Chapter 9 Nutrients and Seagrasses
231
the unstirred boundary layer. The influence of diffusive boundary layers (DBLs) on several biological
processes has received much attention (e.g. Denny
and Wethey, 2001; Koch et al., Chapter 8; Larkum
et al., Chapter 14), and it is known that, under certain conditions, it can curtail solute exchange. The
relevance of such boundary layers in seagrass biology has been examined mostly related to inorganic carbon acquisition (e.g. Koch, 1994; James
and Larkum, 1996; Larkum et al., Chapter 14); however, there is also some evidence of its importance
for nutrients in aquatic macrophytes: for example,
periodic stripping of the boundary layer by passing
waves has been shown to increase nutrient uptake by
a factor of 10 (Stevens and Hurd, 1997; Koch et al.,
Chapter 8).
4. Epiphytes
The second aspect to take into consideration is the
potential interference of epiphytes in nutrient uptake by leaves. The presence of small organisms on
the exchange surface surely adds complexity to the
diffusive process. As epiphytes increase leaf roughness, they could reduce the thickness of the boundary layer under wave or current action, as well as
increasing the boundary layer under unstirred conditions. However, since most of these epiphytes are
algae, they also take up nutrients from water, thus
creating a competitive interference. In any case, and
as far as we are aware, these aspects have not been
investigated in detail (see also Section IV.A, Koch
et al., Chapter 8 and Borowitzka et al., Chapter 19).
5. Assimilation of N and P
Once nutrients have entered the cell, they can be immediately assimilated into organic matter or they can
be stored in inorganic forms for further use. Since
very low intracellular concentration of ammonium
and nitrate have been reported (Invers et al., 2002),
it has to be concluded that, in seagrasses, assimilation, at least for the species investigated, occurs
shortly after uptake. Whereas for ammonium this is
not surprising, given the fact of its known toxicity
(van Katwijk et al., 1997), the lack of nitrate accumulation is in contrast to what happens in algae, where
nitrate is one of the storage forms of N (Lapointe
and Duke, 1984).
Ammonium assimilation takes place through the
GOGAT pathway, i.e. formation of glutamine from
glutamate and ammonium, catalyzed by glutamine
synthetase (GS). This requires energy (1 ATP is
needed for each glutamine synthesized). Ammonium assimilation occurs in both leaves and roots
(and rhizomes), as indicated by in vivo and in vitro
GS activity measurements (Pregnall et al., 1987;
Kraemer and Alberte, 1993; Kraemer et al., 1997;
Kraemer and Mazzella, 1999; Kraemer and Hanisak,
2000; Invers et al., 2002; species involved: Zostera
marina, Thalassia testudinum, Posidonia oceanica,
Cymodocea nodosa, and Zostera noltii).
Nitrate assimilation requires a previous step consisting of nitrate reduction to ammonium, which
involves the participation of two enzymes (nitrate
reductase and nitrite reductase) and NADH or
NADPH; ammonium obtained by these reductions
is then incorporated to organic matter via GOGAT.
Measurements of the activity of the nitrate reductase
in seagrasses (Zostera marina and Halophila stipulacea, Doddema and Howari, 1983; Roth and Pregnall, 1988; Touchette and Burkholder, 2001) have
shown higher values in leaves than in roots, consistent with the relative availability of nitrate to both organs. Nitrate reductase activity is lower in seagrasses
than in algae; this can be a consequence of an adaptation of seagrasses to the major use of ammonium
as inorganic N source, a ‘preference’ afforded by the
ready access to large ammonium pools in the sediment, either directly (through roots) or indirectly
(ammonium release from pore water to the water
column (see Marb` a et al., Chapter 6)).
