Carbon and nitrogen metabolism in the seagrass, Zostera marina L.: Environmental control of enzymes involved in carbon allocation and nitrogen assimilation Brant W. Touchette a, ⁎ , JoAnn M. Burkholder b a Center for Environmental Studies, Elon University, Campus Box 2625, Elon, NC 27244, USA b Center for Applied Aquatic Ecology, North Carolina State University, Raleigh, NC 27695, USA Received 2 April 2007; received in revised form 21 May 2007; accepted 21 May 2007 Abstract This study experimentally examined influences of environmental variables on the activities of key enzymes involved in carbon and nitrogen metabolism of the submersed marine angiosperm, Zostera marina L. Nitrate reductase activity in leaf tissue was correlated with both water-column nitrate concentrations and leaf sucrose levels. Under elevated nitrate, shoot nitrate reductase activity increased in both light and dark periods if carbohydrate reserves were available. When water-column nitrate was low, glutamine synthetase activity in leaf tissue increased with environmental ammonium. In contrast, glutamine synthetase activity in belowground tissues was statistically related to both nitrate and temperature. At the optimal growth temperature for this species (ca. 25 °C), increased water-column nitrate promoted an increase in glutamine synthetase activity of belowground tissues. As temperatures diverged from the optimum, this nitrate effect on glutamine synthetase was no longer evident. Activities of both sucrose synthase and sucrose-P synthase were directly correlated with temperature. Sucrose-P synthase activity also was correlated with salinity, and sucrose synthase activity was statistically related to tissue ammonium. Overall, the enzymatic responses that were observed indicate a tight coupling between carbon and nitrogen metabolism that is strongly influenced by prevailing environmental conditions, especially temperature, salinity, and environmental nutrient levels. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon; Glutamine synthetase; Nitrate reductase; Nitrogen; Sucrose synthase; Sucrose-P synthase; Zostera marina 1. Introduction The submersed marine angiosperm, eelgrass (Zostera marina L.), resides in a habitat that is characteristically harsh and dynamic. This environment is often described as light-limited from extended periods of light attenu- ation (Backman and Barilotti, 1976); carbon-limited due to decreased availability of dissolved inorganic carbon (i.e., CO 2 , HCO 3 - , and CO 3 - 2 ) in aqueous solutions (Goldman and Graham, 1981; Turpin et al., 1985; Levavasseur et al., 1991); and/or nutrient-limited (both N and P; Short, 1983; Murray et al., 1992; van Lent et al., 1995; Udy and Dennison, 1997; Peralta et al., 2003; but see Zimmerman et al., 1987). Reduction of photosynthetically active radiation (PAR) can occur from various natural and anthropogenic factors. For Journal of Experimental Marine Biology and Ecology 350 (2007) 216 – 233 www.elsevier.com/locate/jembe ⁎ Corresponding author. Tel.: +1 336 278 6185; fax: +1 336 278 6258. E-mail address: btouchette@elon.edu (B.W. Touchette). 0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2007.05.034