Chapter 14 Photosynthesis in Seagrasses
329
ratios of above-ground to below-ground parts varies
widely in seagrasses, so that future work should
focus on just how much the proportion of belowground parts influences overall primary production
and changes with depth (see eg. Olesen et al., 2002).
Some seagrasses such as Posidonia oceanica and P.
australis tend to have a very high proportion of underground parts while other seagrasses such as H.
ovalis tend to have a low proportion, which may
be analogous to the division between woody and
herbaceous terrestrial plants. This overriding factor
must strongly influences primary production rates,
but other factors such as nutrient supply and latitude
(Duarte and Chicano, 1999) have so far hindered a
clear assessment of what ultimately controls primary
production in seagrasses compared to, for example,
algae, such as Ulva lactuca, which have a photosynthetic lamina in which the majority of cells are photosynthetic, and with almost no other parts (Longstaff
et al., 2002). Current technology now provides the
means to answer many of these outstanding questions. However, it should be kept clear that in such
studies there are two scales involved, that of photosynthesis and primary production over hours to a
single day, usually under optimal conditions, and that
over a year, subject to herbivory, storm damage and
a variety of other environmental conditions.
VII. Mechanisms of CO 2 Fixation, CAM,
Photorespiration and Oxygen Cycles
It appears that seagrasses inherit a typical CalvinBenson Cycle. The first fixation products (<10 s)
are often typical of a Calvin-Benson Cycle (Benedict et al., 1980 for Thalassia testudinum; Larkum,
unpublished for P. australis). However, in two other
seagrasses (Zostera noltii, Beer et al., 1980, and
Halophila beccarii, Waghmode and Joshi, 1983) evidence has been obtained for early production of
C 4 acids. This latter evidence need not necessarily point to true C 4 metabolism and evidence of
a much more concerted kind (detailed metabolic
schemes and appropriate enzymes, such as PEP
carboxylase, pyruvate-phosphate dikinase, NADP
malate dehydrogenase, and various decarboxylases
such as NADP-specific malic enzyme) would be
needed to indicate that a true C 4 metabolism exists. There is rather more evidence that a concerted
β-carboxylation mechanism exists in some freshwater hydrophytes (Bowes et al., 2002), suggestive of
C 4 metabolism. However, of over 7600 C 4 species
only about 10 aquatic species have been identified
and in several of these the photosynthetic tissue in
question is aerial. The submerged aquatics Hydrilla
verticillata and Egeria densa have been identified
as C 4 NADP-specific malic enzyme species (Bowes
et al., 2002). However, even in those freshwater hydrophytes, which show the most evidence for C 4
metabolism, there is no evidence of Kranz anatomy,
which is so typical of terrestrial C 4 plants. Thus there
may be a possibility for this kind of metabolism
in seagrasses, which also show no sign of Kranz
anatomy (Kuo and den Hartog, Chapter 3).
In the past, C 4 metabolism has been assumed on
the basis of δ
13 C evidence: C3 plants have high values of −20 to −30 whereas C 4 plants have values of −10 to −15. On this basis early assessments indicated that seagrasses were C 4 plants. However, this was before it was realised that large diffusive boundary layers would also generate low negative values of δ
13 C. Based on this, many early
workers questioned the evidence (see references in
Abel and Drew, 1989). More recent analyses have
also excluded the possibility of C 4 metabolism in
seagrasses on the evidence from δ
13 C experiments
(Durako, 1993; Raven, 1997; Raven et al., 2002).
Crassulacean acid metabolism (CAM) is another
means by which terrestrial plants have adapted
the basic Calvin-Benson Cycle by interpolating βcarboxylation metabolism: in this case by temporally
shifting the C i uptake period to night-time. Several
cases of CAM-type metabolism have been found in
marsh plants or freshwater aquatics, the most studied of which is the genus Isoetes (Keeley, 1982).
In terrestrial CAM plants CO 2 is taken up at night
to reduce water loss. In freshwater hydrophytes the
strategy appears to be to take up C i when it is most
freely available, i.e. at night, rather than during the
day when it is restricted and only accessible through a
large DBL. Such a strategy could benefit seagrasses.
Just one report exists on the presence of a CAMlike behaviour of non-photosynthetic cells of leaves
of Thalassodendron ciliatum (Parnik et al., 2002).
However, until more evidence is provided it is safer
to assume that no C 4 - or CAM-type photosynthesis
occurs in seagrasses.
Evidence on the existence of photorespiration in
seagrasses (Fig. 2) is poor, but what evidence there
is seems to suggest that it is rather lower in seagrasses than in terrestrial plants (Abel and Drew,
1989; Frost-Christensen and Sand-Jensen, 1992).
