LaRoche, Julie
Permanent URI for this collectionhttps://hdl.handle.net/10222/22304
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Item Open Access Diazotrophic bacteria respond to Saharan dust additions(2012) Langlois, Rebecca J.; Mills, Matthew M.; Ridame, Celine; Croot, Peter; LaRoche, JulieThree 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.Item Open Access Compartmental models of nitrogen cycling in tropical and temperate marine environments(1987) LaRoche, Julie; Harrison, W. G.Time-course measurements of super(15)N tracer kinetics in particulate organic and in NH super(+) sub(4) pools from tropical and temperate regions were used to test several compartmental models describing the exchange of super(15)N tracer in microplankton communities. Several lines of evidence suggested the involvement of a third, dissolved pool, arbitrarily labelled dissolved organic nitrogen (DON). Although the kinetic patterns of tracer movement were different between the tropical stations and the temperate one, the same 3-compartmental model in which PON and DON can exchange material only through the intermediate of NH super(+) sub(4) gave the best fit. Only the transfer coefficients were modified. Results show that compartmental analysis is useful for the estimation of compartmental transfer rates and for testing the assumptions implicit in any given model.