338
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
the overall primary production of the plant and reduces the compensation depth. The second is that
light harvesting in seagrasses is strongly dependent
on excess chlorophyll proteins (Section VI), whereas
algae have a much greater range of more efficient
light-harvesting strategies. Anatomical and morphological factors in depth limitation are discussed are
discussed in Kuo and den Hartog, Chapter 3.
Duarte (1991) found that the differences in the
depth limits of seagrasses were largely attributable
to differences in light attenuation and could be described by a fairly simple equation:
LogZ c = 0.26 − 1.07 log K
Where Z c is the depth limit of a seagrass (m) and K is
the underwater light attenuation (m
−1 ). As discussed
in Section VI, while the non-photosynthetic parts
of seagrasses probably impose on them a shallower
depth limit as compared to algae, there are probably
a range of morphological types involved, from the
heavy investment in roots and rhizomes of species
like P. oceanica and P. australis, to the much reduced
investment in species like H. ovalis and H. capricorni. This is probably reflected in the much greater
depth limits of the latter species (Duarte, 1991). Olesen et al. (2002) compared changes in anatomy, physiology and growth characteristics (population structure) with depth in the co-occurring P. oceanica and
C. nodosa. C. nodosa showed the greatest flexibility
and reduced its below-ground parts and increased its
leaf area. However, photosynthetic efficiency at low
PAR was about the same for each species. P. oceanica
was able to survive longer at low irradiance due to
low growth and low respiratory rates. The greatest
depth range of any seagrass is ∼90 m (H. capricorni) which represents ∼11% of surface irradiance
(in clear oceanic waters) (Duarte, 1991). For phytoplankton the limit is generally held to be 100 m or
1% of surface irradiance, although the record is held
by a coralline red macroalga (Littler et al. 1995) at
268m m, where the light was 0.0005% of surface
irradiance.
X. Initial Photosynthetic Products
in Seagrass Leaves
By far the largest proportion of carbon fixed photosynthetically by seagrass leaves is rapidly accumulated in the ethanol soluble fraction, primarily as the disaccharide sucrose (see Fig. 5). After
only 5 minutes incubation with
14 C-bicarbonate,
over 50% was incorporated into sucrose and neutral
amino acids in the tropical seagrasses, H. spinulosa
and T. hemprichii (Andrews & Abel, 1979). In a survey of six tropical and temperate species reported by
Abel & Drew (1989), the major
14 C labelled product
after 1 hour was always sucrose in C. nodosa, C rotundata, Phyllospadix torreyi and Thalassodendron
ciliatum or its constituent monosaccharides glucose
and fructose in H. uninervis and S. isoetifolium. In
a more detailed study of C. nodosa, Drew (1983)
found that after 1 hour 97% of photosynthetically
fixed
14 C was in the ethanol-soluble fraction, mostly
as sucrose (89.2%) plus 6.2% as glucose and 2.1%
as fructose.
XI. Translocation and Exudation
Few direct measurements of the movement of photosynthetically fixed carbon within seagrass plants
have been reported. Harrison (1978) showed that,
during a 3 hour incubation, 6.5% of the
14 C fixed
by vegetative shoots of Zostera americana moved
to rhizomes and other shoots, but only 1.6% moved
from flowering shoots. The terminal shoot and any
flowering shoots present on the rhizome were the
major sinks for this translocated carbon. Drew (unpublished) found that 2.5% of the carbon fixed by
leafy shoots of P. oceanica was translocated from
the green leaf tissue during a 4.3 hour incubation
(Fig. 5). Only one third of that passed beyond the unpigmented leaf sheaths into the rhizomes and there
was a lag of 2 hours before any significant movement
was detected.
Exudation was not measured in the experiments
of either Harrison or Drew. However, Wetzel and
Penhale (1979) showed that when
14 C was fixed
in the roots of Zostera marina during a 4 hr incubation, most accumulated in the leaves (82.4%)
and a small amount of dissolved organic carbon
(DOC) was lost from the leaves (0.15%). The
situation in T. testudinum was similar, with 97.7%
being fixed in the leaves and epiphytes and 1.4% lost
as DOC. This was similar to the 1.3% of gross fixation, which Brylinsky (1977) found to be released
from T. testudinum to the surrounding medium. H.
wrightii (Brylinsky, 1977) and C. serrulata (Birch,
pers comm) are also known to release organic carbon to the medium and the photorespiratory product
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
the overall primary production of the plant and reduces the compensation depth. The second is that
light harvesting in seagrasses is strongly dependent
on excess chlorophyll proteins (Section VI), whereas
algae have a much greater range of more efficient
light-harvesting strategies. Anatomical and morphological factors in depth limitation are discussed are
discussed in Kuo and den Hartog, Chapter 3.
