Box 14.2 (continued)
these nutrients for the plants in the form of mineral
salts (fertilizers). It was only in the twentieth century
that the role of phosphate in the transfer of energy has
been highlighted with the discovery of adenosine triphosphate (ATP), which has revolutionized the understanding of the cellular energy metabolism of all living
organisms. The identification of the molecular structure of DNA by Watson and Crick in 1953 formally
established the importance of phosphate in the growth
and development of all organisms. During the past
decades, much research in the field of biochemistry
has continued to demonstrate the importance of phosphate in a wide variety of metabolic processes
(Benitez-Nelson 2000). Since the first work of Redfield
(1934) to the recent synthesis of Sarmiento and Gruber
(2006), a major role has been assigned to phosphate in
the control of the oceanic carbon cycle.
Measurable fractions: Phosphate exists in the ocean
under various forms, mineral and organic, dissolved and
particulate, that would be difficult, if not impossible,
to quantify individually. The classically considered
fractions are dissolved inorganic (mineral) phosphate
(PO 4 ), Dissolved Organic Phosphate (DOP), and
Particulate Organic Phosphate (POP). Dissolved
mineral phosphate essentially corresponds to the
orthophosphoric acid which is essentially in the form
HPO 4
2À in seawater. With an average concentration
of 2.3 μM, dissolved mineral phosphate is by far the
largest reservoir of phosphate in the ocean. The concentration is minimal close to the surface as a result of
biological activity and can reach really low values, of
the order of the nanomole per liter in the highly stratified
oceanic areas. The concentration increases in depth at
the level of the phosphacline and then reach a maximum
value near 1,000 m depth. The concentration is then
usually constant and high toward the bottom. Particulate
organic phosphate (POP) represents essentially living
organisms mainly in the surface ocean (phospholipids
and nucleic acids). A detritical part (from biological
degradation processes, lysis, and cell death) difficult to
measure must nevertheless be considered in this pool.
The concentration varies generally between 0.01 and
0.30 μM even if exceptionally higher values may have
been observed during phytoplankton blooms. POP
decreases with depth reaching hardly measurable values
below 1,000 m.
Dissolved organic phosphate (DOP) corresponds to
the set of molecules originating from mineralization
processes that would be difficult, or even impossible,
to quantify individually. The DOP concentration varies
(continued)
Box 14.2 (continued)
from 0.2 to 1.7 μM in surface waters. In depth, the
concentration decreased to very low values near the
bottom.
The oceanic cycle of phosphate (Box Fig. 14.5) is
presented together with the processes at the origin of
the main fluxes. Inputs into the ocean are mainly coming from rivers through erosion of soils. Atmospheric
depositions are generally low but may be locally significant. Loss, at the scale of the entire ocean,
corresponds to sediment burial. Inputs of phosphate
in the ocean’s surface (using the simple view of an
ocean with 2 reservoirs, surface and depth, separated
by the thermocline) come mainly from the deep ocean
(upwelling, convection, diffusion) and the losses are
dominated by sedimentation of biogenic particles.
Although other chemical processes such as adsorption
of phosphate on atmospheric dust or precipitation of
calcium phosphate under specific conditions should be
reported, the fluxes in the biogeochemical cycle of
phosphate are essentially constrained by biological
processes (assimilation, excretion, regeneration, mineralization) that mainly take place in the ocean’s
surface. Recent discoveries have critically questioned
the understanding of the functional roles of microorganisms in the oceanic phosphate cycle (Thingstad
et al. 1997). Heterotrophic prokaryotes (Eubacteria
and Archaea), originally considered as simple organic
matter remineralizers (from POP to PO 4 ), finally use
mineral phosphate. Their growths can therefore be
limited by its availability, like phytoplankton. The
resulting competition appears as a key process structuring the trophic chain that directly influences the
flux of material exported from the euphotic to the
deep ocean layer (the biological pump, Moutin 2000
and references included, Moutin et al. 2012). Furthermore, it was recently shown that some autotrophic
organisms, originally considered to be only consumers
of mineral phosphate, actively participate in the mineralization of dissolved organic phosphate through
ectoenzyme activities
2 (Box Fig. 14.6). Particularly
(continued)
2 Enzyme excreted by the cell and remaining attached to the
outer membrane (or in the periplasmic space of gram-negative
bacteria). Prokaryotic heterotrophic microorganisms, prokaryotic autotrophic (cyanobacteria) and single-celled eukaryotes
(flagellates, diatoms) have for some species the genetic ability
to produce ectoenzymes such as the alkaline phosphatase or the
phosphonatase. The production of these enzymes is usually
induced by mineral phosphate deficiency, and enables hydrolyze
the phospho-ester (C-O-P) or the phosphonate (C–P) bonds of
organic molecules of the DOP. The mineral phosphate released
can then be incorporated by microorganisms.
