( 21-Jan-2004 / sB/mpT)  

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Objective 3
(click to go to ...)	Autotrophy and the phytoplankton community   Heterotrophy and remineralisation   Remineralisation and iron speciation   Solar radiation

 

 Phytoplankton community structure is a key component  of ecosystem functioning because it controls carbon export by channelling it through direct sedimentation of large or heavy organisms, or through directing it towards the food web

 

            The phytoplankton community structure is sensitive to climate change and floristic regional shifts have already been reported in relation to mixed layer depth (MLD) shoaling and increased stratification (Karl et al., 2001). Changes in the community structure in response to global warming could therefore profoundly affect the capacity of the biological community to draw down atmospheric CO₂ and transport it to the deep ocean (Arrigo et al., 1999). Southern Ocean phytoplankton are especially sensitive to direct temperature effects because antarctic species have temperature optimum for growth ranging between 5 and 8°C (Jacques, 1983). Indirect effects via MLD shoaling are also expected in antarctic waters due to their Fe-limited HNLC status as the Fe content of phytoplankton is influenced by ambient light which implies that the extent of photoacclimation of natural populations may be constrained by the availability of Fe (Strzepek & Price, 2000).

 

 

            Changing the dominant species of the phytoplankton assemblages could impact directly on major nutrient use efficiency in terms of carbon export because elemental ratios have already been shown to be species dependent. Under natural conditions, C/N ratios can change drastically with community structure (Arrigo et al., 1999) and the same holds true for example for Si/C ratios (Quéguiner & Brzezinski, 2002). The latter is also known to vary with trace-element concentrations most notably iron (Hutchins & Bruland, 1998; Takeda, 1998). The efficiency of the biological pump at the first trophic level is then both conditioned to the dominant species as well as by the rate of iron supply.

 

 

The structure of the phytoplankton community  also controls the production of gases important for climate. This has been largely demonstrated with DMS. Dimethylsulfoniopropionate (DMSP) is a very labile compound, and a number of critical steps in its cycling determine the amounts of DMSP-sulphur and carbon that are transformed into dimethylsulfide (DMS) and can be vented to the atmosphere. The amount of DMSP that is synthesised depends on the relative abundance of DMSP-producing algae among primary producers. In many regions of the open ocean, a high proportion of stronger DMSP-producers (dinoflagellates and small haptophytes) coincides with stratified conditions in late spring and summer rather than with seasons of active surface mixing and highest primary production, usually dominated by diatoms, which produce less DMSP.

 

 

 The structure of the phytoplankton community has been therefore placed as a central component in KEOPS. The general objective 3 is split into the following sub-objectives .

 

 

 

 

The key processes to be investigated in 3.2.1 and 3.2.2 are limitation and co-limitation of cell growth, respiration, grazing, remineralization, dissolution, aggregation-desegregation, and viral lysis.

 

 

 

            The recent data on phytoplankton nutritional limitation in the Southern Ocean indicate a significant influence of several potentially limiting factors. The first evidence of  the control of primary production by iron was obtained in 1988/1989 (De Baar et al., 1990 ; Martin & Fitzwater,1990), and the importance of vertical mixing in controlling the underwater light climate was reinforced in 1991 (Mitchell et al., 1991 ; Nelson & Smith, 1991). These limiting factors are not independent. Iron availability has been recognised as a major factor slowing down or even preventing the photo-adaptation of Southern Ocean phytoplankton via pigment synthesis control (Lancelot et al., 2000). The question of the nutritional requirements of diatoms with regard to silicon is still a matter of debate. Silicic acid limitation of silica production has been detected at what would otherwise be considered non-limiting silicic acid concentrations. The first estimates of very high K_S (Jacques, 1983) were obtained well before the development of isotopic tracer studies using stable (³⁰Si) and later on radioactive (³²Si) isotopes. More recent studies using these modern techniques have also indicated surprisingly high K_S values, at least in areas remote from coastal/shelf influence (Caubert, 1998 ; Quéguiner, 2001 ; Nelson et al., 2001). Some environmental observations (Quéguiner, 2001 ; Quéguiner & Brzezinski 2002) as well as shipboard studies (Quéguiner et al., in prep.) have suggested a link between in situ low iron concentrations and high K_S values and it is thus tempting to hypothesize an iron-related control of the silicic acid uptake mechanism similar to that for nitrate utilization or nitrogen fixation. From these considerations it is now clear that the question of the "limiting factor" in the Southern Ocean and elsewhere cannot be considered simply in terms of von Liebig's law, but rather that the factors which control primary production are multiple and interrelated, which leads to the concept of co-limitation of phytoplankton growth by trace-elements, nutrients, and light (Quéguiner et al., 2002). The research proposed below fills the objectives 3 of KEOPS

 

 

 a) Characterisation of the plankton community and the composition of particulate matter.

