Evolution of diatom growth and nutrient concentrations during the culture experiments at 400 μatm pCO₂ and 18°C.

<p>(A) chlorophyll-a, (B) cell number, (C) NO3, (D) PO4, (E) DSi and BSi before dust addition on day 14 (reaching maximum growth), (F) DSi after dust addition on day 14, (G) dissolved Fe⁺², (H) dissolved Fe⁺³, (I) POC and (J) PN for both treatments, without and with dust addition. No dust addition treatment, before and after dust addition treatments and dust dissolution test are shown respectively as open circles, closed black and grey circles, and open diamonds. Vertical dashed lines indicate the day on which the dust (100 mg/L) was added to the dust treatment bottle. For (E), open and close triangles denote BSi concentrations in no dust treatment and dust treatment, respectively.</p

Results of (A) growth rate, (B) cellular chl-a content, (C) cellular POC and PN contents for the exponential growth phase of the four treatments before dust addition.

<p>In (C), filled bars indicate the cellular contents of POC and open bars those of PN. Four treatments represent Present-day (400 μatm pCO₂ and 13°C), High Temperature (400 μatm pCO₂ and 18°C), High pCO₂ (800 μatm pCO₂ and 13°C), and Greenhouse (800 μatm pCO₂ and 18°C), respectively.</p

The evolution of (A) dissolved Fe²⁺ and (B) dissolved Fe³⁺ for the four treatments.

<p>Four treatments correspond to Present-day (400 μatm pCO₂ and 13°C), High Temperature (400 μatm pCO₂ and 18°C), High pCO₂ (800 μatm pCO₂ and 13°C), and Greenhouse (800 μatm pCO₂ and 18°C), respectively. Vertical dashed lines indicate the day on which the dust (100 mg/L) was added.</p

Nutrient drawdown ratios of (A) ΔN/ΔP and (B) ΔN/ΔSi during the exponential growth phase for all four treatments before the dust addition.

<p>Four treatments correspond to Present-day (400 μatm pCO₂ and 13°C), High Temperature (400 μatm pCO₂ and 18°C), High pCO₂ (800 μatm pCO₂ and 13°C), and Greenhouse (800 μatm pCO₂ and 18°C), respectively. The dashed line indicates (A) a N:P molar ratio of 16 and (B) a N:Si molar ratio of 1.</p

Carbonate system parameters in the four pCO₂/temperature treatments.

<p>Carbonate system parameters in the four pCO₂/temperature treatments.</p

Geochemical tracers of N₂-fixation along the DIAPICNA cruise transect.

<p>Longitudinal cross-sections of (A) nitrate concentration, (B) δ¹⁵N_NO3 signal, (C) PN isotopic signature and (D) Δ(15,18)—see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150827#pone.0150827.e002" target="_blank">eq (2)</a>. The position of the Azores Front is indicated by a dashed line, and station i.d. is given on top.</p

Distribution of Carbon fixation and Organic Carbon in <3 μm and >3 μm large particles.

<p>(A) C-fixation rates (μmol m^-3 d^-1), and (B) longitudinal cross-sections of POC (μmol L^-1) depending on the particle size fraction (left: <3 μm; right: >3 μm). The Azores Front position is marked by a bold line.</p

Physical parameters monitored along the DIAPICNA cruise track.

<p>(A) Temperature (°C) longitudinal cross-section, with the position of the Azores Front deduced from the 16°C isotherm at 200m depth (dotted lines); (B) TS diagrams for stations A-E, with overlaid σ_θ density isolines. NACW: North Atlantic Central Water; 15MW: 15°C Mode Water; 18MW: 18°C Mode Water; SAIW: SubArctic Intermediate Water; MedW: Mediterranean Water; LSW: Labrador Sea Water. The black box indicates possible increased influence of Mediterranean Waters at 800–1000m in stations D and E.</p

Sea Surface Height and geostrophic currents during the DIAPICNA cruise.

<p>Station locations (open circles) overlaid on AVISO altimetry-derived geostrophic currents centered between 26/07/2011 and 01/08/2011, with color scale indicating the Sea Surface Height (SSH) in m.</p

Significant correlations between C-, N₂-fixations, UCYN abundance and physical-chemical conditions.

<p>Significant correlations between C-, N₂-fixations, UCYN abundance and physical-chemical conditions.</p