Increased vulnerability of Zostera noltii to stress caused by low light and elevated ammonium levels under phosphate deficiency

Abstract

The effects of light and ammonium levels on net production, fluorescence parameters and non-structural carbohydrates of the seagrass Zostera noltii under different phosphate conditions were studied. A fully factorial design was used with light (low/high levels), ammonium supply and phosphate preculture conditions of the plants as the experimental variables. Both ammonium supply and low light caused negative and synergistic effects on net production, while ammonium toxicity was more severe at high light levels; in this case, it was independent of the non-structural carbohydrate (sucrose and starch) content. Preculturing of plant with added phosphate alleviated the ammonium toxicity, and also attenuated the negative production balance of plants grown at low light levels. The results indicated that phosphate preculture ameliorated the plant's short-term response against the assayed stressors (low light, high ammonium) significantly. An overall consumption of non-structural carbohydrates in response to environmental stressors was recorded throughout the experiment, indicating the importance of carbon and phosphorus reserves to cope with adverse conditions. In addition, phosphate deficiency increased the vulnerability of plants, which could have negative ecological consequences for seagrass species thriving under phosphate deficiency conditions, or in developing seagrass transplantation programs.KEY WORDS: Ammonium toxicity · Carbohydrates · Eutrophication · Fluorescence · Net production · Phosphate limitation · Seagrass Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 365: [67][68][69][70][71][72][73][74][75] 2008 seagrasses has been demonstrated (Burkholder et al. 1992(Burkholder et al. , 1994van Katwijk et al. 1997 and Brun et al. 2002 for ammonium) at concentrations as low as 16 µm ammonium. Seagrass leaves take up ammonium in direct proportion to the concentration in the surrounding water, but are not capable of controlling it (Thursby & Harlin 1982, Iizumi & Hattory 1982, van Katwijk et al. 1997, Touchette & Burkholder 2000. Intracellular ammonium has to be rapidly assimilated into organic compounds to prevent its accumulation to potentially harmful levels, as was shown in an experiment with the seagrass Posidonia oceanica, which maintained low intracellular ammonium levels regardless of the nitrogen enrichment levels at which plants were grown (Invers et al. 2004).Ammonium toxicity mechanisms are complex and not fully understood (Britto & Kronzucher 2002), but the physiological mechanisms are thought to be a combination (among others) of: (1) the uncoupling of ATP production during the photosynthetic electron transport that is triggered by ammonia (Goyal et al. 1982, Marschner 1995, (2) the intracellular depletion of essential cations (such as potassium and magnesium; Kirkby 1968, van Katwijk et al. 1997) that is accompanied by the increase of intracellular inorganic anions (e.g. phosphate; Cruz et al. 1993, van Katwijk et al. 1997, (3) an increa...

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

“…Only 2 studies have so far investigated the effect of lower and more ecologically relevant NH 4 + concentrations. Brun et al (2002) found that leaf-elongation, plastochrone interval and net plant growth in Zostera noltii were affected negatively when exposed to a constant concentration of 16 µM NH 4 + , while Brun et al (2008) reported that ca. 15 µM NH 4 + had a negative effect on net shoot growth and photosynthetic performance (F v /F m ) in Z. noltii.…”

Combined nutrient and macroalgae loads lead to response in seagrass indicator properties

“…In our study, the seagrasses and Ulva were generally still alive after the experimental period in all treatments. Environmental stressors such as nutrient enrichment can change the carbon demand of seagrasses (Brun et al, 2008;Christianen et al, 2011;van Katwijk et al, 2011). The C/N ratio of seagrass leaves is a function of light as well as nutrient availability (Burkholder et al, 2007;Grice et al, 1996).…”

Macroalgae blooms and their effects on seagrass ecosystems

“…Ammonium assimilation into amino acids and other nutrogen-organic compounds requires carbon skeletons and energy, which are from photosynthesis or mobilized from carbon-reserves in seagrasses (Brun et al, 2008;Villazán et al, 2013). Light reduction from macroalgl blooms may increase the toxicity of ammonium on seagrasses (Brun et al, 2008;Villazán et al, 2013).…”