 

Working in the KEOPS study area will offer the opportunity to sample natural phytoplankton communities in contrasting nutritional environments. We will aim to answer to the following specific questions:

 

 

b)      The influence of change in iron concentration,  light and temperature on  the composition of the plankton community and particulate matter composition

 

 

 

 

 

Heterotrophic activity is a critical link in the cycling of photosynthetically fixed organic matter. The fate, i.e. the export or the remineralization of organic carbon and other bioactive elements depends, to a large extent, on the structure and functioning of pelagic food webs. Grazing activity on primary producers is an important process for the degradation of particulate organic matter (POM) and the subsequent transfer of matter and energy to higher trophic levels. As described above, this process is directly linked to phytoplankton community structure, determining whether remineralization or sedimentation of POM prevails. The bulk of marine organic matter (~ 90%), however, exists in the dissolved form (DOM). Different DOM production pathways such as release during photosynthesis, cell death and lysis, release during feeding by heterotrophs and virus-induced lysis and the subsequent biotic and abiotic transformation processes lead to the presence of a variety of compound classes in the marine DOM pool. Heterotrophic bacterioplankton are the principal consumers of DOM (Azam 1983) and bacteria, in turn, are consumed by viruses, protozoa, and metazoan grazers, thereby transferring DOM to diverse plankton groups.

 

About half of marine primary production passes through the reservoir of dissolved organic carbon (DOC) and is processed by heterotrophic bacterioplankton (Ducklow, 2000). A strong spatial and temporal coupling between these processes has been suggested (Kirchman 1991), however, there is increasing evidence indicating that biologically available DOC may accumulate in the photic zone on both daily and seasonal scales (Copin-Montégut 1993, Carlson 1994). This indicates that the factors limiting heterotrophic bacterial activity vary spatially and temporally. Therefore, the degree to which bacterial activity is limited by sources other than carbon influences the coupling between phytoplankton and bacterioplankton activity.

 

In regard to the general objectives of KEOPS listed above, examining heterotrophic processes mineralisation/heterotrophy will mainly contribute to answer question 2.2.  and to elucidate how perturbation of the forcing parameters will affect the transfer of energy and matter through heterotrophic activity. In the framework of KEOPS the focus will be to evaluate the direct and indirect response of heterotrophic activity to iron enrichment because the investigation in the two natural contrasting areas is particularly suitable for this kind of studies. Besides the effect of substrate availibility will be investigated in the two contrasting environments.

 

 

The following topics will be addressed:

 

 

 

A large effort will also be made to investigate how heterotrophy controls the production of gases.

 

a) Carbon dioxide: Net Community Production (NCP) fluxes represent the balance between the processes of photosynthesis and of respiration of the microbial community and consequently allow quantification and qualification of the role of the biological pump of CO₂ (autotrophy versus heterotrophy). Within the framework of KEOPS the major focus will be discern wether NCP is dominated by auto- or heterotrophic processes in the two contratsing environments. NCP will subsequently be related to the major factors controlling community production and respiration, such as the availability of inorganic nutrients, the amount and the biological avaibility of particulate and dissolved organic matter and temperature.

 

Specific questions are :

 

b) DMS and other gases relevant for climate.

As described above (see section xx) the concentration of DMSP in seawaterdepends to a large extent on the structure of the phytoplankton community. The release of DMSP and or DMS from phytoplankton cells is mainly driven by grazing activity and viral lysis. In Antarctic water, it has been shown that iron enrichment wan resukts in development of phytoplanktonic species rich in DMSP. Heterotrophic dinoflagellates and cilliates could have an impact on the composition of the phytoplankton community through selective grazing. DMSP rich species could be potentially an important link between the iron and sulphur cycles in the Southern Ocean. Additionnally, photooxydation has been demonstrated to contribute significantly to the loss of DMS to the atmosphere.

 

The concentration in the surface seawater is therefore determined by the composition of the phytoplankton community and primary productivity, the loss throug grazing and viral lysis, heterotrophic activity and photochemical processes.

 

 The same kind of complexe interactions between various biological processes are also driving the emission of the other biogenic gases to the atmopshere.

