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Marine Ecology Progress Series

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MEPS 470:1-14 (2012)  -  DOI:

Diazotrophic bacteria respond to Saharan dust additions

Rebecca J. Langlois1,2,*, Matthew M. Mills1,3, Celine Ridame1,4, Peter Croot1,5, Julie LaRoche1,2

1Helmholtz Centre for Ocean Research Kiel|GEOMAR, 24105 Kiel, Germany
2Present address: Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
3Present address: Environmental Earth System Science, Stanford University, Stanford, California 94305, USA
4Present address: LOCEAN-IPSL, Université Pierre et Marie Curie, 75252 Paris, France
5Present address: National University of Ireland, Galway, Ireland

ABSTRACT: Three bioassay experiments were performed to study the effects of nutrient and Saharan dust additions on natural diazotrophic communities in the tropical North Atlantic Ocean. Samples for nucleic acid analysis were collected at the beginning and end of 48 h incubations. TaqMan probes specific to 7 diazotrophic phylotypes, viz. filamentous cyanobacteria (Trichodesmium spp.), unicellular cyanobacterial (UCYN) Groups A, B, and C, Gamma A and P Proteobacteria, and Cluster III, were used to quantify nifH DNA abundances. N2 fixation rates were measured in the same experiments using the 15N2 gas bubble injection method. N2 fixation was co-limited by P and Fe. Total nifH abundances increased relative to the control with additions of either Fe or P or both in combination. Additions of dissolved N, alone or in combination with phosphate, induced increases in UCYN-A and Gamma A nifH compared with the control. Saharan dust additions significantly stimulated fixation rates. Abundances of all cyanobacterial and Gamma A nifH phylotypes at least doubled after Saharan dust additions where surface water dissolved Fe concentrations were <2 nmol l−1. Laboratory experiments with cultures of T. erythraeum demonstrated that dust addition promoted colony formation and the persistence of T. erythraeum biomass relative to cultures to which no Fe was added. Our results with both field and laboratory experiments indicate that Saharan dust positively affects diazotrophic phylotype abundances and changes T. erythraeum colony morphology.

KEY WORDS: Nutrient limitation · Nitrogen fixation · North Atlantic · Bioassay · Trichodesmium · UCYN-A · nifH · qPCR

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Cite this article as: Langlois RJ, Mills MM, Ridame C, Croot P, LaRoche J (2012) Diazotrophic bacteria respond to Saharan dust additions. Mar Ecol Prog Ser 470:1-14.

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