Sophie Bonnet

Revisiting atmospheric dust export to the Southern Hemisphere ocean: Biogeochemical implications

Concentrations of dust were 6.1 ± 2.4 ng m 3 for SEPS and 13.0 ± 6.3 ng m 3 for SOKS. Dust fluxes, derived from those concentrations, were 9.9 ± 3.7 m gm 2 d 1 for SEPS and 38 ± 14 m gm 2 d 1 for SOKS and are shown to be representative of actual fluxes in those areas. Dust and iron deposition are up to 2 orders of magnitude lower than former predictions. A map of dust deposition on the Southern Hemisphere is proposed by incorporating those in situ measurements into a dust model. This study confirms that dust deposition is not the dominant source of iron to the large high-nutrient low-chlorophyll Southern Ocean.

Assessment of the dinitrogen released as ammonium and dissolved organic nitrogen by unicellular and filamentous marine diazotrophic cyanobacteria grown in culture

The dinitrogen (N2) released as dissolved nitrogen (DN) has been compared in batch cultures of four marine diazotrophic cyanobacteria: the colony forming Trichodesmium IMS101 and the unicellular strains Cyanothece ATCC51142, Crocosphaera watsonii WH8501 and WH0003. Two approaches were conducted for this purpose. The first approach consisted in the comparison of the total accumulation of fixed N2 in the culture (both in dissolved and particulate pools) with the net N2 fixation rates (i. e. the fixed N2 incorporated only in the particulate fraction after 15N2 incubation). The difference between the two measures accounted for the fixed N2 released as DN. The second approach consisted in the direct measure of the 15N-enrichment of ammonium (NH4+) and dissolved organic N (DON) following the 15N2 incubations. The N2 released as DN accounted for ~0 - 20 % and ~1 % of the N2 fixed after 24 h in the first and second approach, respectively. We show that the recent methodological improvements in the net N2 fixation determination applied in this study tend to reconcile the two approaches that formerly led to contrasted values. However, the large analytical uncertainties of the first approach limit its reliability. Thus, the direct determination of the 15N-enrichment of the dissolved pool remains the best tool to assess the fixed N2 released in the DN pool, in particular as it allows shorter incubation times. There were no clear patterns detected between the filamentous Trichodesmium and unicellular strains, neither in terms of the amount of N2 released as DN nor in terms of the proportion of NH4+ relative to DON. This suggests that N2 release processes are shared among the filamentous and free living diazotrophs.

Chasing after Non-cyanobacterial Nitrogen Fixation in Marine Pelagic Environments

Traditionally, cyanobacterial activity in oceanic photic layers was considered responsible for the marine pelagic dinitrogen (N2) fixation. Other potentially N2-fixing bacteria and archaea have also been detected in the pelagic water column, however, the activity and importance of these non-cyanobacterial diazotrophs (NCDs) remain poorly constrained. In this perspective we summarize the N2 fixation rates from recently published studies on photic and aphotic layers that have been attributed to NCD activity via parallel molecular measurements, and discuss the status, challenges, and data gaps in estimating non-cyanobacterial N2 fixation NCNF in the ocean. Rates attributed to NCNF have generally been near the detection limit thus far (<1 nmol N L−1 d−1). Yet, if considering the large volume of the dark ocean, even low rates of NCNF could make a significant contribution to the new nitrogen input to the ocean. The synthesis here shows that nifH transcription data for NCDs have been reported in only a few studies where N2 fixation rates were detected in the absence of diazotrophic cyanobacteria. In addition, high apparent diversity and regional variability in the NCDs complicate investigations of these communities. Future studies should focus on further investigating impacts of environmental drivers including oxygen, dissolved organic matter, and dissolved inorganic nitrogen on NCNF. Describing the ecology of NCDs and accurately measuring NCNF rates, are critical for a future evaluation of the contribution of NCNF to the marine nitrogen budget.

Iron from a submarine source impacts the productive layer of the Western Tropical South Pacific (WTSP)

In the Western Tropical South Pacific, patches of high chlorophyll concentrations linked to the occurrence of N2-fixing organisms are found in the vicinity of volcanic islands. The survival of these organisms relies on a high bioavailable iron supply whose origin and fluxes remain unknown. Here, we measured high dissolved iron (DFe) concentrations (up to 66u2009nM) in the euphotic layer, extending zonally over 10 degrees longitude (174 E−175 W) at ∼20°S latitude. DFe atmospheric fluxes were at the lower end of reported values of the remote ocean and could not explain the high DFe concentrations measured in the water column in the vicinity of Tonga. We argue that the high DFe concentrations may be sustained by a submarine source, also characterized by freshwater input and recorded as salinity anomalies by Argo float in situ measurements and atlas data. The observed negative salinity anomalies are reproduced by simulations from a general ocean circulation model. Submarine iron sources reaching the euphotic layer may impact nitrogen fixation across the whole region.