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General Context

lundi 12 mai 2014

General context:

The additional CO2 in the atmosphere, mainly resulting from fossil fuel emissions linked to human activities (anthropogenic CO2), is the main cause of global warming. The ocean has acted as a major sink of anthropogenic CO2 (Sabine et al., 2004) preventing a greater accumulation in the atmosphere and therefore a greater increase in the earth’s temperature. The biological pump (Fig. 1) provides the main explanation for the vertical gradient of carbon (C) in the ocean. Its strength and efficiency depends upon the complex balance between organic matter production in the euphotic zone and its remineralisation in both the epipelagic and mesopelagic zones. The biological pump was thought to be in an equilibrium state with an associated near-zero net exchange of CO2 with the atmosphere (Broecker 1991, Murname et al., 1999). Climate alterations are beginning to disrupt this equilibrium and the expected modification of the biological pump will probably considerably influence oceanic C sequestration (and therefore global warming) over a decadal time scale (Sarmiento and Grüber, 2006). The long term decrease of phosphate availability and the shift from previously N to P limited production associated with increasing input of nitrogen by N2 fixation was one of the first evidence of the biological pump alteration observed from the ALOHA station time series in the north Pacific gyre (Karl et al., 1997; 2008).


Fig. 1. Major C fluxes for a biological pump budget. Biological pump: C transfer by biological processes into the ocean interior. DIC: Dissolved Inorganic C, POC: Particulate Organic C, DOC: Dissolved Inorganic C. See Moutin et al., (2012) for a detailed description.

The input of new N to the surface ocean through biological N2 fixation represents a major link between the C and N biogeochemical cycles, i.e between the upper ocean nutrient availability and the biological pump, i.e between ocean and climate. This link was recently shown to play a central role in previous natural climate changes over long time scales (Galbraith et al., 2013, Dernières nouvelles de l’INSU n°200: Le cycle océanique de l’azote face aux changements climatiques, Jeudi, 25 Juillet 2013). It is nevertheless clear that expected climate changes followed by anthropogenic atmospheric CO2 changes may concern shorter time scales: the increase in atmospheric CO2 over the past 200-y is equal to the increase in atmospheric CO2 between glacial to interglacial conditions, which took place over several thousand y (Sarmiento and Grüber, 2006). It is therefore necessary to obtain a precise representation of the N2 fixation process in global biogeochemical models. Even if considerable scientific progresses have been made over the last decades (see reviews from Sohm et al., 2011; Zehr, 2011), many questions remain regarding the impact of this process on biogeochemical cycles and C export.

