Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic.

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.

More than inorganic copper is bioavailable to aquatic mosses at environmentally relevant concentrations.

The present study investigates how dissolved organic matter (DOM) alters copper bioavailability at environmentally relevant concentrations (1-5 microg/L of dissolved copper, 1-4 mg/L of dissolved organic copper). A methodology combining two biological endpoints (short-term and steady-state bioaccumulation of copper by the aquatic moss Fontinalis antipyretica) and a sampling of labile copper with diffusion gradient in thin films (DGT) is proposed for batch experiments conducted with mineral water and various DOM, ethylenediaminetetra-acetic acid (EDTA), humic acid, and natural Seine River (France) extracts (hydrophobic and transphilic fractions). All types of DOM reduce the bioavailability of copper to aquatic mosses, and this reduction was more pronounced for the short-term biological endpoint, which was taken as being representative for environmental exposure. Labile copper sampled with DGT made it possible to estimate short-term bioaccumulation in the case of EDTA and natural Seine River extracts. With humic acid solutions, however, labile copper was lower than bioavailable copper. This result suggests that at realistic metal concentrations and with certain types of natural DOM, bioavailable copper might comprise not only inorganic copper but also some weak organic complexes. Hence, labile copper, in situ sampled with DGT, might not systematically overestimate bioavailable copper, as suggested previously on the basis of in vitro toxicity studies.

Response of the unicellular diazotrophic cyanobacterium Crocosphaera watsonii to iron limitation.

Iron (Fe) is widely suspected as a key controlling factor of N2 fixation due to the high Fe content of nitrogenase and photosynthetic enzymes complex, and to its low concentrations in oceanic surface seawaters. The influence of Fe limitation on the recently discovered unicellular diazotrophic cyanobacteria (UCYN) is poorly understood despite their biogeochemical importance in the carbon and nitrogen cycles. To address this knowledge gap, we conducted culture experiments on Crocosphaera watsonii WH8501 growing under a range of dissolved Fe concentrations (from 3.3 to 403 nM). Overall, severe Fe limitation led to significant decreases in growth rate (2.6-fold), C, N and chlorophyll a contents per cell (up to 4.1-fold), N2 and CO2 fixation rates per cell (17- and 7-fold) as well as biovolume (2.2-fold). We highlighted a two phased response depending on the degree of limitation: (i) under a moderate Fe limitation, the biovolume of C. watsonii was strongly reduced, allowing the cells to keep sufficient energy to maintain an optimal growth, volume-normalized contents and N2 and CO2 fixation rates; (ii) with increasing Fe deprivation, biovolume remained unchanged but the entire cell metabolism was affected, as shown by a strong decrease in the growth rate, volume-normalized contents and N2 and CO2 fixation rates. The half-saturation constant for growth of C. watsonii with respect to Fe is twice as low as that of the filamentous Trichodesmium indicating a better adaptation of C. watsonii to poor Fe environments than filamentous diazotrophs. The physiological response of C. watsonii to Fe limitation was different from that previously shown on the UCYN Cyanothece sp, suggesting potential differences in Fe requirements and/or Fe acquisition within the UCYN community. These results contribute to a better understanding of how Fe bioavailability can control the activity of UCYN and explain the biogeography of diverse N2 fixers in ocean.