Heterotrophic dinitrogen fixation (acetylene reduction) in phosphate-fertilised Microcoleus chthonoplastes microbial mat from the hypersaline inland lake 'la Salada de Chiprana' (NE Spain)

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 cyanobacterium M. chthonoplastes lacks the dinitrogenase reductase nifH gene and with the fact that acetylene reduction rates were strongly stimulated by additions of H-2/CO2, 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.