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. N-2 fixation rates were measured in the same experiments using the N-15(2) gas bubble injection method. N-2 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.
Highlights
Marine nitrogen inventory gain processes are biologically mediated
Samples for molecular analysis were collected at the start and end (48 h) of 3 bioassay experiments performed in Autumn 2002 during the Meteor 55 research cruise in the sub-tropical north Atlantic Ocean (Expts A,B described by Mills et al 2004 and Expts B,C described by Davey et al 2008)
All samples from bioassays experiments conducted at 3 sites in the tropical North Atlantic Ocean (Table 1, Fig. 1) were tested for the presence and abundance of 7 diazotrophic phylotypes
Summary
Marine nitrogen inventory gain processes are biologically mediated. Fixed nitrogen enters the biogeochemical cycle through biological nitrogen (N2) fixation, the reduction of atmospheric N2 gas to ammonium. A few specialized bacteria and Archaea, which contain the highly conserved protein nitrogenase, are able to fix N2. Due to the relatively low abundance of diazotrophs (N2-fixing organisms), tracking their response to changes in environmental conditions has been problematic. Average diazotroph abundances under non-bloom conditions are estimated to be 2.5 × 104 cells l−1 (Langlois et al 2008).
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