Chapter 9 Nutrients and Seagrasses
229
spp. and Thalassodendron ciliatum, seagrass root
tissues are exposed to sediment pore water, which
has usually much higher nutrient concentrations than
the water column, and this makes a marked difference in nutrient acquisition relative to algae.
Water column and sediment pore water largely
differ in geochemical characteristics, and, hence, in
nutrient availability and turnover. In the water column, oxic conditions prevail, and nutrient concentrations are usually low, typically 0–5 µM for nitrate, 0–0.4 µM for phosphate and very variable (but
generally lower for nitrate than ammonium). Phosphate is easily adsorbed to particles, which explains
its lower concentration; however, it is considered
that phosphate is biologically much more reactive,
with residence times shorter than those of nitrogen
species.
In pore water, most of the inorganic nitrogen is in
the form of ammonium, with concentrations ranging from 1 to 1,000 µM, and probably more, while
phosphate reaches up to 20 µM (Udy and Dennison, 1997a; Touchette and Burkholder, 2000) and
nitrate is much more scarce. Nutrient availability
in pore water largely depends on important biogeochemical processes, such as denitrification (which is
important in terrigenous, moderately organic sediments; but see below), nitrogen fixation (occurring
mostly in tropical areas) or phosphorus sequestration by carbonates and/or calcium, typical of biogenic sediments. These processes can cause nutrient
imbalances, and, hence, seagrass growth limitation
by specific elements determined by the nature of the
sediment (e.g. Short, 1987).
Finally, it has to be taken into account that low
molecular weight organic compounds can act as N
and P sources. Concerning nitrogen, urea is an effective nitrogen source in seaweeds, but not in seagrasses. However, some amino acids (glutamic acid)
can contribute to seagrass N nutrition (Bird et al.,
1998). Concerning phosphorus, organic phosphorus, either as dissolved organic phosphorus (DOP)
or particulate organic phosphorus (POP) can be a P
source readily usable through hydrolysis by a number of forms of the enzyme alkaline phosphatase.
Alkaline phosphatase activity has been found in seagrass leaves and roots (P´ erez and Romero, 1993).
However, the real role of phosphatase remains uncertain since the concentration of the substrate for
these enzymes (monoester phosphate) seems to be
very low, at least in natural freshwaters (Hern´ andez,
1992).
B. Nutrient Uptake and Assimilation: Relative
Role of Leaves and Roots
1. Leaf vs. Root Absorption Capacity
The putative ability of seagrasses to exploit the large
nutrient pool of pore waters has led to the notion
that this is the main source of inorganic nutrients for
their growth. This has often been mentioned as an
explanation of their success in colonizing vast areas,
even in oligotrophic waters, and root tissues have
been generally considered more important contributors of overall nutrient acquisition than leaves. There
is, however, evidence that leaf tissues have higher nutrient uptake affinities than root tissues (see below)
at low-nutrient availability (Pedersen et al., 1997;
Lee and Dunton, 1999b), and several studies have
indicated that nutrient uptake by leaf tissues contributed considerably to the total nutrient acquisition
of seagrasses (Iizumi and Hattori, 1982; Short and
McRoy, 1984; Pedersen and Borum, 1992; Stapel
et al., 1996; Terrados and Williams, 1997; Lee and
Dunton, 1999b; Lepoint et al., 2002b).
2. Uptake and Assimilation
Nutrient incorporation is a rather complex process,
which includes two basic coupled steps: uptake and
assimilation. Uptake is usually understood as the internalization of nutrients from the environment to the
plant cells, while assimilation is the incorporation of
such nutrients into the organic matter. Although the
terms uptake and assimilation are sometimes used
indistinctly, they are in fact very different steps in
the process of nutrient acquisition.
In higher plants, inorganic nutrient uptake is
brought about by a variety of plasma membrane
transporters (H
+ ATPases, ABC transport proteins,
etc.). Two different systems (low and high affinity) are involved, both in nitrate and ammonium
uptake (Ourry et al., 1997; Grossman and Takahashi, 2001). Phosphorus is generally taken up using
membrane-spanning proteins (H
+ ATPase) as transporters (Muchhal-Umesh and Raghothama, 1999;
Raghothama, 1999). However, despite the large
body of knowledge on membrane pumps in terrestrial plants, the biochemical and molecular nature
of uptake mechanisms (genes, proteins involved,
etc.) have not yet been investigated in seagrasses,
apart from a single study in Z. marina (Maramatsu
et al., 2002). In contrast, data on nutrient uptake
229
spp. and Thalassodendron ciliatum, seagrass root
tissues are exposed to sediment pore water, which
has usually much higher nutrient concentrations than
the water column, and this makes a marked difference in nutrient acquisition relative to algae.
