D. Lefèvre

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Is DOC the main source of organic matter remineralization in the ocean water column

Recent interpretations of carbon flux data and deep-sea processes have led to a reconsideration of the role of dissolved organic carbon (DOC) in supporting water column remineralization and other mid-water biogeochemical transformations (Suzuki et al., 1985; Cho and Azam, 1988; Karl et al., 1988; Christensen et al., 1989; Naqvi and Shailaja, 1993). To date, there have been no direct comparisons of particulate carbon flux data with water column metabolic rates. Here, for the first time, particulate carbon flux and respiratory electron transport activity (from which metabolic CO2 production is derived), have been monitored simultaneously for one year in the same area of the northwestern Mediterranean Sea. In the aphotic layer (200–1000 m), particulate organic carbon (POC) can support only 20% of the overall organic matter remineralization. Remineralization rates are consistent with recent calculations of DOC exported from the euphotic layer in this area, confirming the vital importance of DOC in maintaining deep-water metabolism. This finding would apply to other regions of mesotrophic and oligotrophic production and thus affect our understanding of carbon recycling in the water column, new production and O2 utilization.

Microbial community production, respiration, and structure of the microbial food web of an ecosystem in the northeastern Atlantic Ocean

increased from winter (101 ± 24 mmol O2 m 2 d 1 ) to spring (153 ± 27 mmol O2 m 2 d 1 ) and then decreased from spring to late summer (44 ± 18 mmol O2 m 2 d 1 ). DCR rates increased from winter (47 ± 18 mmol O2 m 2 d 1 ) to spring (97 ± 7 mmol O2 m 2 d 1 ) and then decreased from spring to late summer (50 ± 7 mmol O2 m 2 d 1 ). The onset of stratification depended on latitude as well as on the presence of mesoscale structures (eddies), and this largely contributed to the variability of GCP. The trophic status of the POMME area was defined as net autotrophic, with a mean annual net community production rate of +38 ± 18 mmol O2 m 2 d 1 , exhibiting a seasonal variation from +2 ± 20 mmol O2 m 2 d 1 to +57 ± 20 mmol O2 m 2 d 1 . This study highlights that small organisms (picoautotrophs, nanoautotrophs, and bacteria) are the main organisms contributing to biological fluxes throughout the year and that episodic blooms of microphytoplankton are related to mesoscale structures.

Deep microbial communities evidenced in the Liguro-Provençal front by their ETS activity

Abstract ETS (electron transport system) activity through the water column (down to 800–1200 m depth) and along a transect across the Liguro-Provencal front (Mediterranean Sea) was monitored over a 2 year period. A time series was run in two zones (peripheral and central) characterizing the area and located on both sides of the front. In contrast with profiles of ETS activity that follow a power function decrease with increasing depth in oligotrophic waters, ETS activity always exhibited a local maximum at 300–400 m depth in the peripheral zone and less frequently in the central one. In the peripheral zone, the mean value of this peak was about 50% higher than in the central zone. This dissymmetry in ETS activity with respect to the front was persistent despite an opposite dissymmetry in the euphotic layer, where the mean value of ETS activity in the central zone was about 30% higher than in the peripheral one. Such a distribution is accounted for by a shoreward transfer of organic matter produced in the central and frontal zones. It also reveals the existence of discrete accumulations of micro-organisms in the aphotic layer, probably in connection with the frontal structure. Principal component analysis of 256 observations made in the 80–1200 m water column quantified the contribution of the front parameters to the ETS activity distribution below the 80 m top layer. The metabolic CO2 production in the 200–800 m water column is higher in the peripheral zone (28.1 g C m−2 y−1) than in the central one (23.9 g C m−2 y−1).

