Assessing The Effect Of Ammonium-Nitrogen On The Photosynthesis And Respiration Of Three Submerged Macrophytes

Inland water bodies are increasingly being threatened by excessive richness of nutrients from anthropogenic activities,impairing the quality of water and disrupting the proper functioning of aquatic ecosystems.Due to this fact,submerged macrophytes have continued to attract much attention in the field of aquatic toxicology,specifically based on their ability to accumulate large amounts of pollutants from contaminated water bodies.In addition to their fundamental role as primary producers and shelter for aquatic fauna,submerged macrophytes also have major influence on the aeration processes of water bodies by absorbing and transforming nutrients which would otherwise accumulate to noxious levels.To a large extent,biogeochemical cycles in the water column and sediments are influenced by absence or presence,as well as the type of submerged macrophytes present.Submerged macrophytes also drive natural phytoremediation processes within aquatic systems in a cheaper and environmentally friendly manner relative to the conventional contaminants clean-up techniques.Generally,plant growth and productivity are often limited by nitrogen availability;therefore submerged plants have developed signaling and assimilation mechanisms specific to their respective nitrogen sources.Submerged macrophytes prefer ammonium to nitrate since the former requires less energy to assimilate.Though an important resource,increased loading of high ammonium concentrations in water bodies has led to the degradation of water quality and lowered the ecological integrity of aquatic systems.Moreover,high [NH₄⁺-N] in water is toxic to submerged macrophytes since it generates physiological stress and inhibits fundamental growth processes.Thus,proper comprehension of the ammonium detoxification mechanisms in submerged macrophytes is vital in understanding basic ecosystem processes such as natural phytoremediation process,wetland restoration as well as management of different macrophyte species.Previous researches related to ammonium detoxification mechanisms have largely studied ammonium uptake and assimilation potential of different species across a range of concentrations.To the best of our knowledge,studies focusing on the effect of varying ammonium concentration on carbon usage of submerged plants are rare.Our current study,therefore,was designed to investigate the effects of different ammonium concentrations on photosynthesis and respiration of submerged macrophytes.To achieve the objectives of our study,we selected a total of 3 species;2from the same genus and 1 from a different genus.Healthy apical shoots of Potamogeton lucens,Potamogeton maackianus and Myriophyllum spicatum were cultured in three lab-scale water columns containing [NH₄⁺-N] at the concentrations of 0,15 and 50 mg/L for a period of 4 days.Interestingly,contrary to the popular understanding that ammonium is usually detrimental to submerged macrophytes at high concentration,our study revealed that the gross photosynthetic rates of P.maackianus and M.spicatum were unchanged across the four concentrations while that of P.lucens reduced by 89% when ammonium was increased from 0 mg/L to 50 mg/L.The results showed that under high ammonium concentration(50 mg/L),Fv/Fm values and total chlorophyll contents of P.lucens decreased by 49% and 73% respectively,while no significant changes were witnessed in M.spicatum.When [NH₄⁺-N] was increased from 0 mg/L to 50 mg/L,starch contents increased 2-folds for M.spicatum but reduced 9-folds for P.lucens.Comparison of the activities of enzyme involved in carbon assimilation indicates insignificant differences for Rubisco across the different ammonium concentrations for all our study species.The ratio of PEPC to Rubisco increased slightly(1.6-1.9)for P.maackinus while that of P.lucens decreased with increase in ammonium content.However,this ratio was 1.9 and 4.0 for M.spicatum at [NH₄⁺-N]of 15 and 50 mg/L respectively.These results suggest that P.maackinus and M.spicatum might perform a type of C4 metabolism when exposed to ammonium stress.Though 2 of our study species are from the same genus,P.lucens and was found to be sensitive to high ammonium while P.maackianus is not.The results of our study,therefore,give convincing evidence on the assertion that though [NH₄⁺-N] is assumed to have deleterious effects on submerged macrophytes at high concentrations,the ultimate effects depend on species' specificity.Inferences derived from this study may help ecologists in macrophyte management for water restoration,especially in eutrophic lakes.