Duarte (1991) found that the differences in the
depth limits of seagrasses were largely attributable
to differences in light attenuation and could be described by a fairly simple equation:
LogZ c = 0.26 − 1.07 log K
Where Z c is the depth limit of a seagrass (m) and K is
the underwater light attenuation (m
−1 ). As discussed
in Section VI, while the non-photosynthetic parts
of seagrasses probably impose on them a shallower
depth limit as compared to algae, there are probably
a range of morphological types involved, from the
heavy investment in roots and rhizomes of species
like P. oceanica and P. australis, to the much reduced
investment in species like H. ovalis and H. capricorni. This is probably reflected in the much greater
depth limits of the latter species (Duarte, 1991). Olesen et al. (2002) compared changes in anatomy, physiology and growth characteristics (population structure) with depth in the co-occurring P. oceanica and
C. nodosa. C. nodosa showed the greatest flexibility
and reduced its below-ground parts and increased its
leaf area. However, photosynthetic efficiency at low
PAR was about the same for each species. P. oceanica
was able to survive longer at low irradiance due to
low growth and low respiratory rates. The greatest
depth range of any seagrass is ∼90 m (H. capricorni) which represents ∼11% of surface irradiance
(in clear oceanic waters) (Duarte, 1991). For phytoplankton the limit is generally held to be 100 m or
1% of surface irradiance, although the record is held
by a coralline red macroalga (Littler et al. 1995) at
268m m, where the light was 0.0005% of surface
irradiance.
X. Initial Photosynthetic Products
in Seagrass Leaves
By far the largest proportion of carbon fixed photosynthetically by seagrass leaves is rapidly accumulated in the ethanol soluble fraction, primarily as the disaccharide sucrose (see Fig. 5). After
only 5 minutes incubation with
14 C-bicarbonate,
over 50% was incorporated into sucrose and neutral
amino acids in the tropical seagrasses, H. spinulosa
and T. hemprichii (Andrews & Abel, 1979). In a survey of six tropical and temperate species reported by
Abel & Drew (1989), the major
14 C labelled product
after 1 hour was always sucrose in C. nodosa, C rotundata, Phyllospadix torreyi and Thalassodendron
ciliatum or its constituent monosaccharides glucose
and fructose in H. uninervis and S. isoetifolium. In
a more detailed study of C. nodosa, Drew (1983)
found that after 1 hour 97% of photosynthetically
fixed
14 C was in the ethanol-soluble fraction, mostly
as sucrose (89.2%) plus 6.2% as glucose and 2.1%
as fructose.
XI. Translocation and Exudation
Few direct measurements of the movement of photosynthetically fixed carbon within seagrass plants
have been reported. Harrison (1978) showed that,
during a 3 hour incubation, 6.5% of the
14 C fixed
by vegetative shoots of Zostera americana moved
to rhizomes and other shoots, but only 1.6% moved
from flowering shoots. The terminal shoot and any
flowering shoots present on the rhizome were the
major sinks for this translocated carbon. Drew (unpublished) found that 2.5% of the carbon fixed by
leafy shoots of P. oceanica was translocated from
the green leaf tissue during a 4.3 hour incubation
(Fig. 5). Only one third of that passed beyond the unpigmented leaf sheaths into the rhizomes and there
was a lag of 2 hours before any significant movement
was detected.
Exudation was not measured in the experiments
of either Harrison or Drew. However, Wetzel and
Penhale (1979) showed that when
14 C was fixed
in the roots of Zostera marina during a 4 hr incubation, most accumulated in the leaves (82.4%)
and a small amount of dissolved organic carbon
(DOC) was lost from the leaves (0.15%). The
situation in T. testudinum was similar, with 97.7%
being fixed in the leaves and epiphytes and 1.4% lost
as DOC. This was similar to the 1.3% of gross fixation, which Brylinsky (1977) found to be released
from T. testudinum to the surrounding medium. H.
wrightii (Brylinsky, 1977) and C. serrulata (Birch,
pers comm) are also known to release organic carbon to the medium and the photorespiratory product