326
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
that the situation in seagrasses is likely to be complex and provide a rich source of studies for several
years to come. The present evidence is summarised
in the next paragraph.
The early work of Millhouse and Strother
(1986a,b, 1987) suggested that there is a saltstimulated HCO
−
3 pump in Z. muelleri as well as
CO 2 uptake. A HCO
−
3 pump has been supported
by a number of more recent investigations (Beer
and Rehnberg, 1997, Beer et al. 2002, Invers et al.
1999a,b, 2001). Beer and Rehnberg (1997) compared photosynthetic rates at pH 8.2 and 9.0. It was
deduced that HCO
−
3 was acquired by an extracellular CA-induced dehydration to CO 2 , prior to C i
uptake. The HCO
−
3 transport appeared to be ATPasemediated. Invers et al. (2001) obtained quantitative
information on the relative role of CO 2 and HCO
−
3
uptake mechanisms by analysing the rate of C i uptake at a range of [CO 2 ] and [HCO
−
3 ]. This gave
clear evidence of a HCO
−
3 mechanism but showed
that P. oceanica and Cymodocea nodosa had higher
capacity for HCO
−
3 uptake than Z. marina or Phyllospadix torreyi. A significant recent advance came
with the finding that external buffers can inhibit the
uptake of C i in Z. marina, Halophila stipulacea and
Ruppia maritima (Hellblom et al., 2001, Beer et al.,
2002). Beer et al. (2002) present further evidence, in
addition to buffering capacity, to suggest three mechanisms of carbon acquisition, (i) a CA-dependent
CO 2 mechanism (P1, Fig. 1), (ii) a proton/HCO
−
3
symport mechanism (P2, Fig. 1) and (iii) a CAlinked HCO
−
3 mechanism carrying CO 2 into the cytoplasm (P3, Fig 1). Clearly there is the possibility
for all these mechanism, and more, based on what
we know from Cyanobacteria (Badger et al., 2002).
A feature mentioned above, and shown in Fig. 1
is the ATP-linked pumping of protons into the cell
wall, and potentially the DBL. This active mechanism is involved in two C i pumping mechanisms,
at least (mechanisms (i) and (ii) above). The role
of Na
+ suggested by Millhouse and Strother (1986
a,b, 1987) could be linked to the pumping of C i (either by a Na
+ ATPase or by a sodium-stimulated
system (see Ritchie et al., 1996) or to an antiport exchange diffusion process such as a Na
+
/H
+
system.
Evidence for acidification of the cell wall and
DBL comes from studies of the buffer Tris in Z.
marina, H. stipulacea and R. maritima (Hellblom
et al., 2001; Beer et al., 2002), where it was concluded that proton extrusion, and the maintenance
of acidic zones in the diffusion boundary layer, is of
importance for supplying C i to the photosynthesising
cells. However, this result is possibly compromised
because of the known inhibitory effect of Tris on
photosynthesis, admittedly in isolated chloroplasts
(Yamashita and Butler, 1968). The presence of an
external carbonic anhydrase has been indicated by
an inhibitory effect of acetozolamide (AZ) on photosynthesis (James and Larkum, 1996; Invers et al.,
1999a,b, Schwarz et al., 2000, Hellblom et al., 2001).
However, this result is also now compromised since
it is known that several C i pumps incorporate a CA,
which is therefore potentially inhibited by an external (non-membrane-penetrating) CA inhibitor such
as AZ (see below).
Despite these advances there is still much left to do
in seagrasses, in documenting DBLs, and C i uptake
mechanisms. The relict cuticle and its effect on resistance to C i movement is an area where no research
has been carried out. Also δ
13 C ratios in seagrasses
are not fully documented (Raven et al., 2002). However, those values that do exist (Amphibolis spp,
Phyllospadix scouleri, Thalassia testudinum, Z. marina, Z. nova-hollandiae) indicate values in the range
−8 to −14. These values are less negative than those
often quoted and much less negative than in extreme
examples (eg. certain red algae with values more
negative than −30 -Raven et al., 2002). These levels
indicate that seagrasses overcome any diffusive limitation of C i uptake due to a DBL and a relict cuticle
by deploying an active C i uptake mechanism, probably by an HCO
−
3 uptake mechanism (Raven et al.,
2002).
V. Rates of C i Uptake
A large number of studies have focussed on P vs E (P
vs I) curves for photosynthesis in seagrasses over the
last 20 years. The results are summarised in Table
1. Unfortunately standardisation of results is clearly
lacking, which makes direct comparisons difficult in
many cases. The major factor as discussed above is
that rates are strongly dependent on stirring: only at
the highest stirring rates will the DBL be thin and allow for maximum photosynthesis. Therefore a number of reported values of P max will have been done
under stirring-limited conditions and this accounts
for some variability in results. Nevertheless the high
photosynthetic rates support previous views on the
high productivity of seagrasses notwithstanding the
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
that the situation in seagrasses is likely to be complex and provide a rich source of studies for several
years to come. The present evidence is summarised
in the next paragraph.