6. The Role of Leaves and Roots
The above-mentioned facts suggest that seagrasses
have a certain plasticity in the acquisition of nutrients. Apparently, they have inherited many of the absorption mechanisms of flowering land plants from
which they evolved at an early stage. However, in
contrast to land plants and as in other hydrophytes,
leaves may play a much greater role in nutrient uptake. The relative importance of leaves and roots in
the acquisition process seems to be modulated according to environmental constraints. Available evidence suggests that, in very general terms (and with
the obvious exception of species attached to rocky
substratum, such as Phyllospadix), leaves and roots
are equally important for nutrient acquisition. This
is the conclusion for Zostera marina based on a numerical model (Zimmerman et al., 1987), and for
231
the unstirred boundary layer. The influence of diffusive boundary layers (DBLs) on several biological
processes has received much attention (e.g. Denny
and Wethey, 2001; Koch et al., Chapter 8; Larkum
et al., Chapter 14), and it is known that, under certain conditions, it can curtail solute exchange. The
relevance of such boundary layers in seagrass biology has been examined mostly related to inorganic carbon acquisition (e.g. Koch, 1994; James
and Larkum, 1996; Larkum et al., Chapter 14); however, there is also some evidence of its importance
for nutrients in aquatic macrophytes: for example,
periodic stripping of the boundary layer by passing
waves has been shown to increase nutrient uptake by
a factor of 10 (Stevens and Hurd, 1997; Koch et al.,
Chapter 8).
4. Epiphytes
The second aspect to take into consideration is the
potential interference of epiphytes in nutrient uptake by leaves. The presence of small organisms on
the exchange surface surely adds complexity to the
diffusive process. As epiphytes increase leaf roughness, they could reduce the thickness of the boundary layer under wave or current action, as well as
increasing the boundary layer under unstirred conditions. However, since most of these epiphytes are
algae, they also take up nutrients from water, thus
creating a competitive interference. In any case, and
as far as we are aware, these aspects have not been
investigated in detail (see also Section IV.A, Koch
et al., Chapter 8 and Borowitzka et al., Chapter 19).
5. Assimilation of N and P
Once nutrients have entered the cell, they can be immediately assimilated into organic matter or they can
be stored in inorganic forms for further use. Since
very low intracellular concentration of ammonium
and nitrate have been reported (Invers et al., 2002),
it has to be concluded that, in seagrasses, assimilation, at least for the species investigated, occurs
shortly after uptake. Whereas for ammonium this is
not surprising, given the fact of its known toxicity
(van Katwijk et al., 1997), the lack of nitrate accumulation is in contrast to what happens in algae, where
nitrate is one of the storage forms of N (Lapointe
and Duke, 1984).
Ammonium assimilation takes place through the
GOGAT pathway, i.e. formation of glutamine from
glutamate and ammonium, catalyzed by glutamine
synthetase (GS). This requires energy (1 ATP is
needed for each glutamine synthesized). Ammonium assimilation occurs in both leaves and roots
(and rhizomes), as indicated by in vivo and in vitro
GS activity measurements (Pregnall et al., 1987;
Kraemer and Alberte, 1993; Kraemer et al., 1997;
Kraemer and Mazzella, 1999; Kraemer and Hanisak,
2000; Invers et al., 2002; species involved: Zostera
marina, Thalassia testudinum, Posidonia oceanica,
Cymodocea nodosa, and Zostera noltii).
Nitrate assimilation requires a previous step consisting of nitrate reduction to ammonium, which
involves the participation of two enzymes (nitrate
reductase and nitrite reductase) and NADH or
NADPH; ammonium obtained by these reductions
is then incorporated to organic matter via GOGAT.
Measurements of the activity of the nitrate reductase
in seagrasses (Zostera marina and Halophila stipulacea, Doddema and Howari, 1983; Roth and Pregnall, 1988; Touchette and Burkholder, 2001) have
shown higher values in leaves than in roots, consistent with the relative availability of nitrate to both organs. Nitrate reductase activity is lower in seagrasses
than in algae; this can be a consequence of an adaptation of seagrasses to the major use of ammonium
as inorganic N source, a ‘preference’ afforded by the
ready access to large ammonium pools in the sediment, either directly (through roots) or indirectly
(ammonium release from pore water to the water
column (see Marb` a et al., Chapter 6)).
6. The Role of Leaves and Roots
The above-mentioned facts suggest that seagrasses
have a certain plasticity in the acquisition of nutrients. Apparently, they have inherited many of the absorption mechanisms of flowering land plants from
which they evolved at an early stage. However, in
contrast to land plants and as in other hydrophytes,
leaves may play a much greater role in nutrient uptake. The relative importance of leaves and roots in
the acquisition process seems to be modulated according to environmental constraints. Available evidence suggests that, in very general terms (and with
the obvious exception of species attached to rocky
substratum, such as Phyllospadix), leaves and roots
are equally important for nutrient acquisition. This
is the conclusion for Zostera marina based on a numerical model (Zimmerman et al., 1987), and for