Review of nitrogen and phosphorus metabolism in seagrasses

Abstract

Within the past few decades, major losses of seagrass habitats in coastal waters impacted by cultural eutrophication have been documented worldwide. In confronting a pressing need to improve the management and protection of seagrass meadows, surprisingly little is known about the basic nutritional physiology of these critical habitat species, or the physiological mechanisms that control their responses to N and P gradients. The limited available evidence to date already has revealed, for some seagrass species such as the north temperate dominant Zostera marina, unusual responses to nutrient enrichment in comparison to other vascular plants. Seagrasses derive N and P from sediment pore water (especially ammonium) and the water column (most nitrate). The importance of leaves versus roots in nutrient acquisition depends, in part, on the enrichment conditions. For example, a shift from reliance on sediment pore water to increased reliance on the overlying water for N and P supplies has been observed under progressive water-column nutrient enrichment. Seagrasses may be N-limited in nutrient-poor waters with sandy or (less so) organic sediments, and P-limited in carbonate sediments. On the basis of data from few species, seagrasses enrichment (as low as 3.5-7 mM NO , 3-5 weeks), which is actively taken up without apparent 3 product feedback inhibition. Inhibition by elevated nitrate has also been reported, with description of the underlying physiological mechanisms, in certain macroalgae and microalgae. In Z. marina, this effect has been related to the high, sustained energy demands of nitrate uptake, and to inducement of internal carbon limitation by the concomitant 'carbon drain' into amino acid assimilation. In contrast, nitrate enrichment can stimulate growth of Z. marina when the sediment, rather than the water column, is the source. Because seagrass species have shown considerable variation in nutritional response, inferences about one well-studied species, from one geographic location, should not be applied a priori to that species in other regions or to seagrasses in general.Most of the available information has been obtained from study of a few species, and the basic nutritional physiology of many seagrasses remains to be examined and compared across geographic regions. Nonetheless, the relatively recent gains in general understanding about the physiological responses of some seagrass species to nutrient gradients already have proven valuable in both basic and applied research. For example, physiological variables such as tissue C:N:P content have begun to be developed as integrative indicators of nutrient conditions and anthropogenic nutrient enrichment. To strengthen insights for management strategies to optimize seagrass survival in coastal waters adjacent to exponential human population growth and associated nutrient inputs, additional emphasis is critically needed to assess the role of variable interactions-among inorganic as well as organic N, P and C, environmental factors such as temperature, ...

Nitrogen fixation in a chemoautotrophic lucinid symbiosis

“…In marine systems, the main sources of nitrogen available for bacterial growth (besides N 2 ) are NH 4 + and free amino acids 34 , as well asto a lesser extent-NO 3 − , NO 2 − and urea 35 . In seagrass bed sediments in particular, NH 4 + is considered to be the dominating nitrogen source, with concentrations of up to 175 µM in some areas 36 . We detected NH 4 + levels from 1 to 10 µM in the T. testudinum sediment directly surrounding C. orbicularis, with the highest concentrations measured in the deeper layers of the sediment (Fig.…”

“…These values are low compared to previously reported NH 4 + concentrations of up to 120 µM in T. testudinum sediments in subtropical Florida 37 . However, sediment pore water ammonium concentrations have been shown to vary substantially between sampling sites and times 36,38 . In tropical T. testudinum seagrass beds of the Caribbean, NH 4 + levels were observed to be particularly low, that is, below 25 µM in the upper 10 cm of sediment 39 .…”

Cover versus recovery: Contrasting responses of two indicators in seagrass beds

“…Concentrations for the high treatment were calculated as a function of nitrogen addition since nitrogen is documented as the limiting nutrient for seagrasses in many temperate systems (Zimmerman et al, 1987;Touchette and Burkholder, 2000). Nutrient additions were chosen to be really high, comparable to levels used in agriculture in the Shandong province (Ju et al, 2006).…”

“…However when nutrients, particularly nitrogen, are in excess, the plant physiological and growth response are negatively affected. This is due to nitrogen toxicity, and in aquatic or marine systems additional algal overgrowth, which both lead to a higher demand of carbon skeletons, thus causing physiological imbalance, ultimately leading to mortality and collapse (Pearson and Stewart, 1993;Marschner, 1995; for seagrasses reviewed in: Touchette and Burkholder, 2000;Burkholder et al, 2007). Along this eutrophication trajectory, prior to collapse, increasing plant cover thus may coincide with increasing physiological imbalance, which likely diminishes plant recovery potential.…”

Applications of C and N stable isotopes to ecological and environmental studies in seagrass ecosystems

“…On the contrary, use of benthic plants allows a temporal integration of the 15 N source signal because of their longer turnover time (Costanzo et al, 2001). However, sea-grasses often display very complex strategies to meet their nutrient requirements, particularly of nitrogen (Touchette and Burkholder, 2000). Some species or populations rely almost exclusively on nutrients of the water column, others on the nutrients of the pore water pool, while a majority of species rely on a variable mixture of these two sources.…”