Journal of Experimental Marine Biology and Ecology 250 (2000) 169–205 www.elsevier.nl / locate / jembe Overview of the physiological ecology of carbon metabolism in seagrasses * Brant W. Touchette , JoAnn M. Burkholder Department of Botany Box 7510, North Carolina State University, Raleigh, NC 27695-7510, USA Abstract The small but diverse group of angiosperms known as seagrasses form submersed meadow communities that are among the most productive on earth. Seagrasses are frequently light-limited and, despite access to carbon-rich seawaters, they may also sustain periodic internal carbon limitation. They have been regarded as C3 plants, but many species appear to be C3–C4 intermediates and / or have various carbon-concentrating mechanisms to aid the Rubisco enzyme in carbon acquisition. Photorespiration can occur as a C loss process that may protect photosynthetic electron transport during periods of low CO availability and high light intensity. Seagrasses can 2 22 21 also become photoinhibited in high light (generally . 1000 mEm s ) as a protective mechanism that allows excessive light energy to be dissipated as heat. Many photosynthesis– irradiance curves have been developed to assess light levels needed for seagrass growth. However, most available data (e.g. compensation irradiance I ) do not account for belowground tissue c respiration and, thus, are of limited use in assessing the whole-plant carbon balance across light gradients. Caution is recommended in use of I (saturating irradiance for photosynthesis), since k seagrass photosynthesis commonly increases under higher light intensities than I ; and in k estimating seagrass productivity from H (duration of daily light period when light equals or sat exceeds I ) which varies considerably among species and sites, and which fails to account for k light-limited photosynthesis at light levels less than I . The dominant storage carbohydrate in k seagrasses is sucrose (primarily stored in rhizomes), which generally forms more than 90% of the total soluble carbohydrate pool. Seagrasses with high I levels (suggesting lower efficiency in C c acquisition) have relatively low levels of leaf carbohydrates. Sucrose-P synthase (SPS, involved in sucrose synthesis) activity increases with leaf age, consistent with leaf maturation from carbon sink to source. Unlike terrestrial plants, SPS apparently is not light-activated, and is positively influenced by increasing temperature and salinity. This response may indicate an osmotic adjustment in marine angiosperms, analogous to increased SPS activity as a cryoprotectant response in terrestrial non-halophytic plants. Sucrose synthase (SS, involved in sucrose metabo- lism and degradation in sink tissues) of both above- and belowground tissues decreases with tissue age. In belowground tissues, SS activity increases under low oxygen availability and with increasing temperatures, likely indicating increased metabolic carbohydrate demand. Respiration *Corresponding author. Tel.: 11-919-515-2726; fax: 11-919-513-3194. E-mail address: joann burkholder@ncsu.edu (B.W. Touchette). ] 0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00196-9