Geochemistry, Phytoplankton & Color of the Ocean (GeP&CO)


( 25-Aoû-2003 /yD)


The process studies initiated by JGOFS have permitted a better understanding of the coupling between the dynamics of the ocean, and the main biogeochemical fluxes. They have also shown that new production, that is the main, primary flux, is controlled not only by nitrate availability, but also by iron, and by silica. In addition, non-redfieldian processes such as biocalcification or diazotrophy, have specific characteristics, and are important enough in some areas to modify the ratios between chemical constituents involved in oceanic biogeochemical fluxes. The phytoplankton species are responsible for these variable biogeochemical capacities. A well-known example is given by biocalcification, made by the Coccolithophorids that build calcareous pieces in addition to their usual photosynthesis. Biocalcification tends to decrease the alkalinity of seawater. The Trichodesmium (cyanophyceae) are another example of a specific chemistry : they are able to fulfil their nitrogen requirements with N2 instead of oxidized nitrogen forms, thus contributing to increase the global reactive nitrogen stock in the ocean.

The priority for the JGOFS scientific community is now to carry out the synthesis of results obtained in the field during the preceding decade, and to improve the modelling of biogeochemical fluxes. For this new challenge, data that document these specificities are available only in those areas and time periods where JGOFS time series or process studies took place, but they are lacking over most of the ocean. The objective of GeP&CO is to describe and understand the variability, in space and time, of phytoplankton, in wide oceanic areas, from a biogeochemical point of view. GeP&CO is strongly characterized by sampling along a merchant ship lane. This kind of sampling is a low-cost solution to collect time series data on long oceanic transects, but is limited to the surface mixed layer. This layer however plays a dominant role : it exchanges energy and gases with the atmosphere, and absorbs most of the light energy that is necessary for photosynthesis. The systems encountered will be characterized using their photosynthetic pigments, measured by HPLC and by spectrofluorimetry, using quantitative inventories of the picoplankton by flow cytometry en flux, and using some important chemical properties such as macronutrients concentration, total CO2 and alkalinity, colored dissolved organic matter. Bio-optical measurements (light absorption, seawater reflectances) will also be made in order to validate on a regional basis the products delivered from sea color sensors on satellites. These measurements can be useful for the improvement of algorithms intended to detect blooms of functional groups with important geochemical consequences from satellite sea color data (blooms of coccolithophoridae or Trichodesmium).
These objectives make up the core of GeP&CO. Some additional measurements will also be done, in order to optimize the observations on the merchant ship lane. Part of them are easy to carry out, at low coast, and provide information useful for the understanding of the observations (sea surface temperature and salinity), while other ones have no or few relationship with the GeP&CO objectives (atmospheric 13C).
The shipping ligne that has been selected starts from Le Havre every three months, and goes to Nouméa, via Panama and Tahiti. It samples the north Atlantic, the Sargasso Sea, the Gulf of Mexico, the Gulf of Panama, the equatorial upwelling in the Pacific and the south Pacific subtropical gyre. A scientific observer embarks on each trip to carry out the sampling and to watch over the instruments. GeP&CO started in October 1999. 12 cruises (one very three month) were scheduled. The tenth cruise started on December 31 of 2001, in conformity with the initial plan.
The analyses of samples have generally been made within a few months after the end of each cruise. Counts of coccolithophorids however necessitate much more time and, at present, are only available for the first four cruises in the North Atlantic. Preliminary results show that the variability of photosynthetic pigments ratios is low. Pigments that are indicators for phytoplankton functional groups (fucoxanthin for diatoms, or 19’hexano-yloxyfucoxanthin for coccolithophorids) are found to be ubiquitous, with maximum abundance in the North Atlantic during the spring bloom.
Two methodology studies are in progress. The first one shows that the ancient fluorescence – acidification technique does not lead to large errors in the estimate of chlorophyll a concentration given the actual assemblages of pigments that emit red fluorescence when excited by blue light by. The reason for this small error is the scarcity of chlorophyll b and of pheophytin a in the phytoplankton at the surface. The second one presents a new spectrofluorimetric method for the routine measurement of phycoerythrin.
The objectives and methods of GeP&CO, and access to the data, are available at