The early work of Millhouse and Strother
(1986a,b, 1987) suggested that there is a saltstimulated HCO
−
3 pump in Z. muelleri as well as
CO 2 uptake. A HCO
−
3 pump has been supported
by a number of more recent investigations (Beer
and Rehnberg, 1997, Beer et al. 2002, Invers et al.
1999a,b, 2001). Beer and Rehnberg (1997) compared photosynthetic rates at pH 8.2 and 9.0. It was
deduced that HCO
−
3 was acquired by an extracellular CA-induced dehydration to CO 2 , prior to C i
uptake. The HCO
−
3 transport appeared to be ATPasemediated. Invers et al. (2001) obtained quantitative
information on the relative role of CO 2 and HCO
−
3
uptake mechanisms by analysing the rate of C i uptake at a range of [CO 2 ] and [HCO
−
3 ]. This gave
clear evidence of a HCO
−
3 mechanism but showed
that P. oceanica and Cymodocea nodosa had higher
capacity for HCO
−
3 uptake than Z. marina or Phyllospadix torreyi. A significant recent advance came
with the finding that external buffers can inhibit the
uptake of C i in Z. marina, Halophila stipulacea and
Ruppia maritima (Hellblom et al., 2001, Beer et al.,
2002). Beer et al. (2002) present further evidence, in
addition to buffering capacity, to suggest three mechanisms of carbon acquisition, (i) a CA-dependent
CO 2 mechanism (P1, Fig. 1), (ii) a proton/HCO
−
3
symport mechanism (P2, Fig. 1) and (iii) a CAlinked HCO
−
3 mechanism carrying CO 2 into the cytoplasm (P3, Fig 1). Clearly there is the possibility
for all these mechanism, and more, based on what
we know from Cyanobacteria (Badger et al., 2002).
A feature mentioned above, and shown in Fig. 1
is the ATP-linked pumping of protons into the cell
wall, and potentially the DBL. This active mechanism is involved in two C i pumping mechanisms,
at least (mechanisms (i) and (ii) above). The role
of Na
+ suggested by Millhouse and Strother (1986
a,b, 1987) could be linked to the pumping of C i (either by a Na
+ ATPase or by a sodium-stimulated
system (see Ritchie et al., 1996) or to an antiport exchange diffusion process such as a Na
+
/H
+
system.
Evidence for acidification of the cell wall and
DBL comes from studies of the buffer Tris in Z.
marina, H. stipulacea and R. maritima (Hellblom
et al., 2001; Beer et al., 2002), where it was concluded that proton extrusion, and the maintenance
of acidic zones in the diffusion boundary layer, is of
importance for supplying C i to the photosynthesising
cells. However, this result is possibly compromised
because of the known inhibitory effect of Tris on
photosynthesis, admittedly in isolated chloroplasts
(Yamashita and Butler, 1968). The presence of an
external carbonic anhydrase has been indicated by
an inhibitory effect of acetozolamide (AZ) on photosynthesis (James and Larkum, 1996; Invers et al.,
1999a,b, Schwarz et al., 2000, Hellblom et al., 2001).
However, this result is also now compromised since
it is known that several C i pumps incorporate a CA,
which is therefore potentially inhibited by an external (non-membrane-penetrating) CA inhibitor such
as AZ (see below).
Despite these advances there is still much left to do
in seagrasses, in documenting DBLs, and C i uptake
mechanisms. The relict cuticle and its effect on resistance to C i movement is an area where no research
has been carried out. Also δ
13 C ratios in seagrasses
are not fully documented (Raven et al., 2002). However, those values that do exist (Amphibolis spp,
Phyllospadix scouleri, Thalassia testudinum, Z. marina, Z. nova-hollandiae) indicate values in the range
−8 to −14. These values are less negative than those
often quoted and much less negative than in extreme
examples (eg. certain red algae with values more
negative than −30 -Raven et al., 2002). These levels
indicate that seagrasses overcome any diffusive limitation of C i uptake due to a DBL and a relict cuticle
by deploying an active C i uptake mechanism, probably by an HCO
−
3 uptake mechanism (Raven et al.,
2002).
V. Rates of C i Uptake
A large number of studies have focussed on P vs E (P
vs I) curves for photosynthesis in seagrasses over the
last 20 years. The results are summarised in Table
1. Unfortunately standardisation of results is clearly
lacking, which makes direct comparisons difficult in
many cases. The major factor as discussed above is
that rates are strongly dependent on stirring: only at
the highest stirring rates will the DBL be thin and allow for maximum photosynthesis. Therefore a number of reported values of P max will have been done
under stirring-limited conditions and this accounts
for some variability in results. Nevertheless the high
photosynthetic rates support previous views on the
high productivity of seagrasses notwithstanding the