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importance of trophic interactions in maintaining the viability of seagrass
ecosystems. Finally, we will examine seagrasses as biogeochemical agents that stabilize coastal sediments; sequester CO 2 through direct burial of organic carbon in
coastal ocean sediments, recently termed “blue carbon” (McLeod et al. 2011); and
trap remineralized CO 2 by generating alkalinity through carbonate dissolution and
hydrogen sulfide precipitation in sedimentary pore waters.
8.2 Seagrasses as Biological Systems
Seagrasses represent an ecological assemblage of about 66 species of angiosperms
distributed across four taxonomic families within the superorder Alismatiflorae
(Monocotyledonae, den Hartog and Kuo 2006). The somewhat obscure evolutionary origins appear to involve independent derivations from freshwater and estuarine
ancestors sometime in the Lower Cretaceous (Les et al. 1997). Despite this polyphyletic origin, seagrasses share many common properties with each other that
facilitate an aquatic life history. These traits include (1) the full submergence of all
plant structures; (2) tolerance of saline water; (3) a secure anchoring system consisting of metabolically functional roots and rhizomes capable of assimilating nutrients
and tolerating anoxic, permanently flooded sediments; and (4) hydrophilous pollination mechanisms (Larkum et al. 2006; Jackson et al. 2009).
8.2.1 Key Anatomical Features and Functions
The seagrass leaf has been heavily modified from the terrestrial archetype to accommodate a submerged aquatic existence. Chloroplasts are concentrated in the epidermis rather than the mesophyll (Kuo and den Hartog 2006) (Fig 8.1a). The cuticle is
extremely thin and water permeable to permit gas, solute, and water exchange with
the epidermis. There are no stomata. However, the thin water-permeable cuticle
renders seagrasses highly vulnerable to desiccation upon exposure to air (Leuschner
et al. 1998; Björk et al. 1999). These leaf modifications are more reminiscent of
macrophytic algae than the terrestrial vascular plants from which seagrasses are
derived and provide a strong example of convergent evolution by the seagrasses and
seaweeds to the solution of common problems. The photosynthetic apparatus also
shows a high capacity for photoacclimation to different light environments, which
protects it from photoinhibition in the bright, blue light of clear tropical waters and
facilitates light harvesting in the dim, green waters of many temperate environments
(Ralph et al. 2002; Cummings and Zimmerman 2003).
Gas-filled lacunae (i.e., aerenchyma) run continuously from the leaves through
the rhizomes to the roots. They provide buoyancy to maintain the leaves in a vertical
orientation that minimizes self-shading and optimizes photosynthesis of the entire
canopy (Zimmerman 2006). The lacunae transport life-sustaining oxygen to roots
8 Systems Biology and the Seagrass Paradox…
importance of trophic interactions in maintaining the viability of seagrass
ecosystems. Finally, we will examine seagrasses as biogeochemical agents that stabilize coastal sediments; sequester CO 2 through direct burial of organic carbon in
coastal ocean sediments, recently termed “blue carbon” (McLeod et al. 2011); and
trap remineralized CO 2 by generating alkalinity through carbonate dissolution and
hydrogen sulfide precipitation in sedimentary pore waters.
8.2 Seagrasses as Biological Systems
Seagrasses represent an ecological assemblage of about 66 species of angiosperms
distributed across four taxonomic families within the superorder Alismatiflorae
(Monocotyledonae, den Hartog and Kuo 2006). The somewhat obscure evolutionary origins appear to involve independent derivations from freshwater and estuarine
ancestors sometime in the Lower Cretaceous (Les et al. 1997). Despite this polyphyletic origin, seagrasses share many common properties with each other that
facilitate an aquatic life history. These traits include (1) the full submergence of all
plant structures; (2) tolerance of saline water; (3) a secure anchoring system consisting of metabolically functional roots and rhizomes capable of assimilating nutrients
and tolerating anoxic, permanently flooded sediments; and (4) hydrophilous pollination mechanisms (Larkum et al. 2006; Jackson et al. 2009).
8.2.1 Key Anatomical Features and Functions
The seagrass leaf has been heavily modified from the terrestrial archetype to accommodate a submerged aquatic existence. Chloroplasts are concentrated in the epidermis rather than the mesophyll (Kuo and den Hartog 2006) (Fig 8.1a). The cuticle is
extremely thin and water permeable to permit gas, solute, and water exchange with
the epidermis. There are no stomata. However, the thin water-permeable cuticle
renders seagrasses highly vulnerable to desiccation upon exposure to air (Leuschner
et al. 1998; Björk et al. 1999). These leaf modifications are more reminiscent of
macrophytic algae than the terrestrial vascular plants from which seagrasses are
derived and provide a strong example of convergent evolution by the seagrasses and
seaweeds to the solution of common problems. The photosynthetic apparatus also
shows a high capacity for photoacclimation to different light environments, which
protects it from photoinhibition in the bright, blue light of clear tropical waters and
facilitates light harvesting in the dim, green waters of many temperate environments
(Ralph et al. 2002; Cummings and Zimmerman 2003).
Gas-filled lacunae (i.e., aerenchyma) run continuously from the leaves through
the rhizomes to the roots. They provide buoyancy to maintain the leaves in a vertical
orientation that minimizes self-shading and optimizes photosynthesis of the entire
canopy (Zimmerman 2006). The lacunae transport life-sustaining oxygen to roots
8 Systems Biology and the Seagrass Paradox…