1. Rationale

The oceanic carbon cycle is mainly controlled by photosynthesis that occurs in the photic layer. A simple representation of this process by models uses chlorophyll, light, and nitrogen availability. Nitrogen is generally available as nitrate, nitrite, ammonia or urea. However, since the biological carbon sink is determined by new production, nitrate is considered as the main limiting nutrient in the ocean, and this sink is approximated using nitrate fixation and the C : N Redfield ratio.
More than ten years of oceanic studies coordinated by JGOFS lead to a more complex scheme. The nitrate-based cycle cannot explain all the details of the variability of the carbon cycle, that result from processes whose importance was evidenced during the last decade. Some phototrophs are thus able to fulfil their nitrogen requirements using dissolved atmospheric N2 instead of nitrate or other more reactive forms (Trichodesmium). Other species or groups of species are able to build calcium carbonate pieces (coccolithophorids). These studies have also shown that most phototrophs also necessitate iron in addition to their nitrogen needs, and that this element, that has a low solubility, plays an decisive role in biogeochemical cycles. Diatoms that can grow rapidly need silica and are also strongly dependent on iron availability. On the contrary, the procaryote Prochlorococcus sp. Is able to grow at very low iron concentration. Knowledge of the specific biogeochemical behavior of different phytoplankton groups, and of the factors that control them, is thus an important challenge, which is at the core of the EDOCC initiative (Ecological Determinants of Oceanic Carbon Cycling).
GeP&CO intends to detect and to describe the variability of phytoplankton populations, and to understand how the physics of the ocean controls this variability. This is a prerequisite to realistic simulation of the oceanic carbon cycle. Indeed, while all phytoplankton species have a photosystem in which chlorophyll captures the energy of photons to reduce carbon dioxide, the rate, and pathways for using this energy vary according to phytoplankton groups. To improve the simulation of biogeochemical cycles in the photic layer, Bisset et al. (1999) for instance have developed a model with four phytoplankton compartments : mixed layer Prochlorococcus, deep Prochlorococcus, Synechococcus , and others. To summarize, while it is important to estimate primary production using chlorophyll concentration and light, it is important as well to know which groups of phytoplankton are doing phototsynthesis. Examples are given below.

Diazotrophy :
Some phototrophs species (the cyanobacteria Trichodesmium and Rickelia intracellularis) have the capacity to use nitrogen in the N2 form. Consequently, they are able to grow and make new production in places where nitrate and ammonia are lacking. Thus, atmospheric nitrogen can increase the stock of reactive nitrogen in the ocean, and this flux must be considered for the long-term budget of nitrogen and carbon in the ocean (Falkowski, 1997), the counterpart being denitrification that occurs in oxygen depleted areas. According to Codispoti (1995), 25 1012g of atmospheric nitrogen are fixed each year in the ocean, most of it by Trichodesmium (Capone et al. 1997). Many questions subsist however if one wants to estimate the global role of diazotrophy in the ocean (Hood et al., 2000). Diazotrophy makes it possible for ecosystems to export carbon without any nitrate supply. This is a plausible explanation to the summer decrease in the CO2 partial pressure at time series stations BATS and ALOHA (Marchal et al., 1996 ; Karl et al., 1997 ; Abell et al., 2000). Similarly, Walsh et al. (1999) had to introduce nitrogen fixing cyanobacteria in their model of the Cariaco bassin (Venezuela) in order to improve the simulation of biogeochemical variability in this region. Cyanobacteria have in common specific pigments of the phycobiliproteins group, that give to these species characteristic optical properties of absorption (Subramaniam et al., 1999a). The best known are Trichodesmiums. When they form blooms at the surface, they have a strong absorption in the near infra-red (in the same way as terrestrial plants), and a strong retrodiffusion caused by gas vacuoles. These properties may provide the possibility to detect them from satellites (Subramaniam et al., 1999b).

