Macroalgal Ecophysiology in Response to High Ammonium Concentrations and Different Irradiances

Abstract

Reducing the net release of nutrients into the coastal environment has become an important issue of ecological and societal relevance. Efforts to remove excess nutrients have been made to utilize chemical or physical treatments, but they have not been successfully implemented, due to negative affect to the environment. To utilize abundant macroalgae may become a sustainable alternative to eliminate eutrophic conditions, and additionally produce a valuable by-product. However, it is necessary to deepen the sustainable concept into focused studies on macroalgal ecophysiological response to obtain a better understanding on macroalgal performance under environmental alterations. Numerous studies have examined ammonium utilization by marine macroalgae, with the most current research focusing on low ammonium concentrations. There is comparatively few information on macroalgal resistance to high ammonium concentrations, a condition which is usually found in aquaculture systems. Thus, studies in three different experimental series under laboratory conditions have been conducted to investigate the maximum uptake and physiological tolerance to high ammonium concentration treatments, including ammonium toxicity symptoms and protein content, to test the feasibilities as nutrient bioabsorber. Dictyota bartayresiana J.V. Lamoroux and Gracilaria verrucosa (Hudson) Papenfuss were compared in relation to their ammonium uptake capacities in the first study. A series of perturbation experiments was conducted in a 360 minute experimental period. Ammonium concentrations used were 0, 50, 100, 200, and 400 µM under 70 µmol photons/m²/s irradiation. In general, G. verrucosa showed higher nutrient uptake and photosynthetic performance. Final Fv/Fm of G. verrucosa remained high in all ammonium treatments, in contrast with D. bartayresiana. G. verrucosa showed no saturation point over various time intervals. In contrast, D. bartayresiana showed a saturation point and discoloration after the first 15 minutes. A high ammonium supply and light availability might support photosynthesis and growth of G. verrucosa, but on the contrary, indicated ammonium toxicity in D. bartayresiana. To investigate the maximum physiological tolerance of G. verrucosa under a very high ammonium concentrations and different irradiances was the focus of the second study. In the first experiment with 70 µmol photons/m²/s irradiation, uptake rate of G. verrucosa showed a surge phase in the first two days, and was saturated after day 4, while in the experiment with 300 µmol photons/m²/s irradiation, uptake saturation was shown from day 2 of the experimental period. The final Fv/Fm of both irradiances decreased significantly with increasing ammonium concentration treatments and irradiation. Tissue loss started from day 7 when exposed to high ammonium concentration treatment for both irradiances. Ammonium toxicity symptoms were found in G. verrucosa with ammonium addition greater than 800 µM for both irradiations. High ammonium concentration treatments and different irradiance significantly affected accumulative biomass, total growth rate, pigments, and nutrient in tissue. The third study was conducted to obtain information on biochemical levels (e.g., protein content) to high irradiance and ammonium stress. A series of experiments under irradiation of 70 and 300 µmol photons/m²/s with 1200 µM ammonium concentration treatment had been conducted in a climate room for 7 days. In general, G. verrucosa showed low performance in most observed parameters, earlier than previously found. Massive discoloration as a symptom of infectious disease was found by day 4 in the macroalgae under 300 µmol photons/m²/s irradiation. Protein content in all treatments was lower than 0.5 µg/µL extract. Overall, the most relevant findings of these studies were that 1) G. verrucosa exhibited a high performance in ammonium uptake and storage capacity, 2) unsaturated ammonium uptake shown by G. verrucosa was not a passive diffusion process, but actively transported due to charged ions of ammonium and huge storage capacity in macroalgal tissue, and 3) a very high ammonium concentration interacted with high irradiance to affect nutrient uptake, growth rate, and photosynthetic performance of G. verrucosa, and lead to toxicity symptoms. The overall outcome of these studies advanced a different perspective on relevance of nutrient abundance and macroalgae, although in some points illustrate that high ammonium concentrations may contribute to substantial negative consequences to the macroalgae. To involve G. verrucosa in an integrated applied system to control the nutrients excess and to achieve an appropriate balance of uptake and growth ratio would be the next important step.