Aumento de fijación de nitrógeno en diazótrofos unicelulares versus Trichodesmium tras un evento de deposición de polvo atmosférico en las Islas Canarias

Enhancement of nitrogen fixation rates by unicellular diazotrophs versus Trichodesmium after a dust deposition event in the Canary Islands Mar Benavides •Reduction of atmospheric nitrogen to ammonium N2NH4+ •Diazotrophic organisms (diazo = nitrogen, trophykos = nutrition) Cyanobacteria, Bacteria, Archaea •N2fixation fuels up to 50% of open ocean primary production (Capone et al., 2005) •NitrogenasenifH gene •Diazotrophs limited by Fe and P Introduction Unicellular cyanobacteria Eg. Cyanothece sp. Symbiotic cyanobacteria Eg. Richelia intracellularis Colonial cyanobacteria Eg. Trichodesmium thiebautii Riemann et al., 2010 Introduction UCYN-A UCYN-B Moisander et al., 2010 •Trichodesmium only appears in tropical and subtropical areas •Wide distribution of the nifH gene but… •Lack of N2fixation data The importance of unicellular diazotrophs Introduction Canary Current •Strong surface heating throughout the year •Quasi-permanent thermoclineoligotrophic waters •Mixing January-March‘late winter bloom’ •Dust depositionP and Fe Arístegui et al., 2009 •Station 2 Sampling each 15 days / 3 months LuCIFer Project Objectives • Short-term variability of N2fixation activity in Canary Current waters • Influence of Saharan dust deposition on diazotrophic activity and diversity N2fixation •Acetylene Reduction Assay (ARA) (Stal, 1988) H−CΞC−HNΞN Conversion factor C2H2:N2= 4:1 •Surface water (Niskin 30 L) •On-deck incubations (light/dark) Methods 20x 10x Abundance of Trichodesmium sp. •Plankton net 50 μm •Vertical casts to 100 m •ARA >50 µm >50 µm N2fixation rates = Trichodesmium N2fixation rates •Correlations between >50 µm N2fixation rates and Trichodesmium abundance r2=0.86 (light) r2=0.69 (dark) p<0.05 Surface seawater N2fixation rates = Unicellular diazotroph N2fixation rates •Low probability of catching Trichodesmium with Niskin bottles when its abundance is low (Chang, 2000) Unicellular diazotroph abundance Tyramide Signal Amplification – Fluorescent In Situ Hybridization (TSA-FISH) Methods •Filtrate seawater on PC filters •Probe Nitro821 nifH gene •Classification Leica DM 2500 Size Type <1 µm Small Free-living >1 - <3 µm Medium Attached to organic matter >3 µm Large Symbiont Methods Dust deposition and composition •Aerosol Optical DepthGiovanni NASA •PM10, PM2.5Gobierno de Canarias Calidad del Aire •Metal analysis Hydrography and Chl a •CTD SBE 25 •Turner AU-10 Fluorometer Results & Discussion Temperature, MLD, Chl a, nutrients •Higher temperatures than previous years •Stratification •Lower Chl a •Low nutrient concentrations No clear ‘late winter bloom’ Gracias!