Biocalcification :
Another group, the coccolithophorids, builds calacareous coccoliths in addition to fixing carbon in the organic form. This has consequences on the equilibrium of carbonates that differ from those of photosynthesis, resulting in a decrease of seawater valkalinity (Balch and Kilpatrick, 1996). This process is named biocalcification. It is difficult to measure, and is only represented in an approximate and empirical way in models of the oceanic carbon cycle (Yamanaka and Tajika, 1996, Le Quéré, 1999). Biocalcification was estimated to be about 3 to 12 % of the primary production flux in the equatorial Pacific during the EqPac cruises (Balch and Kilpatrick, 1996). In the Arabian Sea, biocalcification amounted to 1 to 5 % (Balch et al., 2000). Coccolithophorids blooms have characteristic optical properties caused by the strong retrodiffusion by coccoliths (Brown and Yoder, 1994 ; Balch et al., 1999 ; Tyrell et al., 1999), and can be detected from satellite. The coupled effects of photosynthesis and biocalcification decrease alkalinity, which generally leads to a rise of CO2 partial pressure (Holligan et al., 1994). As a feed back, a rise of pCO2 seems to stimulate photosynthesis in two species of coccolithophorids, but decreases their biocalcification rate (Riebesell et al., 2000). Coccolithophorids dominate in some areas, especially in frontal zones at temperate latitude (Eynaud et al., 1999). They are abundant in the diet of filter feeders (Gorsky et al., 1999).
Biocalcification is also worked out by zooplanktonic foraminifers and pteropods that seem to be controlled by the occurrence of phytoplankton blooms (Schiebel et al., 2001).

Iron limitation :
While iron is more and more considered as a key element for phytoplankton growth (Martin and Fitzwater, 1988; Price et al. 1994), species have specific responses to iron limitation : diatoms are very sensitive. Oppositely, Prochlorococcus sp. Seem to be able to grow fast in the absence of iron (Di Tullio et al., 1993). High Prochlorococcus growth rates indeed have been observed in the iron poor central equatorial Pacific, (one doubling each day) demonstrating that these organisms are not or few affected (Vaulot et al., 1995). Other cyanobacteria such as Synechococcus produce siderophores that help them to compete against other species when iron is lacking (Whilhelm and Trick, 1994). Iron limitation does not seem to affect the C / N assimilation ratio, but it decreases the rate of photosynthesis. The field data on iron concentration at sea begin to be abundant enough for the study of global trends (Lefevre and Watson, 1999; Fung et al., 2000), and models have been developed that consider the iron demand by the phytoplankton (Loukos et al., 1997; Hurtt and Armstrong, 1999). The effect of iron however varies according to phytoplankton groups : the IRONEX II experiment showed that the diatoms have the fasted growth rate when their iron requirements are fulfilled, while the Prochlorococcus do not react to addition of iron (Cavender-Bares et al., 1999). The consequences of iron limitation however are not fully understood. For instance, it has been observed in the equatorial Pacific that iron addition did increase the size of Prochlorococcus cells, and the amount of chlorophyll per cell, but that the number of Prochlorococcus in seawater did not change significantly, as cell mortality probably counterbalanced their division rate (Mann and Chisolm, 2000). At the ocan surface, iron has Aeolian or upwelling origin (Fung et al., 2000), and its availability is thus controlled by physical processes. In addition, there seems to exist a biological control, as some phytoplankton groups seem to have the capacity for pre-conditionning seawater : prokaryotic algae indeed seem to block iron for themselves by emitting siderophores that absorb iron and that they only can assimilate. A similar process, with de porphyrins instead of siderophores, would benefit to eukaryotes only (Hutchins et al., 1999). Attempts to represent the role of "ligands" produced bys phytoplankton groups in models have been made (Archer et al., 2000).

