg volatilization in Chlorella pyrenoidosa. Cultures of the microalga showed a lag phase of four days when spiked with 1.25 mg ml –1 HgCl 2 and volatilized Hg 2+ to Hg(0) from the medium by means of an intracellular reducing factor of low molecular weight (less than 1200 Da) in an eight-day experiment. Complete Hg volatilization, which usually takes only a few minutes in bacteria, required some days in C. pyrenoidosa. Marine eukaryotic phytoplankton also reduce inorganic mercury to Hg(0) (Mason et al. 1995a). In Thalassiosira weissflogii, Dunaliella terti- olecta and Pavlova lutheri the mechanism of Hg volatilization is still unknown and the volatiliza- tion rate is lower than in marine cyanobacteria. In 1 1 1 1 1 1 © 1997 Rapid Science Publishers Ltd London BioMetals Vol 10 1997 85 Tolerance to mercury chloride in Scenedesmus strains Elena Capolino*, Mario Tredici*, Milva Pepi† & Franco Baldi† *Centro di Studio dei Microrganismi Autotrofi – CNR, Firenze and †Dipartimento di Biologia Ambientale, Università di Siena, Siena, Italy Received 13 June 1996; accepted for publication 28 October 1996 Mercury chloride toxicity was investigated in two strains of Chlorella and in a strain of Scenedesmus isolated from polluted areas in Tuscany (Italy). No Hg resistance was found in the autotrophic microorganisms isolated, but Scenedesmus sp. strain AR-2489, isolated from the Arno river, was able to grow at concen- trations of up to 5 g ml –1 of Hg. This concentration was twice that which inhibited growth of the two Chlorella strains and Scenedesmus acutus 8M, the reference strain from a culture collection. Photosynthesizing cells of Scenedesmus sp. AR-2489 showed reduced Hg uptake, with the highest percentage of Hg removal from the medium. Loss of Hg was not due to Hg(0) volatilization, as shown by a compar- ison test with the broad-spectrum Hg-resistant Pseudomonas putida FB1. The metabolic differences between Scenedesmus sp. strain AR-2489 and Siacatus strain 8M were: (1) higher growth rate (doubling time of 6.0 h versus 10.6 h); (2) higher O 2 production rate (maximum 2 mol h –1 mg –1 dry weight); and (3) higher intra- cellular pH during growth. The latter was imaged with a green fluorescence molecular probe (BCEFC–AM) and observed by scanning confocal laser microscopy (SCLM). The distribution of red-autofluorescence chlorophyll-a showed that strain AR-2489 had a rougher and hence more extended specific chloroplast surface than strain 8M. Hg tolerance in strain AR-2489 was related to the rapid increase in dissolved O 2 in the medium and in intracellular pH; this caused a loss of soluble mercury transformed to insoluble mercury hydroxide, which is thermodynamically more stable at alkaline pH in highly oxygenated systems. Keywords: inorganic mercury, microalgae, pH molecular probe, photosynthesis rates, scanning confocal laser microscopy BioMetals 1997, 10, 85–94 Address for correspondence: F. Baldi, Dipartimento di Biologia Ambientale, Università di Siena, Via P.A. Mattioli, 4 , 53100 Siena, Italy. Tel: (+39) 577 298859; Fax (+39) 577 298860; e-mail baldi@unisi.it