Salt Ecosystems Section Heterotrophic dinitrogen fixation (acetylene reduction) in phosphate-fertilised Microcoleus chthonoplastes microbial mat from the hypersaline inland lake ‘la Salada de Chiprana’ (NE Spain) Rutger de Wit 1,3* , Luisa I. Falco´n 1,2 & Claude Charpy-Roubaud 2 1 Laboratoire d’Oce´anographie Biologique, UMR 5805 CNRS & Universite´ Bordeaux I, Arcachon 33120, France 2 IRD, Station Marine d’Endoume, Marseille 13007, France 3 UMR 5119 CNRS-Universite´ Montpellier II ‘Ecosyste`mes lagunaires’, Universite´ Montpellier II, Case 093, 34095 Montpellier Cedex 05, France (*Author for correspondence: Tel.: +33 4 67 14 34 29; E-mail: rde-wit@univ-montp2.fr) Received 15 November 2002; in revised form 13 July 2004; accepted 13 July 2004 Key words: methanogenic archaea, heterotrophic diazotrophs, microbial mat consortia Abstract Microcoleus chthonoplastes dominated microbial mats are conspicuous along the shallow littoral zone in Lake Chiprana, a hypersaline lake located in the Ebro river basin in north-eastern Spain. Pigment data show that these mats included diatom species and anoxygenic phototrophs, Chloroflexus-type bacteria and purple bacteria. In situ, these mats showed low rates of dinitrogen fixation (acetylene reduction). Acetylene reduction was stimulated about 30-fold in excised mats after moderate phosphate fertilisation during 2 weeks incubation in a mesocosm. Pigment analyses showed that this treatment had little impact on the phototrophic community structure, except that it induced a decrease of Chloroflexus-type bacteria. The use of metabolic inhibitors indicated that methanogenic archaea and aerobic heterotrophic bacteria were the major dinitrogen fixers in this system. This is in agreement with the fact that the mat-building cyanobac- terium M. chthonoplastes lacks the dinitrogenase reductase nifH gene and with the fact that acetylene reduction rates were strongly stimulated by additions of H 2 /CO 2 , methanol, fructose and sucrose, but not by lactate, acetate, formate and glucose. No significant differences where found for acetylene reduction rates when comparing light and dark incubations of these microbial mats. However, acetylene reduction rates were enhanced in the light when the near infrared (NIR) light was filtered out, which arrested anoxygenic photosynthesis. We suggest, therefore, that the chemoheterotrophic dinitrogen fixing bacteria were in competition with anoxygenic phototrophic bacteria for organic substrates, while the latter did not contribute to dinitrogen fixation in the mat. Introduction Lake Chiprana is located in an endorheic basin in the southern region of the Ebro valley in north- eastern Spain. The lake is permanent with a maximum depth of 5.5 m and hypersaline (on average 78 g/l salinity) with Na + and Mg 2+ as the major cations and SO 2 4 and Cl ) as the major anions (Vidonde et al., 1993; Jonkers et al., 2003). In the late 1980s, microbial mats of M. chtho- noplastes covered extensive parts of shallow zones up to 1 m depth. The lake suffered from eutro- phication in the early 1990s when it received high quantities of nutrients, particularly phosphorus, from the neighboring lake, ‘Laguna de las Roces’, through a small canal (Diaz et al., 1998). This nutrient input, in concert with a decrease of the population densities of Artemia parthenogenetica, Hydrobiologia (2005) 534: 245–253 Ó Springer 2005