Anaerobic ammonium oxidation mediated by Mn-oxides: from sediment to strain level.

Nitrite and (29)N(2) productions in slurry incubations of anaerobically sediment after (15)NO(3) or (15)NH(4) labelling in the presence of Mn-oxides suggested that anaerobic Mn-oxides mediated nitrification coupled with denitrification in muddy intertidal sediments of Arcachon Bay (SW Atlantic French coast). From this sediment, bacterial strains were isolated and physiologically characterized in terms of Mn-oxides and nitrate reduction as well as potential anaerobic nitrification. One of the isolated strain, identified as Marinobacter daepoensis strain M4AY14, was a denitrifier. Nitrous oxide production by this strain was demonstrated in the absence of nitrate and with Mn-oxides and NH(4) amendment, giving indirect proof of anaerobic nitrate or nitrite production. Anaerobic Mn-oxide-mediated nitrification was confirmed by (29)N(2) production in the presence of (15)NO(3) and (14)NH(4) under denitrifying conditions. Anaerobic nitrification by M4AY14 seemed to occur only in the absence of nitrate, or at nitrate levels lower than that of Mn-oxides. Most of the other isolates were affiliated with the Shewanella genus and were able to use both nitrate and Mn-oxides as electron acceptors. When both electron acceptors were present, whatever their concentrations, nitrate and Mn-oxide reduction co-occurred. These data indicate that bacterial Mn-oxide reduction could be an important process in marine sediments with low oxygen concentrations, and demonstrate for the first time the role of bacteria in anaerobic Mn-mediated nitrification.

[Introduction and establishment processes of marine species: a study case with the Japanese brown kelp Undaria pinnatifida]. / Processus et dynamique d'installation des espèces introduites en milieu marin: une illustration avec l'algue brune asiatique Undaria pinnatifida.

The number of biological introductions has increased since the 1970's and is now considered as the second major cause of the biodiversity erosion, after fragmentation or disappearance of habitat. Beyond the threat they represent for the ecosystem equilibrium, introduced species are interesting models to study fundamental issues in ecology and evolution like the processes of dispersal and adaptation to novel environments. In this context, species introduced over a large geographic range and spectrum of habitats provide an excellent opportunity for comparing the mechanisms that promote introduction and settlement between different environments. In this paper, based on a case study, the worldwide introduction of the brown alga Undaria pinnatifida, and on the use of molecular tools, we aim at examining several processes promoting or occurring during biological introductions. Our results showed that i) multiple processes can account for the success of the pandemic introduction of this alga, highlighting the necessity to study introduced species in relation with the ecosystem they invaded, ii) the recurrence of introductions is a critical component in the dynamics of settlement and iii) human activities can play a major role not only during the primary introduction but also for the sustainable settlement of introduced species in natural environments by providing reservoir of migrants. Taken together, these results demonstrate that the complexity of mechanisms occurring in biological invasion require spatial but also long-term analysis.