 

 

The related specific questions to be addressed within the framework of KEOPS are

 

 

 

 

 

 

 Deficiency in bioavailable iron and grazing pressure have been proposed as alternative explanations for the existence of HNLC regions (Martin et al., 1990; Cullen, 1991; Banse, 1992). However it seems that both explanations should be regarded as complementary, rather than alternative (Cullen et al., 1992). When phytoplankton are Fe-limited, they are dominated by small species that may grow at rapid but generally less than physiologically maximal growth rate and are grazed to low stable abundances by microzooplankton (Landry et al., 1997). Moreover, zooplankton grazing can strongly influence the fate of trace metals associated with phytoplankton biomass (Hutchins and Bruland, 1994; Lee and Fisher, 1994; Hutchins et al., 1995; Barbeau et al., 1996; Wang et al., 1996). Efficient cropping of bacteria and diatoms by small and large protistan grazers lead to a rapid remineralisation of iron and biogenic silica. Hutchins et al. (1995) showed that about 50% of Fe assimilated by diatoms can be regenerated by copepod grazing in the dissolved phase within few hours. In the southern Ocean, during the in-situ Fe fertilisation experiment (SOIREE, Boyd et al., 2000), Bowie et al. (2001) showed that 60% and 10% of Fe assimilated by phytoplankton were regenerated by micrograzers and copepods, respectively, and that Fe regeneration by grazers was tightly coupled to uptake by phytoplankton and bacteria. Fe can also be regenerated in the dissolved phase by viral lysis (Gobler et al., 1997). Viruses are a major cause of mortality for microorganisms (Bergh et al., 1989). They are present in surface seawater at concentrations of >10⁴ to 10⁸ viruses ml^-1 (Bratbak et al., 1994), and may release more than 10% of ambient Fe concentrations (Gobler et al., 1997). In some environments, where new sources of Fe to the surface waters are low, biologically regeneration of Fe could be a major process in the biogeochemical cycle  of Fe and may therefore help to relieve Fe limitation (Barbeau et al., 1996). To date, however, no studies have investigated the speciation and bioavailability of Fe regenerated by biological activity in the Southern Ocean. In order to better understand and quantify the feed-back of biological activity on the biogeochemical cycling of Fe, we propose,  to address the following  questions in the framework of KEOPS.

 

 

 

 

 

 

Stratospheric ozone depletion and the resulting increase in solar short-wavelength ultraviolet (UV) radiation have stimulated research on the possible effects of UV radiation on aquatic ecosystems. The seasonal ozone depletion has been observed to be most pronounced over Antarctica, suggesting that UV radiation is an important physical variable in this marine environment. UVB radiation accounts for only < 1% of the total radiation intensity reaching the Earth’s surface, nevertheless, it is a highly reactive component of sunlight. Direct exposure to UV radiation has been shown to be detrimental to aquatic organisms (Herndl, 1993, Cullen, 1994, Sommaruga, 1996). Exposure to UV radiation has been shown to be detrimental to aquatic organisms (Herndl, 1993, Cullen, 1994, Sommaruga, 1996).

 

 

Solar radiation can significantly impact the biogeochemical cycling of carbon and other important elements such as nitrogen, phosphate, sulfur and different trace elements (see review by Mopper and Kieber 2000). In the context of carbon cycling the two major processes identified thus far are the direct loss of DOC due to photomineralization (Salonen and Vähätalo 1994; Miller and Zepp 1995; Graneli et al. 1996) and photochemical transformations of dissolved organic matter (DOM) thereby affecting the subsequent biological mineralization of DOC (see review by Kieber 2000). It has become evident that the source and therefore the composition of DOM determine to a large extent the impact of photochemical processes on the loss and turnover of DOC. Recent studies clearly demonstrate that the biological reactivity of DOM following exposure to irradiation is inversely related to its biological reactivity prior to irradiation (Benner and Biddanda 1998; Obernosterer et al. 1999; Tranvik and Bertilsson 2001). This indicates that initially biologically reactive DOM can be rendered biologically more resistant upon exposure to solar radiation, while DOM of initially low biological reactivity can be, at least partly, photochemically transformed to compounds of higher biological reactivity. The impact of increasing UV radiation on these photo-induced processes still needs to be established.

 

In addition to the impact of photochemical processes on the carbon cycle, they could also be important for the control of the production of other gases. For example,  laboratory have shown that the photo-degradation of dissolved organic carbon (DOC) was the likely process responsible for at least  the production of alkenes in surface seawater. This is also confirmed studies of Bonsang et al. (1992) on the production of unsaturated hydrocarbons in seawater.

 

Light driven transformations can also result in a dramatic shift in steady state soluble iron in marine waters. Photoredox behavior of Fe(III) in marines waters its likely associated with absorbtion of light by Fe(III)-organic chromophores resulting in the production of Fe(II) species. (Waite 2001). Recently it has been demonstrated that photolysis of Fe(III)-siderophores complexes leads to the formation of lower-affinity Fe(III) ligands and the reduction of Fe(III)  (Barbeau et al. 2002). The photochemistry of Fe(III) macrocyclic complexes, especially those with porphyrin-type ligands have been extensively studies but no further studies have been carried out in seawater.

 

During KEOPS, we will focus on the following specific questions