title: Mesopelagic N2 Fixation Related to Organic Matter Composition in the Solomon and Bismarck Seas (Southwest Pacific).
abstract: Dinitrogen (N2) fixation was investigated together with organic matter composition in the mesopelagic zone of the Bismarck (Transect 1) and Solomon (Transect 2) Seas (Southwest Pacific). Transparent exopolymer particles (TEP) and the presence of compounds sharing molecular formulae with saturated fatty acids and sugars, as well as dissolved organic matter (DOM) compounds containing nitrogen (N) and phosphorus (P) were higher on Transect 1 than on Transect 2, while oxygen concentrations showed an opposite pattern. N2 fixation rates (up to ~1 nmol N L-1 d-1) were higher in Transect 1 than in Transect 2, and correlated positively with TEP, suggesting a dependence of diazotroph activity on organic matter. The scores of the multivariate ordination of DOM molecular formulae and their relative abundance correlated negatively with bacterial abundances and positively with N2 fixation rates, suggesting an active bacterial exploitation of DOM and its use to sustain diazotrophic activity. Sequences of the nifH gene clustered with Alpha-, Beta-, Gamma- and Deltaproteobacteria, and included representatives from Clusters I, III and IV. A third of the clone library included sequences close to the potentially anaerobic Cluster III, suggesting that N2 fixation was partially supported by presumably particle-attached diazotrophs. Quantitative polymerase chain reaction (qPCR) primer-probe sets were designed for three phylotypes and showed low abundances, with a phylotype within Cluster III at up to 103 nifH gene copies L-1. These results provide new insights into the ecology of non-cyanobacterial diazotrophs and suggest that organic matter sustains their activity in the mesopelagic ocean.
authors.affiliation: Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 98848, Nouméa, New Caledonia.
authors.forename: Mar
authors.initials: M
authors.lastname: Benavides
authors.affiliation: Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, Massachusetts 02747, United States of America.
authors.forename: Pia H
authors.initials: PH
authors.lastname: Moisander
authors.affiliation: Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 13288, Marseille, France.
authors.forename: Hugo
authors.initials: H
authors.lastname: Berthelot
authors.affiliation: Research Group for Marine Geochemistry, Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Carl-von-Ossietzky-Strasse 9-11, D-26129 Oldenburg, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Dittmar
authors.affiliation: Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 13288, Marseille, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Grosso
authors.affiliation: Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 98848, Nouméa, New Caledonia.
authors.forename: Sophie
authors.initials: S
authors.lastname: Bonnet
mesh_terms: D020561:Alphaproteobacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D020562:Betaproteobacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000737:chemistry; D020564:Deltaproteobacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D020563:Gammaproteobacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation / Q000502:physiology*; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism; D010140:Pacific Ocean; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D012623:Seawater / Q000737:chemistry / Q000382:microbiology
chemical_list: D001426:Bacterial Proteins; D002241:Carbohydrates; D010088:Oxidoreductases; C029130:nitrogenase reductase; D009584:Nitrogen