534
J.-C. Bertrand et al.
these nutrients for the plants in the form of mineral
salts (fertilizers). It was only in the twentieth century
that the role of phosphate in the transfer of energy has
been highlighted with the discovery of adenosine triphosphate (ATP), which has revolutionized the understanding of the cellular energy metabolism of all living
organisms. The identification of the molecular structure of DNA by Watson and Crick in 1953 formally
established the importance of phosphate in the growth
and development of all organisms. During the past
decades, much research in the field of biochemistry
has continued to demonstrate the importance of phosphate in a wide variety of metabolic processes
(Benitez-Nelson 2000). Since the first work of Redfield
(1934) to the recent synthesis of Sarmiento and Gruber
(2006), a major role has been assigned to phosphate in
the control of the oceanic carbon cycle.
Measurable fractions: Phosphate exists in the ocean
under various forms, mineral and organic, dissolved and
particulate, that would be difficult, if not impossible,
to quantify individually. The classically considered
fractions are dissolved inorganic (mineral) phosphate
(PO 4 ), Dissolved Organic Phosphate (DOP), and
Particulate Organic Phosphate (POP). Dissolved
mineral phosphate essentially corresponds to the
orthophosphoric acid which is essentially in the form
HPO 4
2À in seawater. With an average concentration
of 2.3 μM, dissolved mineral phosphate is by far the
largest reservoir of phosphate in the ocean. The concentration is minimal close to the surface as a result of
biological activity and can reach really low values, of
the order of the nanomole per liter in the highly stratified
oceanic areas. The concentration increases in depth at
the level of the phosphacline and then reach a maximum
value near 1,000 m depth. The concentration is then
usually constant and high toward the bottom. Particulate
organic phosphate (POP) represents essentially living
organisms mainly in the surface ocean (phospholipids
and nucleic acids). A detritical part (from biological
degradation processes, lysis, and cell death) difficult to
measure must nevertheless be considered in this pool.
The concentration varies generally between 0.01 and
0.30 μM even if exceptionally higher values may have
been observed during phytoplankton blooms. POP
decreases with depth reaching hardly measurable values
below 1,000 m.
Dissolved organic phosphate (DOP) corresponds to
the set of molecules originating from mineralization
processes that would be difficult, or even impossible,
to quantify individually. The DOP concentration varies
(continued)
Box 14.2 (continued)
from 0.2 to 1.7 μM in surface waters. In depth, the
concentration decreased to very low values near the
bottom.
The oceanic cycle of phosphate (Box Fig. 14.5) is
presented together with the processes at the origin of
the main fluxes. Inputs into the ocean are mainly coming from rivers through erosion of soils. Atmospheric
depositions are generally low but may be locally significant. Loss, at the scale of the entire ocean,
corresponds to sediment burial. Inputs of phosphate
in the ocean’s surface (using the simple view of an
ocean with 2 reservoirs, surface and depth, separated
by the thermocline) come mainly from the deep ocean
(upwelling, convection, diffusion) and the losses are
dominated by sedimentation of biogenic particles.
Although other chemical processes such as adsorption
of phosphate on atmospheric dust or precipitation of
calcium phosphate under specific conditions should be
reported, the fluxes in the biogeochemical cycle of
phosphate are essentially constrained by biological
processes (assimilation, excretion, regeneration, mineralization) that mainly take place in the ocean’s
surface. Recent discoveries have critically questioned
the understanding of the functional roles of microorganisms in the oceanic phosphate cycle (Thingstad
et al. 1997). Heterotrophic prokaryotes (Eubacteria
and Archaea), originally considered as simple organic
matter remineralizers (from POP to PO 4 ), finally use
mineral phosphate. Their growths can therefore be
limited by its availability, like phytoplankton. The
resulting competition appears as a key process structuring the trophic chain that directly influences the
flux of material exported from the euphotic to the
deep ocean layer (the biological pump, Moutin 2000
and references included, Moutin et al. 2012). Furthermore, it was recently shown that some autotrophic
organisms, originally considered to be only consumers
of mineral phosphate, actively participate in the mineralization of dissolved organic phosphate through
ectoenzyme activities
2 (Box Fig. 14.6). Particularly
(continued)
2 Enzyme excreted by the cell and remaining attached to the
outer membrane (or in the periplasmic space of gram-negative
bacteria). Prokaryotic heterotrophic microorganisms, prokaryotic autotrophic (cyanobacteria) and single-celled eukaryotes
(flagellates, diatoms) have for some species the genetic ability
to produce ectoenzymes such as the alkaline phosphatase or the
phosphonatase. The production of these enzymes is usually
induced by mineral phosphate deficiency, and enables hydrolyze
the phospho-ester (C-O-P) or the phosphonate (C–P) bonds of
organic molecules of the DOP. The mineral phosphate released
can then be incorporated by microorganisms.
534
J.-C. Bertrand et al.