Limitation by silica :
The phytoplankton group that can grow at fatest rate is the diatoms (Goldman, 1993; Nelson and Brzezinski, 1997). Their growth is limited both by iron, which they need at high concentration, and by silica, which they need to build their skeleton. Silica is abundant in cold seas, and becomes rapidly limitant in tropical regions where new production thus must be done by other phytoplankton groups, which have a smammer growth rate (Nelson and Tréguer, 1992). Consequently, silica that is needed by diatoms controls the ocean’s biological carbon sink (Tréguer and Pondaven, 2000). When iron is lacking, the diatoms tend to build thicker à fabriquer des skeletons (Takeda, 1998), because the time between cells division is greater. Each year after the spring bloom, the diatoms produce the largest part of carbon export to depth in the North Atlantic (Lochte et al., 1993). They are less numerous in tropical seas (except in coastal upwellings), but here too, they are thought to be responsible for most of the new production (Bender and McPhaden, 1990 ; Goldman, 1993). Still, silica is limitant in tropical regions, even in the equatorial Pacific upwelling ( Blain et al., 1997 ; Dugdale and Wilkerson, 1998). The diameter of diatoms is generally between 20 and 200 µ, so that they cannot be detected by routine flow cytometry analysis. However, they contain fucoxanthin that is rare in other groups. Considering the important role of diatoms in new production, as well as their high fuxocanthin content, Claustre (1994) proposed that the ratio (fuxocanthin + péridinin) / (total pigments) be a proxi of the f ratio (new production to total production ratio).

The production of dimethylsulfide :
Some algae give a remarkable example of the role that biological processes may have on climate : as a protection against grazers, they have an enzyme (DMSP-lyase) that produces a repellant derived from dymethyl sulfonium proponiate (DMSP) that some grazers (especially protozoans) avoid (Wolfe et al. 1997), and that is finally degraded into dimethylsulfide (DMS). The later is an important source of aerosols and condensation nuclei for the formation of clouds. This is accounted for in some atmosphere models for climate change, but the variability of DMS sources, and what causes that variability, are poorly known. While it is clearly established that primary producers are at the origin of marine dimethylsulfide emission, the phytoplankton groups contribute in different manners. The coccolithophorids are known to be the main producers (Malin, 1997). In addition to the phytoplankton composition, its physiological state would also be important. Fuhrman (1999) indeed shows a direct effect of viruses on DMS production. Consequently, it is not possible to forecast the DMS concentration using only the chlorophyll concentration. The abnormally high abundance of DMS, while correlated to chlorophyll concentration, that was observed by Gui-Peng et al. (1999), may be explained by such physiological factors.

Thus, in addition to the ocean’s variability in terms of chlorophyll and macronutrients concentration, there also exists an enormous variability in the phytoplankton composition that results in a variety of biogeochemical processes. An illustration can be given, starting from iron, silica and nitrate rich waters. Under such conditions, diatoms will grow first, occasioning an intense biological sink of carbon (this can be seen during the North Atlantic spring bloom : Lochte et al., 1993). When silica shortage occurs, followed by iron, nitrate, and phosphorus shortage, there is a succession of different groups of algae, and finally, in ultra oligotrophic areas, populations are dominated by picoplanckon, and especially by the genus Prochlorococcus (Landry et al., 1997). Between these two extremes, the capacity of the ocean to export organic carbon to depth decreases by several orders o f magnitude.
JGOFS has defined oceanic provinces that encompass the diversity of biological systems. Process studies carried out in these provinces led to considerable improvement of knowledge. However, these studies are limited in space and/or time, and observations are lacking in many parts of the ocean. At present, 3D ocean biogeochemical models, as well as satellite sea color data, are the only tools to enable global studies and interpolation. These tools however still ignore most of the biological variability, and GeP&CO aims to document this variability over large space and time scales.


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