Elevated ammonium concentrations and low light form a dangerous synergy for eelgrass Zostera marina

Abstract

We studied the effect of ecologically relevant ammonium concentrations and light on several morphological and physiological properties, nitrogen metabolism and carbon reserves of eelgrass Zostera marina L. Eelgrass was grown under mesocosm conditions at 3 levels of ammonium enrichment (target concentrations of 0, 10 and 25 µM) and 2 levels of light (low and high light). High ammonium supply combined with low light had a negative effect on several morphological and physiological response parameters, while no such effects were found when ammonium was supplied under high light. N enrichment caused an increase in the content of total N, intracellular ammonium, free amino acids and residual N in the plants and this response was more pronounced under low-light conditions than under high light. The soluble proteins content decrea sed, in contrast with external ammonium enrichment. The accumulation of free amino acids and residual N in NH 4 + -enriched plants was followed by a substantial drop in carbohydrate reserves (sucrose and starch), which was larger in plants grown under low-light conditions. Our results indicate that N enrichment increases the demand for C skeletons and energy, and that photosynthesis cannot supply enough C and energy to cover that demand under low-light conditions. Eelgrass plants exposed to reduced light conditions, for example close to their depth limit or when covered by drift macroalgae, may thus be especially susceptible to enhanced ammonium concentrations. Our study demonstrates that ammonium toxicity may explain why eelgrass and other seagrasses deteriorate under nutrient-rich, low-light conditions.

Additive response to multiple environmental stressors in the seagrass Zostera marina L.

“…The amount of glutamate (major product of assimilation and source for most amino acids) and glutamine (major N reserve in plants) decreased, in contrast, and the authors concluded that eelgrass slows down N uptake and assimilation when light is insufficient. The N content in plants from our LL + HN treatment increased however from 2.5% to 3.5% of DW over the course of the experiment, which corresponds the response observed by Villazán et al () who also showed that the increase in N largely could be explained from accumulation of amino acids. We cannot explain why our findings disagree with those of Hasler‐Sheetal et al (), but their plants contained very high levels of N (ca.…”

“…c,d), which may seem strange if the metabolism of organic N‐compounds was limited by LL. However, Villazán et al () found a similar pattern and showed that the increase in total N in shade plants covered over a drop in amino acids and proteins while the content of , , and unidentified N‐compounds increased dramatically.…”

“…High mobilization of C reserves combined with high acquisition of C resulted in the highest quantity of available C among all 2‐factor treatments. The amount of C allocated to growth constituted however less than 50% of the available C. We know of no studies that have evaluated the processes underlying the combined response to elevated heat and N, but we suggest that the remaining 50% of the available C was allocated to protection and repair processes in an attempt to alleviate the negative effects of HT (e.g., Bergmann et al ; Winters et al ; Gu et al ) while at the same time a significant amount of energy and C must have been used for assimilation of since these plants received the same N content as in those from the HN treatment (Marschner ; Villazán et al ; Hasler‐Sheetal et al ). The respiration rate in plants from the HT + HN treatment was similar to that in the control so any additional energetic expenditure related to these processes must either have taken place earlier in the experiment, or plants may alternatively have undergone structural alterations that could serve to reduce respiratory losses at the plant level.…”

“…Prolonged exposure to excess is toxic to plants although the underlying mechanisms remain partly unresolved (Britto et al ). Plants exposed to high levels accumulate intra‐cellular , obtain lower pigment concentrations, higher levels of oxidative damages, and impaired photosynthesis and growth (e.g., Brun et al ; Villazán et al ; Zhou et al ). Various processes may serve to reduce high concentrations of in the cytosol.…”

“…25 μM just after addition). The dose and target concentration used in the HN treatments were chosen because these are known to cause adverse effects on eelgrass (Van Katwijk et al ; Villazán et al ).…”