How do microbial communities keep living in the Mediterranean outflow within northeast Atlantic intermediate waters

Abstract As part of the France-JGOFS programme, the Mediterranean outflow within the Atlantic Ocean was investigated during MEDATLANTE cruises (January and August 1989). The present work is focused on the export and fate into the Atlantic of the microbial communities carried away from the Mediterranean Sea by this outflow that generates Meddies (Mediterranean eddies). The respiratory activity of micro-organisms (≤200 μm) was monitored through their electron transport system (ETS) activity in Meddy “Nicole” (36°25′N, 10°35′W), Meddy “Yseult” (36°20′N, 16°W), and in waters of Mediterranean origin (MOW) at 36°57′N, 17°02′W. On each cruise, profiles of ETS activity through the water column (0–2000 m) were established in the Mediterranean Sea to serve as a reference for the ETS activity profiles in the Atlantic. ETS activity within the Meddies or MOW was similar to that in the Mediterranean Sea at the same period and significantly higher (up to three times) than that calculated for intermediate Atlantic waters. Within investigated Meddies and MOW, rates of metabolic CO2 production varied from 64.8 mg C m−2 day−1 (Meddy Nicole) to 200.5 mg C m−2 (Meddy Yseult). This represents up to 10 times the rate of metabolic CO2 production within the corresponding 500–1500 m layer in the Sargasso Sea. Considering the life-time of micro-organisms (a few days) and the age of Meddies and MOW (up to 1 year), the microbial communities evidenced by their ETS activity at stations far from the Strait of Gibraltar must have either reproduced themselves or been replaced by opportunistic communities. The initial loading of the outflow in fresh organic material, close to Gibraltar Strait, cannot account for the relatively higher rate of mineralization by microorganisms. We suggest a hypothesis based on the dynamics of Meddies.

Observation of oxygen ventilation into deep waters through targeted deployment of multiple Argo-O 2 floats in the north-western Mediterranean Sea in 2013

During the winter 2013, an intense observation and monitoring was performed in the north-western Mediterranean Sea to study deep water formation process that drives thermohaline circulation and biogeochemical processes (HYMEX SOP2 and DEWEX projects). To observe intensively and continuously the impact of deep convection on oxygen (O 2 ) ventilation, an observation strategy was based on the enhancement of the Argo-O 2 floats to monitor the offshore dense water formation area (DWF) in the Gulf of Lion prior to and at the end of the convective period (December 2012 to April 2013). The intense O 2 measurements performed through shipborne CTD casts and Argo-O 2 floats deployment revealed an O 2 inventory rapidly impacted by mixed layer (ML) deepening on the month scale. The open-sea convection in winter 2013 ventilated the deep waters from mid-February to the end of May 2013. The newly ventilated dense water volume, based on an Apparent Oxygen Utilization (AOU) threshold, was estimated to be about 1.5 × 10 13 m 3 during the DWF episode, increasing the deep O2 2 concentrations from 196 to 205 µmol kg −1 in the north-western basin.

A flow cytometric approach to assess phytoplankton respiration.

Microbial respiration in the ocean is considered as the major process representative of the organic matter biological oxidation. The corresponding metabolic CO2 production was estimated to be about 22 Pg C y(-1). However, the in situ respiration rate is generally too low (by several orders of magnitude) to be accessible to the available direct measurement methods. Some fluorescent probes, such as DiOC6(3) (Molecular Probes, USA) have been shown to be very sensitive to changes in the proton electrochemical potential difference (DeltamuH+), characterising mitochondrial and plasmic membranes bearing the cell respiratory system in eukaryotic and prokaryotic cells respectively. In mitochondria, DeltamuH+ is linked to the flux of oxygen uptake by a linear relationship. To our knowledge, no such relationship has been established in the case of whole marine cells. In the present work, we addressed the dark respiration rate of the Chlorophyceae Dunaliella tertiolecta (Butcher) in axenic cultures, both directly by using a highly sensitive oxygraph (Oroboros) and by staining cells with DiOC6(3). We found and standardized a linear relationship between oxygen uptake by D. tertiolecta and its green fluorescence induced by DiOC6(3), enabling the determination by flow cytometry of the respiration rate of D. tertiolecta.

