title: Tolerance to mercury chloride in Scenedesmus strains.
abstract: 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 concentrations of up to 5 micrograms 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 comparison 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 O2 production rate (maximum 2 mumol h-1 mg-1 dry weight); and (3) higher intracellular 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 O2 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.
authors.affiliation: Centro di Studio dei Microrganismi Autotrofi, CNR, Firenze, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Capolino
authors.forename: M
authors.initials: M
authors.lastname: Tredici
authors.forename: M
authors.initials: M
authors.lastname: Pepi
authors.forename: F
authors.initials: F
authors.lastname: Baldi
mesh_terms: D002708:Chlorella / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D000077194:Chlorophyll A; D000460:Chlorophyta / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D002736:Chloroplasts; D003470:Culture Media; D004202:Disinfectants / Q000378:metabolism / Q000633:toxicity*; D005456:Fluorescent Dyes; D005618:Fresh Water / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007558:Italy; D008627:Mercuric Chloride / Q000378:metabolism / Q000633:toxicity*; D018613:Microscopy, Confocal; D010101:Oxygen Consumption / Q000187:drug effects; D010788:Photosynthesis / Q000187:drug effects; D010860:Pigments, Biological / Q000378:metabolism; D011549:Pseudomonas / Q000187:drug effects / Q000378:metabolism; D012015:Reference Standards; D012989:Soil Pollutants / Q000378:metabolism / Q000633:toxicity; D013045:Species Specificity; D014835:Volatilization; D014874:Water Pollutants, Chemical / Q000378:metabolism / Q000633:toxicity*
chemical_list: D003470:Culture Media; D004202:Disinfectants; D005456:Fluorescent Dyes; D010860:Pigments, Biological; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D002734:Chlorophyll; D008627:Mercuric Chloride; D000077194:Chlorophyll A