Water column and sediment pore water largely
differ in geochemical characteristics, and, hence, in
nutrient availability and turnover. In the water column, oxic conditions prevail, and nutrient concentrations are usually low, typically 0–5 µM for nitrate, 0–0.4 µM for phosphate and very variable (but
generally lower for nitrate than ammonium). Phosphate is easily adsorbed to particles, which explains
its lower concentration; however, it is considered
that phosphate is biologically much more reactive,
with residence times shorter than those of nitrogen
species.
In pore water, most of the inorganic nitrogen is in
the form of ammonium, with concentrations ranging from 1 to 1,000 µM, and probably more, while
phosphate reaches up to 20 µM (Udy and Dennison, 1997a; Touchette and Burkholder, 2000) and
nitrate is much more scarce. Nutrient availability
in pore water largely depends on important biogeochemical processes, such as denitrification (which is
important in terrigenous, moderately organic sediments; but see below), nitrogen fixation (occurring
mostly in tropical areas) or phosphorus sequestration by carbonates and/or calcium, typical of biogenic sediments. These processes can cause nutrient
imbalances, and, hence, seagrass growth limitation
by specific elements determined by the nature of the
sediment (e.g. Short, 1987).
Finally, it has to be taken into account that low
molecular weight organic compounds can act as N
and P sources. Concerning nitrogen, urea is an effective nitrogen source in seaweeds, but not in seagrasses. However, some amino acids (glutamic acid)
can contribute to seagrass N nutrition (Bird et al.,
1998). Concerning phosphorus, organic phosphorus, either as dissolved organic phosphorus (DOP)
or particulate organic phosphorus (POP) can be a P
source readily usable through hydrolysis by a number of forms of the enzyme alkaline phosphatase.
Alkaline phosphatase activity has been found in seagrass leaves and roots (P´ erez and Romero, 1993).
However, the real role of phosphatase remains uncertain since the concentration of the substrate for
these enzymes (monoester phosphate) seems to be
very low, at least in natural freshwaters (Hern´ andez,
1992).
B. Nutrient Uptake and Assimilation: Relative
Role of Leaves and Roots
1. Leaf vs. Root Absorption Capacity
The putative ability of seagrasses to exploit the large
nutrient pool of pore waters has led to the notion
that this is the main source of inorganic nutrients for
their growth. This has often been mentioned as an
explanation of their success in colonizing vast areas,
even in oligotrophic waters, and root tissues have
been generally considered more important contributors of overall nutrient acquisition than leaves. There
is, however, evidence that leaf tissues have higher nutrient uptake affinities than root tissues (see below)
at low-nutrient availability (Pedersen et al., 1997;
Lee and Dunton, 1999b), and several studies have
indicated that nutrient uptake by leaf tissues contributed considerably to the total nutrient acquisition
of seagrasses (Iizumi and Hattori, 1982; Short and
McRoy, 1984; Pedersen and Borum, 1992; Stapel
et al., 1996; Terrados and Williams, 1997; Lee and
Dunton, 1999b; Lepoint et al., 2002b).
2. Uptake and Assimilation
Nutrient incorporation is a rather complex process,
which includes two basic coupled steps: uptake and
assimilation. Uptake is usually understood as the internalization of nutrients from the environment to the
plant cells, while assimilation is the incorporation of
such nutrients into the organic matter. Although the
terms uptake and assimilation are sometimes used
indistinctly, they are in fact very different steps in
the process of nutrient acquisition.
In higher plants, inorganic nutrient uptake is
brought about by a variety of plasma membrane
transporters (H
+ ATPases, ABC transport proteins,
etc.). Two different systems (low and high affinity) are involved, both in nitrate and ammonium
uptake (Ourry et al., 1997; Grossman and Takahashi, 2001). Phosphorus is generally taken up using
membrane-spanning proteins (H
+ ATPase) as transporters (Muchhal-Umesh and Raghothama, 1999;
Raghothama, 1999). However, despite the large
body of knowledge on membrane pumps in terrestrial plants, the biochemical and molecular nature
of uptake mechanisms (genes, proteins involved,
etc.) have not yet been investigated in seagrasses,
apart from a single study in Z. marina (Maramatsu
et al., 2002). In contrast, data on nutrient uptake