Chapter 10 Oxygen Movement in Seagrasses
257
adding titanium citrate buffer to the root medium
thereby increasing measured rates of oxygen release
from the roots (Sorrell and Armstrong, 1994). However, if simulating natural conditions properly the
split chamber techniques provide the most reliable
estimates of whole plant oxygen transport.
C. Gas Extraction Techniques
Changes in internal pools of oxygen in plants may
be more directly assessed by extracting oxygen from
the lacunar spaces of different tissues (e.g. Oremland and Taylor, 1976; Carlson et al., 1988). This
technique is usually destructive in the sense that tissues have to be cut or squeezed to harvest internal
gases, but the method does allow assessment of diel
changes in lacunar oxygen and, in addition, extracted
air-samples can be analyzed for concentrations of
internal CO 2 , N 2 and CH 4 using infrared gas analysis and gas chromatography at high precision (Hartman and Brown, 1967; Oremland and Taylor, 1976;
Larkum et al., 1989). However, the gas extraction
technique does not allow on-line recording of internal gas dynamics as functions of changes in environmental conditions and the technique has poor
temporal and spatial resolution.
D. Microtechniques
Microelectrodes, compared to other techniques, do
provide much more elegant opportunities for online assessment of internal oxygen conditions within
plants (e.g. Armstrong et al., 1994; Armstrong
et al., 2000; Greve et al., 2003). Some of the earliest microelectrodes for measuring internal oxygen contents in plant tissues were polarographic
electrodes requiring external reference electrodes,
and therefore oxygen could only be assessed in
the liquid phase and not in air-filled lacunae (e.g.
Bowling, 1973). The appearance of fast-responding
and stirring-independent Clark-type oxygen microelectrodes with built-in guard cathodes provided
the first means of microscale oxygen measurements
in both liquid and gas phase at high spatial and
temporal resolutions (Revsbech, 1989). The technique allows measurements of internal plant gradients and oxygen profiles on root surfaces at spatial
scales of 10 µm or less (Caffrey and Kemp, 1991;
Armstrong et al., 1994; Christensen et al., 1994;
Pedersen et al., 1998; Greve et al., 2003) and at
temporal scales of less than 1 s. Hence, rates of oxygen release or consumption can be continuously and
precisely assessed in specific tissues under natural
or manipulated conditions. The high spatial resolution, however, has the drawback that the overview
of whole plant metabolism or oxygen release is
lost. Such processes are better determined by using
chamber techniques under proper mimicry of natural
conditions.
Microoptodes also provide means of measuring
oxygen and other compounds at high spatial and temporal resolution, and, in addition, the optode technique has been developed to allow two dimensional
recording of changes in oxygen conditions in sediments, microbial mats and in the rhizosphere of
aquatic plants (Glud et al., 1996). The planar optodes, potentially, provide excellent means for assessing spatial differences in oxygen concentrations
around roots and rhizomes of intact plants as a function of experimentally altered conditions for plant
photosynthesis.
III. Oxygen Sources
The supply of oxygen to support aerobic metabolism
within seagrass tissues derives from internal oxygen
produced by photosynthesis and from passive diffusion of oxygen from water column or sediment, when
oxygen partial pressures in the external media surpass plant oxygen partial pressures (Fig. 1). Photosynthesis of seagrasses mainly takes place in the epidermal cells with high chlorophyll contents assumed
to represent an adaptation to the low light conditions
often experienced by submerged macrophytes (Kuo
and McComb, 1989; Larkum et al., 1989). Rates of
photosynthesis on a dry weight basis are relatively
low for seagrasses and other hydrophytes compared
to terrestrial plants (Bowes, 1985; Nielsen and SandJensen, 1989; Larkum et al., Chapter 14). However,
high rates of oxygen evolution take place in individual leaves which is immediately apparent as formation of numerous gas bubbles during calm, sunny
days (Zieman, 1974). In addition, leaf biomass of
some seagrass beds may be very high and gross
primary production can exceed 10 g O 2 m
−2 d
−1
(Ziegler and Benner, 1998; Hemminga and Duarte,
2000).
