This article is protected by German copyright law. You may copy and distribute this article for your personal use only. Other use is only allowed with written permission by the copyright holder. Radiochim. Acta 93, 683–689 (2005)  by Oldenbourg Wissenschaftsverlag, München Resistance, accumulation and transformation of selenium by the cyanobacterium Synechocystis sp. PCC 6803 after exposure to inorganic Se VI or Se IV By B. Gouget 1 , ∗ , L. Avoscan 1 , G. Sarret 2 , R. Collins 1 and M. Carrière 1 1 Laboratoire Pierre Süe, CEA-CNRS UMR 9956, CEA/Saclay, 91191 Gif-sur-Yvette, France 2 Environmental Geochemistry Group, LGIT, University of Grenoble and CNRS, BP 53, 38041 Grenoble Cedex 9, France (Received January 3, 2005; accepted April 8, 2005) Selenium / Speciation / Bacteria / Synechocystis / XANES / X-ray absorption spectroscopy Summary. Our purpose was to investigate the ability of Synechocystis sp. PCC 6803, a photosynthetic prokaryote iso- lated from fresh water, to resist, incorporate and reduce the oxidized forms of selenium including selenite and selenate, the major selenium species present in aquatic systems. Selenium speciation and the chemical intermediates during selenium transformation were determined by X-ray absorption near edge structure (XANES) spectroscopy. The possible internal- isation pathways involving selenium and the metabolic fate of selenate and selenite were examined. Selenate metabolism seemed to proceed via the sulfate reduction pathway result- ing in the formation of the R-Se-H, R-Se-R and R-Se-Se-R species. The transformation of selenate to toxic amino acids may explain the high sensitivity of Synechocystis to selenate. Several mechanisms of selenium reduction seem to com- pete during selenite assimilation. A specific mechanism may transform internalised selenite into selenide and, subsequently induce the biosynthesis of selenoproteins. A non-specific mechanism may interfere with thiols, such as glutathione in the cell cytoplasm, or with proteins in the periplasm of the bacteria, notably thioredoxins. Several hypotheses concern- ing the complex transformation of selenium in Synechocystis could therefore be proposed. Introduction Selenium, as part of the 21 st amino acid – selenocysteine – present in many enzymes, is essential to all living or- ganisms (50–200 μ g/d for humans) [1]. At high concen- trations (900 μ g/d), selenium becomes very toxic and the range of concentrations between essentiality and toxicity is extremely narrow [2]. The formation of dysfunctional en- zymes and the disruption of biochemical processes within the cell have been implicated in selenium toxicity. A signifi- cant loss of enzyme function may result from the synthesis of polypeptides in which some, or all, of the sulfur amino acids have been replaced by their selenium analogues [3]. In contrast, certain microorganisms have been found to re- sist and accumulate large quantities of toxic compounds *Author for correspondence (E-mail: gouget@drecam.cea.fr). such as selenium. These bacteria have developed a variety of resistance mechanisms, for instance, the oxidation, re- duction or methylation of inorganic and organic selenium species [4, 5]. Some bacteria can use the selenium oxyan- ions, primarily selenate, as terminal electron acceptors [for instance, 6]. Each mechanism involved in the detoxification of selenium leads to a modification in the speciation of this element. As many cyanobacterial strains have been isolated from metal polluted water resources, and since they are ubiqui- tous organisms in aquatic ecosystems, they provide an ex- cellent model to study the toxicological effects of metals or metalloids. The cyanobacterium Synechocystis sp. PCC 6803 [7] has been shown to display nine genes involved in zinc, cobalt and nickel resistance [8] as well as three genes conferring resistance to arsenic and antimony [9]. Whereas several investigations on selenium toxicity and bioaccumu- lation in aquatic microorganisms have been conducted (for instance, [10]), the ability of Synechocystis to resist and as- similate selenate (Se VI O 4 2− ) or selenite (Se IV O 3 2− ), the dom- inant forms of selenium present in aquatic systems [11], has never been previously considered. An understanding of how cyanobacteria resist, incorporate and transform selen- ium oxyanions can help to develop new strategies for sel- enium detoxification or removal from contaminated aquatic ecosystems. The present work investigated the resistance of Syne- chocystis to both selenate and selenite in terms of growth kinetics and bioaccumulation. Its ability to reduce these oxyanions during bioassimilation is described. Selenium speciation in the bacteria was determined through the use of X-ray absorption near edge structure (XANES) spec- troscopy. This technique is well established for probing the oxidation state and atomic structure of selenium in biologi- cal samples [12, 13]. Materials and methods Bacterial strain and growth conditions Synechocystis sp. strain PCC 6803, purchased from the American Type Culture Collection, was grown photoau- totrophically in the liquid minimal mineral medium BG-11