329
ratios of above-ground to below-ground parts varies
widely in seagrasses, so that future work should
focus on just how much the proportion of belowground parts influences overall primary production
and changes with depth (see eg. Olesen et al., 2002).
Some seagrasses such as Posidonia oceanica and P.
australis tend to have a very high proportion of underground parts while other seagrasses such as H.
ovalis tend to have a low proportion, which may
be analogous to the division between woody and
herbaceous terrestrial plants. This overriding factor
must strongly influences primary production rates,
but other factors such as nutrient supply and latitude
(Duarte and Chicano, 1999) have so far hindered a
clear assessment of what ultimately controls primary
production in seagrasses compared to, for example,
algae, such as Ulva lactuca, which have a photosynthetic lamina in which the majority of cells are photosynthetic, and with almost no other parts (Longstaff
et al., 2002). Current technology now provides the
means to answer many of these outstanding questions. However, it should be kept clear that in such
studies there are two scales involved, that of photosynthesis and primary production over hours to a
single day, usually under optimal conditions, and that
over a year, subject to herbivory, storm damage and
a variety of other environmental conditions.
VII. Mechanisms of CO 2 Fixation, CAM,
Photorespiration and Oxygen Cycles
It appears that seagrasses inherit a typical CalvinBenson Cycle. The first fixation products (<10 s)
are often typical of a Calvin-Benson Cycle (Benedict et al., 1980 for Thalassia testudinum; Larkum,
unpublished for P. australis). However, in two other
seagrasses (Zostera noltii, Beer et al., 1980, and
Halophila beccarii, Waghmode and Joshi, 1983) evidence has been obtained for early production of
C 4 acids. This latter evidence need not necessarily point to true C 4 metabolism and evidence of
a much more concerted kind (detailed metabolic
schemes and appropriate enzymes, such as PEP
carboxylase, pyruvate-phosphate dikinase, NADP
malate dehydrogenase, and various decarboxylases
such as NADP-specific malic enzyme) would be
needed to indicate that a true C 4 metabolism exists. There is rather more evidence that a concerted
β-carboxylation mechanism exists in some freshwater hydrophytes (Bowes et al., 2002), suggestive of
C 4 metabolism. However, of over 7600 C 4 species
only about 10 aquatic species have been identified
and in several of these the photosynthetic tissue in
question is aerial. The submerged aquatics Hydrilla
verticillata and Egeria densa have been identified
as C 4 NADP-specific malic enzyme species (Bowes
et al., 2002). However, even in those freshwater hydrophytes, which show the most evidence for C 4
metabolism, there is no evidence of Kranz anatomy,
which is so typical of terrestrial C 4 plants. Thus there
may be a possibility for this kind of metabolism
in seagrasses, which also show no sign of Kranz
anatomy (Kuo and den Hartog, Chapter 3).
In the past, C 4 metabolism has been assumed on
the basis of δ
13 C evidence: C3 plants have high values of −20 to −30 whereas C 4 plants have values of −10 to −15. On this basis early assessments indicated that seagrasses were C 4 plants. However, this was before it was realised that large diffusive boundary layers would also generate low negative values of δ
13 C. Based on this, many early
workers questioned the evidence (see references in
Abel and Drew, 1989). More recent analyses have
also excluded the possibility of C 4 metabolism in
seagrasses on the evidence from δ
13 C experiments
(Durako, 1993; Raven, 1997; Raven et al., 2002).
Crassulacean acid metabolism (CAM) is another
means by which terrestrial plants have adapted
the basic Calvin-Benson Cycle by interpolating βcarboxylation metabolism: in this case by temporally
shifting the C i uptake period to night-time. Several
cases of CAM-type metabolism have been found in
marsh plants or freshwater aquatics, the most studied of which is the genus Isoetes (Keeley, 1982).
In terrestrial CAM plants CO 2 is taken up at night
to reduce water loss. In freshwater hydrophytes the
strategy appears to be to take up C i when it is most
freely available, i.e. at night, rather than during the
day when it is restricted and only accessible through a
large DBL. Such a strategy could benefit seagrasses.
Just one report exists on the presence of a CAMlike behaviour of non-photosynthetic cells of leaves
of Thalassodendron ciliatum (Parnik et al., 2002).
However, until more evidence is provided it is safer
to assume that no C 4 - or CAM-type photosynthesis
occurs in seagrasses.
Evidence on the existence of photorespiration in
seagrasses (Fig. 2) is poor, but what evidence there
is seems to suggest that it is rather lower in seagrasses than in terrestrial plants (Abel and Drew,
1989; Frost-Christensen and Sand-Jensen, 1992).