A. Oxygen Evolution by Photosynthesis
Gross photosynthesis of seagrass leaves exceeds respiratory demands by almost an order of magnitude
257
adding titanium citrate buffer to the root medium
thereby increasing measured rates of oxygen release
from the roots (Sorrell and Armstrong, 1994). However, if simulating natural conditions properly the
split chamber techniques provide the most reliable
estimates of whole plant oxygen transport.
C. Gas Extraction Techniques
Changes in internal pools of oxygen in plants may
be more directly assessed by extracting oxygen from
the lacunar spaces of different tissues (e.g. Oremland and Taylor, 1976; Carlson et al., 1988). This
technique is usually destructive in the sense that tissues have to be cut or squeezed to harvest internal
gases, but the method does allow assessment of diel
changes in lacunar oxygen and, in addition, extracted
air-samples can be analyzed for concentrations of
internal CO 2 , N 2 and CH 4 using infrared gas analysis and gas chromatography at high precision (Hartman and Brown, 1967; Oremland and Taylor, 1976;
Larkum et al., 1989). However, the gas extraction
technique does not allow on-line recording of internal gas dynamics as functions of changes in environmental conditions and the technique has poor
temporal and spatial resolution.
D. Microtechniques
Microelectrodes, compared to other techniques, do
provide much more elegant opportunities for online assessment of internal oxygen conditions within
plants (e.g. Armstrong et al., 1994; Armstrong
et al., 2000; Greve et al., 2003). Some of the earliest microelectrodes for measuring internal oxygen contents in plant tissues were polarographic
electrodes requiring external reference electrodes,
and therefore oxygen could only be assessed in
the liquid phase and not in air-filled lacunae (e.g.
Bowling, 1973). The appearance of fast-responding
and stirring-independent Clark-type oxygen microelectrodes with built-in guard cathodes provided
the first means of microscale oxygen measurements
in both liquid and gas phase at high spatial and
temporal resolutions (Revsbech, 1989). The technique allows measurements of internal plant gradients and oxygen profiles on root surfaces at spatial
scales of 10 µm or less (Caffrey and Kemp, 1991;
Armstrong et al., 1994; Christensen et al., 1994;
Pedersen et al., 1998; Greve et al., 2003) and at
temporal scales of less than 1 s. Hence, rates of oxygen release or consumption can be continuously and
precisely assessed in specific tissues under natural
or manipulated conditions. The high spatial resolution, however, has the drawback that the overview
of whole plant metabolism or oxygen release is
lost. Such processes are better determined by using
chamber techniques under proper mimicry of natural
conditions.
Microoptodes also provide means of measuring
oxygen and other compounds at high spatial and temporal resolution, and, in addition, the optode technique has been developed to allow two dimensional
recording of changes in oxygen conditions in sediments, microbial mats and in the rhizosphere of
aquatic plants (Glud et al., 1996). The planar optodes, potentially, provide excellent means for assessing spatial differences in oxygen concentrations
around roots and rhizomes of intact plants as a function of experimentally altered conditions for plant
photosynthesis.
III. Oxygen Sources
The supply of oxygen to support aerobic metabolism
within seagrass tissues derives from internal oxygen
produced by photosynthesis and from passive diffusion of oxygen from water column or sediment, when
oxygen partial pressures in the external media surpass plant oxygen partial pressures (Fig. 1). Photosynthesis of seagrasses mainly takes place in the epidermal cells with high chlorophyll contents assumed
to represent an adaptation to the low light conditions
often experienced by submerged macrophytes (Kuo
and McComb, 1989; Larkum et al., 1989). Rates of
photosynthesis on a dry weight basis are relatively
low for seagrasses and other hydrophytes compared
to terrestrial plants (Bowes, 1985; Nielsen and SandJensen, 1989; Larkum et al., Chapter 14). However,
high rates of oxygen evolution take place in individual leaves which is immediately apparent as formation of numerous gas bubbles during calm, sunny
days (Zieman, 1974). In addition, leaf biomass of
some seagrass beds may be very high and gross
primary production can exceed 10 g O 2 m
−2 d
−1
(Ziegler and Benner, 1998; Hemminga and Duarte,
2000).
A. Oxygen Evolution by Photosynthesis
Gross photosynthesis of seagrass leaves exceeds respiratory demands by almost an order of magnitude