The role of N2 fixation in the oligotrophic ocean and overview of previous cruises in the SW Pacific:
The efficiency of oceanic C sequestration depends upon many factors, among which is the availability of nutrients to support phytoplankton growth in the photic zone of the surface ocean (Fig. 1). Large amounts of N are required for phytoplankton growth, as it is an essential component of proteins, nucleic acids and other cellular constituents. Fixed N in the form of nitrate (NO3-) or ammonium (NH4+) is directly usable for growth, but concentrations of fixed N are low (<0.1 µmol L-1) in the oligotrophic ocean and often growth-limiting in most of the open ocean euphotic zone (Falkowski et al., 1998). Dissolved dinitrogen (N2) gas in seawater, on the other hand, is very abundant in the euphotic zone (ca. 450 µmol L-1) and could constitute a nearly inexhaustible N source for the marine biota. Some prokaryotic organisms (Bacteria, Cyanobacteria, Archaea) called ‘N2-fixers’ (or diazotrophs) are able to use this gaseous N source since they can break the triple bond between the two N atoms of the N2 molecule, and convert it into a usable form (i.e. NH3) for assimilation thanks to the nitrogenase enzyme system (Zehr et al., 1998; 2001). By doing this conversion, they release dissolved N in surface waters. At the global scale, they provide 100-200.1012 g of N to the surface ocean every year (Mahaffey et al., 2005). It is the major external source of N for the ocean, significantly larger than atmospheric and riverine sources (Codispoti et al., 2001; Grüber and Sarmiento, 2002; Grüber, 2004). N2-fixing organisms act thus as ‘natural fertilizers’, and contribute to sustain life and potentially C export in coastal and oceanic environments.
Most of the ocean (60%, Longhurst et al., 1995) is comprised by low-nutrient, low-biomass oligotrophic ecosystems, which constitute the largest coherent biomes on our planet. They support a large part (40%) of the photosynthetic C fixation in the ocean (Antoine et al., 1996). This C fixation is mainly performed by pico-plankton (smaller than 2-3 µm in diameter) that are generally thought to represent a negligible fraction of the total particulate organic C (POC) export flux due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Consequently, the efficiency of the biological C pump in these oligotrophic systems has long been considered to be low as the greatest proportion of fixed C is thought to be recycled in the surface layer and rapidly re-exchanged with the atmosphere.
Recent studies have challenged this view and indicate that all primary producers, including picoplanktonic cells, contribute to export from the surface layer of the ocean at rates proportional to their production rates (Richardson & Jackson, 2007). Export mechanisms differ compared to larger cells as picoplanktonic export is mainly due to packaging into larger particles and via grazing and/or aggregation processes (Jackson et al., 1990; 2001; Lomas et al., 2010). More recently, Close et al. (2013) pointed out that 40-70% of picoplanktonic cells are small enough to escape detection under the most common definition of suspended particulate organic matter (POM). Thanks to a coupling between lipid profiles, radiocarbon and stable isotopic signatures of lipids from the North Pacific Subtropical gyre (Hawaii Ocean Times-series station), they showed that the transfer of submicron POM from the surface to mesopelagic waters is an important contributor to the global biological pump. Besides submicron POM export, Karl et al. (2012) have reported an efficient summer export flux of C (3 times greater than the mean wintertime particle flux) at the same HOT station thanks to a 13-year sediment trap experiment. This summer export flux has been attributed to an increase in biomass and productivity of symbiotic N2-fixing cyanobacteria associated with diatoms, which have high sinking and low remineralization rates during downward transit. This efficient POM export was thus not related to submicron particles but rather to large-size phytoplankton that could grow in the oligotrophic waters of the NPG thanks to new N provided by N2 fixation (Karl et al., 2012). Consequently, the conventional view of low POM export in oligotrophic areas should be revised to include alternative pathways for picoplankton export and the role of N2 fixation in potentially promoting C export.
The SW Pacific is an ideal location to study this question, as it is considered to be one of the highest areas of global N2 fixation (Capone et al., 1997; Sohm et al., 2011). It exhibits among the highest abundances, diversity of N2-fixing organisms (Moisander et al., 2010) and activity (Montoya et al., 2004; Bonnet et al., 2009) in the world ocean. While average N2 fixation rates range from 50-200 µmol N.m-2.d-1 in the tropical North Atlantic (Capone et al., 2005) and Pacific (Karl et al., 1997; Dore et al., 2002), they reach 126-4000 µmol N.m-2.d-1 (Montoya et al., 2004) and 50-360 µmol N.m-2.d-1 (Bonnet et al., 2009) in the SW Pacific, in the Arafura Sea (off Australia, TRICHONESIA cruise) and the western warm pool (EUC-Fe cruise), respectively. The seasonal DIAPALIS cruises (INSU-PROOF DIAPAZON programme) in the Loyalty channel off New Caledonia also reported high rates ranging from 151 to 703 µmol N.m-2.d-1 (Garcia et al., 2007), the highest being measured in austral summer which has been directly linked to phosphate availability (Van den Broeck et al., 2004; Moutin et al., 2005). The seasonal distribution of N2 fixation is corroborated by in situ and satellite observations (TRICHOSAT algorithm, Dupouy et al., 2011) of recurrent large Trichodesmium blooms during the austral summer (October-March) during the 1998-2010 period in the Melanesian archipelagos around New Caledonia (Vanuatu, Fiji Islands). In addition to Trichodesmium, high abundances of unicellular diazotrophic cyanobacteria have been reported in the SW Pacific (Campbell et al., 2005; Bonnet et al., 2009; Hewson et al., 2009; Moisander et al., 2010). Campbell et al., (2005, TRICHONESIA cruise) reported 1.6 103 of Crocosphaera-like cells (UCYN-B) between New Caledonia and Fiji, which is on the same order of magnitude as Synechococcus abundance, the most widespread non-diazotrophic cyanobacterium in the ocean (Partensky et al., 1999). The newly discovered and uncultured UCYN-A (Zehr et al., 2001; 2008) also displays extremely high abundances (105-106 nifH copies.l-1, Moisander et al., 2010, KM0703 cruise; Bonnet et al., Unpublished, BIFURCATION cruise) in the SW Pacific (between New Caledonia and Australia), but they seem to have different ecological niches compared to Trichodesmium and UCYN-B (Moisander et al., 2010).
When going eastward towards the South Pacific gyre, Halm et al., (2011) have reported rates of 12 – 190 µmol N m-2 d-1 on the western border of the gyre and Raimbault and Garcia, (2008) and Moutin et al., (2008) reported rates of 60 ± 30 µmol N m-2 d-1 in the central gyre during the BIOSOPE cruise (INSU-LEFE BIOSOPE programme), indicating a decreasing gradient of N2 fixation from W to E and low N2 fixation rates relative to other ocean gyre ecosystems. The organisms responsible for these fluxes are different from common autotrophic diazotrophs such as Trichodesmium or Crocosphaera, and are mainly affiliated with heterotrophic proteobacteria and low abundances of UCYN-A (Bonnet et al., 2008, BIOSOPE cruise; Halm et al., 2011).
The W to E zonal gradient of N2 fixation and the distinct diversity of N2-fixing organisms along this gradient provide a unique opportunity to study how production, mineralisation and export of organic matter depends upon N2 fixation in contrasting nutrient conditions. Comparisons between different systems along a zonal gradient of trophic status and N2 fixation will provide new insights for identifying and understanding fundamental interactions between marine biogeochemical C, N, P, Si, Fe-cycles and oligotrophic ecosystems.