N. Wannicke

Impact of diazotrophy on N stable isotope signatures of nitrate and particulate organic nitrogen: case studies in the north-eastern tropical Atlantic Ocean

During two independent cruises in the north-eastern tropical Atlantic Ocean, we applied two different approaches to investigate the impact of diazotrophy on nitrogen stable isotope signatures in nitrate and particulate organic nitrogen (PON) of the food-web constituents. The first approach, used during the Poseidon cruise 348 in the Mauritanian upwelling, investigated the long-term influence of diazotrophy on the natural abundance of δ15N-NO− 3 and PON. The second approach, adopted during the Cape Verde field cruise, applied stable isotope tracer addition experiments. These served to determine the instantaneous transfer of diazotrophic N to the higher trophic level. Both approaches showed that N2 fixation was compatible with the pattern and the magnitude of the isotopic depletion of dissolved NO− 3 during the Mauritanian upwelling cruise, as well as PON in zooplankton and phytoplankton during the Cape Verde cruises. An N-budget using 15N incorporation rates and diazotrophic N2 fixation rates showed that 6 % of the daily N2 fixation was potentially taken up by the mesozooplankton community. Direct grazing accounted for 56 % of gross mesozooplanktonic N incorporation, while 46 % occurred due to channelling through the microbial loop.

Atmospheric CO2 availability does not equate to increased nodularin production in diazotrophic cyanobacteria, but does induce varying responses in net photosynthesis and N2 fixation rates.

Increasing levels of CO2 in the atmosphere are suggested to favour increased incidences of cyanobacterial blooms in water bodies, with a potential concomitant increase in toxin production. As nitrogen fixing cyanobacteria are independent of nitrate and ammonium, this pilot study investigated whether elevated atmospheric CO2 levels (eCO2), could increase toxin production and net photosynthesis (NP) rates in both terrestrial and aquatic diazotrophic cyanobacteria. Both toxin and non-toxin producing strains of Nostoc and Nodularia were grown at present atmospheric levels (PAL) of CO2 or near future elevated (eCO2) and net photosynthesis (NP) determined. Short term responses demonstrated CO2 associated increases and decreases in NP, with N. harveyana SAG44.85 showing little change in its NP at eCO2. Long term responses recorded increases in NP for all species in response to eCO2, except for N. harveyana on day 7. Nitrogen fixation rates were significantly higher by approx. 10 fold in the aquatic Nodularia species compared to the terrestrial Nostoc species tested. Moreover, nitrogen fixation rates were not significantly higher at eCO2, except for N. harveyana. There was no direct correlation between increased nodularin production and eCO2 in neither aquatic, nor terrestrial nodularin producing species, however there was a significant correlation between nodularin content and POC:PON ratio for the terrestrial Nostoc sp. 73.1 not observed for the aquatic Nodularia spumigena CCY9414.