Chapter 1
38
Metal
Protein(s)
Function(s)
Mn
O2-evolving
enzyme
Oxidation of water during photosynthesis
Superoxide
dismutase
Disproportionation of superoxide
to hydrogen peroxide and O
2
Arginase
Hydrolysis of arginine to ornithine and urea
Phosphotransferases
Phosphorylation reactions
Ni
Urease
Hydrolysis of urea
Superoxide dismutase
Disproportionation of superoxide to hydrogen peroxide and O
2
Cu
Plastocyanin
Photosynthesis electron transport
Cytochrome
oxidase
Mitochondrial electron transport
Ascorbate oxidase
Ascorbic acid oxidation and reduction
Superoxide dismutase
Disproportionation of superoxide to hydrogen peroxide and O
2
Multicopper ferroxidase
High-affinity transmembrane Fe transport
Co
Vitamin B12
C
and H transfer reactions
Cd
Carbonic anhydrase
Hydration and dehydration of carbon dioxide
Mo
Nitrate reductase
Conversion of nitrate to ammonia
Nitrogenase
N fixation
Table 1.1: Common metalloproteins present within marine phytoplankton and associated functions
(from Twining and Baines, 2013).
Some oceanic areas are particularly productive owing to the availability of all essential
elements described above due to their close vicinity to the sources (e.g. estuaries, coastal
areas) or favored by physical processes (e.g. coastal upwelling, mixing of nutrient-rich water
masses).
However, two broad regimes of phytoplankton nutrient limitations exist in the upper ocean
(Figure 1.7). The first one is dependent on nitrogen and phosphorus availability, which mainly
occurs at low latitudes characterized by a slow supply of nutrients from the sub-surface. These
areas are defined as low nutrient low chlorophyll (LNLC). The second one is intimately linked
to iron availability, which is limiting in vast regions of Southern Ocean, Eastern Equatorial
Pacific, North Pacific and North Atlantic, where in contrast to Fe, N and P nutrients are not
limiting. These areas are defined as high nutrient low chlorophyll (HNLC).
Vitamins and other micro nutrients can also limit marine phytoplankton (Moore et al., 2013;
Morel, 2003; Swift, 1981; Wu et al., 2000; Figure 1.7).
Chapter 1
39
Figure 1.7: Patterns of nutrient limitation.
Backgrounds indicate annual average surface concentrations
of nitrate (a) and phosphate (b) in µmol.kg
-1
.To assist comparison, nitrate is scaled by the mean N:P
ratio of organic matter (that is divided by 16). Symbols indicate the primary (central circles) and
secondary (outer circles) limiting nutrients as inferred from chlorophyll and/or primary productivity
increases following artificial amendment of: N (green), P (black), Fe (red), Si (orange), Co (yellow), Zn
(cyan) and vitamin B12 (purple). Divided circles indicate potentially co-limiting elements. White outer
circles indicate that no secondary limiting nutrient was identified, which will be because of the lack of a
test (from Moore et al., 2013).
These nutrient limitations directly impact the phytoplankton development and more generally,
the ecosystem structures, resulting in a change of the 106C:16N:1P Redfield ratio. Indeed,
spatial variations in the elemental stoichiometry of phytoplankton have been observed due to
supply and limitation of nutrients but also due to specific uptake by the different phytoplankton
communities (Hagstrom et al., 2016; Martiny et al., 2013; Rembauville et al., 2016a; Teng et
al., 2014;
Weber and Deutsch, 2010).
It is typically the case for the Southern Ocean which is isolated from any major iron sources,
inducing a limited primary production (Martin, 1990). However, in this HNLC Southern Ocean,
diatoms (the major phytoplankton group in this region) can develop important adaptive
strategies to these particular conditions (Boyd et al., 2007; de Baar, 2005), such as a heavily
silicified and thick shell, a slower growth rate and a high iron storage capacity (Boyd, 2013).
A nutrient limitation, as seen above, drives the succession of the phytoplankton communities.
In the North Atlantic, for example, a strong diatom bloom usually thrives during spring,
consuming the available nutrients. When silicic acid levels are depleted, there is then a
transition from diatoms to coccolithophores (Henson et al., 2006; Sanders et al., 2014).
Moreover, at the end of the productive period, a Fe limitation or co-limitation with silicic acid
Chapter 1
40
has also been reported (Nielsdottir et al., 2009) inducing seasonal HNLC conditions in this
area.
These different limitations have broad implications for the oceanic ‘biological pump’ that links
nutrient and carbon cycling.
-
Suspended “ballast” mineral
Ballast minerals, including lithogenic minerals (e.g. dust, clays) and biogenic minerals (BSi and
CaCO
3
), are important for the settling of POM to depth. Indeed, the density of the organic
matter (≈ 1.05 g.cm
-3
) is almost the same as seawater (≈ 1.03 g.cm
-3
) and thus particles require
a “ballast” effect to sink. Biogenic mineral ballast are mainly calcium carbonate (CaCO
3
) and
biogenic silica (bSiO
2
) that form
the shells of coccolithophores, pteropods, foraminifers for the
former, and of diatoms and radiolarians respectively (Lam and Bishop, 2007). Calcite density
is 2.71 g.cm
-3
, opal density is 2.1 g.cm
-3
and lithogenic material density can reach 2.65 g.cm
-3
(e.g. quartz; Klaas and Archer, 2002). Some authors have proposed a relationship between
the POC flux and the flux of ballast minerals in deep sediment traps (Armstrong et al., 2002;
Francois et al., 2002; Klaas and Archer, 2002; Figure 1.8) as well as in the upper water column
(Sanders et al., 2010; Thomalla et al., 2008).
Figure 1.8: Correlations between POC and mineral fluxes collected in 52 sediment traps below 1000 m
and around the world ocean (Klaas
and Archer, 2002).
The biogenic and lithogenic minerals have the potential to control the fraction of primary
production that reaches the seabed by protecting the organic matter from oxidation, by initiating