170 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
in seagrasses is primarily influenced by temperature and, in belowground tissues, by oxygen availability. Aboveground tissues involved in C assimilation and other energy-costly processes
generally have higher respiration rates than belowground mostly storage tissues. Respiration rates increase with increasing temperature in excess of 408C and increasing water-column nitrate
enrichment Z
. marina, which may help to supply the energy and carbon needed to assimilate and reduce nitrate. Seagrasses translocate oxygen from photosynthesizing leaves to belowground
tissues for aerobic respiration. During darkness or extended periods of low light, belowground tissues can sustain extended anerobiosis. Documented alternate fermentation pathways have
yielded high alanine, a metabolic ‘strategy’ that would depress production of the more toxic product ethanol, while conserving carbon skeletons and assimilated nitrogen. In comparison to the
wealth of information available for terrestrial plants, little is known about the physiological ecology of seagrasses in carbon acquisition and metabolism. Many aspects of their carbon
metabolism — controls by interactive environmental factors; and the role of carbon metabolism in salt tolerance, growth under resource-limited conditions, and survival through periods of dormancy
— remain to be resolved as directions in future research. Such research will strengthen the understanding needed to improve management and protection of these environmentally important
marine angiosperms.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Carbon; Light; Photosynthesis; Respiration; Seagrass; Temperature
1. Introduction
Seagrasses are a small but diverse group of mostly submersed marine angiosperms which inhabit environments that are characterized by periodic light limitation. These
monocots are taxonomically restricted to two families, 12 genera, and 55 species here, including the species Ruppia maritima, all of which have evolved from land pre-
decessors that returned to the sea approximately 100 million years ago during the Cretaceous McRoy and Helfferich, 1977; Larkum and den Hartog, 1989. Few
angiosperms have evolved the osmoregulatory capacity to exist in marine waters. Nevertheless, the high productivity of seagrass meadows, sometimes exceeding 15 g C
22 21
m d
, places them among the most productive of all marine ecosystems Phillips and McRoy, 1980; Hillman et al., 1989. Seagrass meadows provide both habitat and a
nutritional base for finfish, shellfish, waterfowl, and herbivorous mammals Klumpp et ˜
al., 1989; Phillips and Menez, 1998. Seagrass meadows also function in stabilizing bottom sediments and clearing the water of suspended sediments and nutrients Terrados
and Duarte, 2000. Most seagrasses grow rooted in nutrient-rich sediments of shallow coastal lagoons and
embayments, where the water column sustains periodic increased turbidity from sediment loading resuspension, phytoplankton, and macroalgal growth Harlin, 1993;
Morris and Tomasko, 1998. Epiphytic algal development on the macrophyte leaves can significantly reduce available light for growth, as well Sand-Jensen, 1977. Moreover,
during low tide some intertidal species undergo hours of atmospheric exposure with associated desiccation and increased UV radiation Trocine et al., 1981; Leuschner et al.,
1998.
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
171
Since the sediments typically provide high supplies of most nutrients, with the water column as a secondary source Short and McRoy, 1984; Harlin, 1993, seagrasses
generally are not primarily nutrient-limited Zimmerman et al., 1987. However, seagrasses obtain their carbon supply from the water rather than the sediments Sand-
Jensen, 1977. Since carbon dioxide diffuses through water |10 000-fold more slowly than through air Stumm and Morgan, 1996, carbon acquisition is more difficult for
submersed plants. Whereas there is a wealth of information about the ecology of seagrasses under varying light regimes, there has been no effort to present an overview
and conceptual framework about carbon uptake and metabolism in seagrasses from physiological and ecological perspectives. Here, we synthesize available information on
the interplay between carbon and light in the carbon metabolism of this ecologically important group of aquatic angiosperms.
2. Seagrass photosynthesis
