Chapter 1
35
Macronutrients
Redfield et al. (1963) discovered that relatively invariant elemental ratios of C, N, and P found
in marine organisms are intimately entwined with the co-variation (106C:16N:1P) of these
elements in the ocean.
Dissolved phosphorous is present in seawater under the phosphate form (PO
4
3-
; < 0.1 – 3.0
µmol.L
-1
). Important inputs of PO
4
3-
are delivered through riverine runoff, atmospheric
deposition and submarine groundwater discharge (e.g. Paytan and McLaughlin, 2007). In polar
areas, ice sheets could also bring large quantities of P in solution (Hawkings et al., 2016).
These PO
4
3-
sources fuel phytoplankton blooms in surface waters but in some regions, like in
the subtropical Sargasso Sea, P has been reported to limit the phytoplankton developments
(Wu et al., 2000). Phosphorus is easily remineralized in the water column (Clark and Ingall,
1998), and its preferential remineralization compared to C is explained by the more labile
nature of organic compounds containing P (Berner, 1980).
Inorganic nitrogen is mainly present in the marine system as nitrate (NO
3
-
) but also under other
forms such as ammonium (NH
4
+
), nitrite (NO
2
-
), nitrous oxide (N
2
O), nitric oxide (NO) and
dinitrogen (N
2
) and is brought through the rivers, the atmospheric depositions or the N
2
fixation
(Capone et al., 2005; Gruber, 2008). Ammonium (< 0.3 – 1.2 µmol.L
-1
) is the preferred source
of nitrogen for phytoplankton because its assimilation requires less energy (Zehr and Ward,
2002). Nitrate uptake requires more energy but since nitrate concentrations are more abundant
in the ocean (< 1 - 35 µmol.L
-1
), most phytoplankton have the enzymes to initiate the reduction
of NO
3
-
in order to grow (nitrate reductase), with the exception of prokaryote species such as
Synechococcus (Moore et al., 2002). Phytoplankton can also use NO
2
-
as a nitrogen source
but its concentration in the ocean is usually low (< 0.2 – 1.5 µmol.L
-1
). Another important source
of N, when the other forms are exhausted or when the N/P ratio is not “Redfieldian”, (with P
being in excess compared to N in Redfield stoichometry) is via the biological N
2
fixation which
transforms N
2
into PON. Organisms able to fix dissolved N
2
, called diazotrophs, are found
among free and/or symbiotic cyanobacteria some filamentous such as bloom forming
Chapter 1
36
Trichodesmium spp. (Capone et al., 1997; Carpenter et al., 1999), Richelia (Foster et al., 2007;
Foster and Zehr, 2006) and others unicellullar diazotrophic cyanobacteria (Zehr et al., 2001)
as well as heterotrophic bacteria (Zehr et al., 1998). Once phytoplankton has satisfied its
nitrogen demand for growth, the different dissolved nitrogen forms are transformed in PON.
Most of the PON is returned back to dissolved inorganic nitrogen by remineralization processes
generated by bacteria: ammonification transforming PON to NH
4
+
, nitrification transforming
NH
4
+
to NO
2
-
(ammonium oxidation) and then NO
2
-
to NO
3
-
(nitrite oxidation or true nitrification).
In the ocean, the fluxes of PON to the seabed are small. The major sink for nitrogen is the
denitrification transforming the NO
3
-
into N
2
in low oxygen environment (oxygen minimum
zones, shelf or margin sediments), leading to a nitrogen loss through the atmosphere
(Galloway et al., 2004; Gruber, 2004). In addition, nitrogen is also lost as N
2
O produced during
the nitrification as well as during denitrification.
Silicic acid (H
4
SiO
4
or dSi) is also considered as a macronutrient and is particularly essential
for some plankton taxa known as silicifiers such as diatoms (autotrophic phytoplankton),
silicoflagellates and radiolarians (heterotrophic zooplankton), as they use it to build their shells
which is then referred as biogenic silica (BSi or bSiO
2
). Silicic acid is brought to the ocean
essentially via the rivers (about 66% of the total net inputs, Tréguer and De La Rocha, 2013)
and its concentrations vary from < 0.2 to 170 µmol.L
-1
. In the world ocean, 56% of BSi is
recycled in the upper 100m and the remaining fraction is transported to the deep ocean where
BSi can also undergo dissolution. Only 3% of BSi is estimated to reach the seafloor (Tréguer
and De La Rocha, 2013).
Micronutrients
Other elements, such as the following metals: Fe, Zn, Mn, Ni, Cu, Co, Cd, Mo that are present
at subnanomolar concentrations in seawater exert a key role in many metabolic processes
(Table 1.1) and are referred as key micronutrients. As a result, the oceanic trace element
biogeochemical cycles have been intensely studied over the past decade (e.g. see review in
SCOR working group, 2007). Their sources are very diverse being atmospheric (e.g. Jickells
Chapter 1
37
et al., 2005; Shelley et al., 2015), riverine (e.g. de Baar and Jong, 2001), sedimentary (e.g.
Bruland and Lohan, 2008), hydrothermal (e.g. Tagliabue et al., 2010), cold seep vents (e.g.
Lemaitre et al., 2014), ice melting (e.g. Lannuzel et al., 2011), anthropogenic (e.g. Gao et al.,
2014), groundwater discharge (e.g. Trezzi et al., 2016), extraterrestrial (e.g. Johnson, 2001)
or volcanic activity (e.g. Achterberg et al., 2013), shelves (Elrod et al., 2004). However, only a
small fraction of trace metals originating from these sources is soluble in seawater and
accessible to the phytoplankton. In the surface, a significant fraction of trace metals is bound
to strong organic complexes, that increase their solubility and can favor their bioavailability
(e.g. Gledhill and Buck, 2012; Rue and Bruland, 1995). Some trace metals are easily
scavenged onto particles and are subsequently removed from the surface waters when
particles sink. In the mesopelagic layer, trace metals are also affected by bacterial activity, at
different remineralization rates. For example, Twining et al. (2014) have shown that Ni or Zn
were remineralized faster than Fe.
Metal
Protein(s)
Function(s)
Fe
Cytochromes
Electron transport in photosynthesis and respiration
Ferredoxin
Electron transport in photosynthesis and N fixation
Other Fe-S proteins
Electron transport in photosynthesis and respiration
Nitrate and nitrite
reductase
Conversion of nitrate to ammonia
Chelatase
Porphyrin and phycobiliprotein synthesis
Nitrogenase
N fixation
Catalase
Conversion of hydrogen peroxide to water
Peroxidase
Reduction of reactive oxygen species
Superoxide dismutase
Disproportionation of superoxide to hydrogen peroxide and O
2
Zn
Carbonic anhydrase
Hydration and dehydration of carbon dioxide
Alkaline phosphatase
Hydrolysis of phosphate esters
RNA polymerase
Nucleic acid replication and transcription
tRNA synthetase
Synthesis of tRNA
Reverse transcriptase
Synthesis of single-stranded DNA from RNA
Carboxypeptidase
Hydrolysis of peptide bonds
Superoxide dismutase
Disproportionation of superoxide to hydrogen peroxide and O
2
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