Chapter 10
Oxygen Movement in Seagrasses
Jens Borum*, Kaj Sand-Jensen, Thomas Binzer and Ole Pedersen
Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51,
DK-3400 Hillerød, Denmark
Tina Maria Greve
National Environmental Research Institute, Ministry of Environment and Energy,
Vejlsøvej 25, DK-8600 Silkeborg, Denmark
I. General Introduction
Seagrasses are, like all vascular plants, obligate aerobes, which require a continuous supply of oxygen
to sustain aerobic metabolism of both above- and
below-ground tissues. Compared to their leaves, seagrass roots and rhizomes may experience oxygen deprivation for shorter periods, but these below-ground
tissues exhibit physiological adaptations which allow them to rely temporarily on anaerobic fermentative metabolism (Pregnall et al., 1984; Smith et al.,
1988). Aerobic respiration is energetically about 10
times more efficient than fermentative processes,
which tend to accumulate ethanol, acetate, and other
potentially toxic metabolites representing a threat
to tissue survival (Smith et al., 1988; Crawford and
Braendle, 1996). The meristematic tissues, located
in the transition between water column and sediment,
are especially vulnerable to low oxygen supply and
exposure to anaerobic metabolites due to their high
metabolic activity and the continuous oxygen supply required for mitotic growth. In addition to the
importance of oxygen inside seagrass tissues, maintenance of oxic conditions around roots may provide efficient protection against invasion of reduced
toxic compounds and metal ions from the surrounding sediment (Armstrong et al., 1992; Crawford and
Braendle, 1996; see also Marb´ a et al., Chapter 6).
Accordingly, there are several benefits to plant performance in maintaining a rich oxygen supply to all
tissues including roots and rhizomes.
∗ Author for correspondence, email: jBorum@bi.ku.dk
In most terrestrial plants oxygen is readily supplied from the atmosphere and from aerated soils.
However, emergent wetland plants rooted in waterlogged soils or submerged macrophytes, such as seagrasses, must temporarily or permanently endure
conditions with low supplies of oxygen from the
surrounding environment. Coastal marine sediments
are mostly anoxic and highly reduced because of the
degradation of organic matter within the sediment
and slow oxygen diffusion from the water column.
Hence the sediment represents a strong oxygen sink
rather than a source, and oxygen must be supplied to
below-ground tissues of seagrasses either by photosynthesis or by oxygen diffusing from the water column through leaves to rhizomes and roots (Pedersen
et al., 1998). The slow diffusion of oxygen in water
(10,000 times slower than in air) contributes to the
potential risk of oxygen deprivation in submerged
plants. Firstly, transport of oxygen from the water
column through the relatively thick diffusive boundary layers around leaves is impaired by the slow rate
of diffusion, and secondly, liquid phase oxygen diffusion inside plants is grossly inadequate to support
oxygen transport over long distances such as those
between leaves and root tips. Therefore, submerged
plants have become anatomically adapted to oxygen
shortage by developing aerenchymatic tissues with
continuous air-filled lacunae running from leaves to
roots (Armstrong, 1979; Larkum et al., 1989; Kuo
and den Hartog, Chapter 3).
This chapter aims to present the current status
of knowledge with respect to oxygen production,
consumption and transport within seagrasses. We
255–270.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.
Oxygen Movement in Seagrasses
Jens Borum*, Kaj Sand-Jensen, Thomas Binzer and Ole Pedersen
Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51,
DK-3400 Hillerød, Denmark
Tina Maria Greve
National Environmental Research Institute, Ministry of Environment and Energy,
Vejlsøvej 25, DK-8600 Silkeborg, Denmark
I. General Introduction
Seagrasses are, like all vascular plants, obligate aerobes, which require a continuous supply of oxygen
to sustain aerobic metabolism of both above- and
below-ground tissues. Compared to their leaves, seagrass roots and rhizomes may experience oxygen deprivation for shorter periods, but these below-ground
tissues exhibit physiological adaptations which allow them to rely temporarily on anaerobic fermentative metabolism (Pregnall et al., 1984; Smith et al.,
1988). Aerobic respiration is energetically about 10
times more efficient than fermentative processes,
which tend to accumulate ethanol, acetate, and other
potentially toxic metabolites representing a threat
to tissue survival (Smith et al., 1988; Crawford and
Braendle, 1996). The meristematic tissues, located
in the transition between water column and sediment,
are especially vulnerable to low oxygen supply and
exposure to anaerobic metabolites due to their high
metabolic activity and the continuous oxygen supply required for mitotic growth. In addition to the
importance of oxygen inside seagrass tissues, maintenance of oxic conditions around roots may provide efficient protection against invasion of reduced
toxic compounds and metal ions from the surrounding sediment (Armstrong et al., 1992; Crawford and
Braendle, 1996; see also Marb´ a et al., Chapter 6).
Accordingly, there are several benefits to plant performance in maintaining a rich oxygen supply to all
tissues including roots and rhizomes.
∗ Author for correspondence, email: jBorum@bi.ku.dk
In most terrestrial plants oxygen is readily supplied from the atmosphere and from aerated soils.
However, emergent wetland plants rooted in waterlogged soils or submerged macrophytes, such as seagrasses, must temporarily or permanently endure
conditions with low supplies of oxygen from the
surrounding environment. Coastal marine sediments
are mostly anoxic and highly reduced because of the
degradation of organic matter within the sediment
and slow oxygen diffusion from the water column.
Hence the sediment represents a strong oxygen sink
rather than a source, and oxygen must be supplied to
below-ground tissues of seagrasses either by photosynthesis or by oxygen diffusing from the water column through leaves to rhizomes and roots (Pedersen
et al., 1998). The slow diffusion of oxygen in water
(10,000 times slower than in air) contributes to the
potential risk of oxygen deprivation in submerged
plants. Firstly, transport of oxygen from the water
column through the relatively thick diffusive boundary layers around leaves is impaired by the slow rate
of diffusion, and secondly, liquid phase oxygen diffusion inside plants is grossly inadequate to support
oxygen transport over long distances such as those
between leaves and root tips. Therefore, submerged
plants have become anatomically adapted to oxygen
shortage by developing aerenchymatic tissues with
continuous air-filled lacunae running from leaves to
roots (Armstrong, 1979; Larkum et al., 1989; Kuo
and den Hartog, Chapter 3).
This chapter aims to present the current status
of knowledge with respect to oxygen production,
consumption and transport within seagrasses. We
255–270.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.