Nutrients in mode waters of the northeast Atlantic

Programme Ocean Multidisciplinaire Meso Echelle (POMME) seasonal surveys in the northeast Atlantic (39°N-44.5°N; 16.5°W-20.3°W) in 2001 are used to investigate the subduction of nutrients in the subsurface mode waters. Isopycnal subsurface distributions are used to estimate inorganic nutrients at the time of late winter restratification. These nutrient concentrations were close to winter near-surface concentrations, indicating a moderate consumption of nitrate (0.2-0.3 μM kg−1) and dissolved inorganic carbon (2-3 μM kg−1) in the surface layer before the effective subduction. Spring survey nutrient concentrations on isopycnal surfaces are lower north of 41.7°N indicating younger waters than further south. The seasonal increase of subsurface nutrients from spring to late summer diminishes from the shallower isopycnals to the deeper ones of the mode waters. It is also larger north of 41.7°N than south of it with values as large as 2 μM kg−1 for nitrate and 10 μM kg−1 for inorganic carbon. This evolution is mostly attributed to remineralization processes, both from falling particles (at least 15%) and from preformed dissolved organic matter (at most 30%). Ratios of nutrient changes to oxygen changes are often larger than Redfield ratios for nitrate (N:apparent oxygen utilization (AOU) at least 1:7) and phosphate (P:AOU at least 1:150) consistent with favored remineralization of P and N over C, both for dissolved and particulate organic pools.

Mixotrophic metabolism by natural communities of unicellular cyanobacteria in the western tropical South Pacific Ocean: Mixotrophy in natural marine cyanobacteria

Cyanobacteria are major contributors to ocean biogeochemical cycling. However, mixotrophic metabolism and the relative importance of inorganic and organic carbon assimilation within the most abundant cyanobacteria are still poorly understood. We explore the ability of Prochlorococcus and Synechococcus to assimilate organic molecules with variable C:N:P composition and its modulation by light availability and photosynthetic impairment. We used a combination of radiolabelled molecules incubations with flow cytometry cell sorting to separate picoplankton groups from the western tropical South Pacific Ocean. Prochlorococcus and Synechococcus assimilated glucose, leucine and ATP at all stations, but cell-specific assimilation rates of N and P containing molecules were significantly higher than glucose. Incubations in the dark or with an inhibitor of photosystem II resulted in reduced assimilation rates. Light-enhanced cell-specific glucose uptake was generally higher for cyanobacteria (∼50%) than for the low nucleic acid fraction of bacterioplankton (LNA, ∼35%). Our results confirm previous findings, based mainly on cultures and genomic potentials, showing that Prochlorococcus and Synechococcus have a flexible mixotrophic metabolism, but demonstrate that natural populations remain primarily photoautotrophs. Our findings indicate that mixotrophy by marine cyanobacteria is more likely to be an adaptation to low inorganic nutrient availability rather than a facultative pathway for carbon acquisition.

Longitudinal contrast in Turbulence along a ∼ 19S section in the Pacific and its consequences on biogeochemical fluxes

Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific (Moutin and Bonnet, 2015). Small-scale dynamics and turbulence in the first 800m surface layer were characterized based on hydrographic and current measurements at fine vertical scale and turbulence measurements at cm scale using a vertical microstructure profiler. The possible impact of turbulence on biogeochemical budgets in the surface layer was also addressed in this region of increasing oligotrophy to the East. The dissipation rate of turbulent kinetic energy, , showed an interesting 5 contrast along the longitudinal transect with stronger turbulence in the West, i.e. the Melanesian Archipelago, compared to the East, within the South Pacific Subtropical Gyre, with a variation of by a factor of 3 within [100m−500m]. The layer with enhanced turbulence decreased in vertical extent traveling eastward. This spatial pattern was correlated with the energy level of the internal wave field, higher in the West compared to the East. The difference in wave energy mostly resulted from enhanced wind power input into inertial motions in the West. Moreover, three long duration stations were sampled along the cruise 10 transect, each over three inertial periods. The analysis from the western long duration station gave evidence of an energetic baroclinic near-inertial wave that was responsible for the enhanced , observed within a 50m-250m layer, with a value of 810−9Wkg−1, about 8 times larger than at the eastern long duration stations. Averaged nitrate turbulent diffusive fluxes in a 100-m layer below the top of the nitracline were about twice larger west of 170W due to the higher vertical diffusion coefficient. In the photic layer, the depth-averaged nitrate turbulent diffusive flux strongly decreased eastward with an 15 averaged value of 11μmolm−2d−1 West of 170W to be compared with the 3μmolm−2d−1 averaged value East of 170W. Contrastingly phosphate turbulent diffusive fluxes were significantly larger in the photic layer. This input may have an important role in sustaining the development of N2-fixing organisms that were shown to be the main primary contributors to the biological pump in the area. The time-space intermittency of mixing events, intrinsic to turbulence, was underlined but its consequences on micro-organisms would deserve a dedicated study. 20 Copyright statement.