Physiological And Biochemical Responses Of Seaweed To Nutrient And Water Exchange

Seaweed is a promising agency for coastal eutrophication remediation, and the remediation ability of seaweed and how it response to environmental factors are hot topics. The growth and bioremediation ability of seaweed are effected by various environmental factors, including temperature, light, nutrient, CO2, pH, water flow, salinity, light-dark ratio, density etc. Among them, adequate nutrients supply (i.e. nitrogen, phosphours) is of primary importance to maintain the high biomass accumulation of seaweed. Moreover, water exchange can not only maintain continuous nutrient supply, but also alleviate the pH rising due to dissolved carbon consumption for photosynthesis, as well as increase dissolved carbon content, remove dead algae cells, metabolites and dirt. However, to our knowledge, the effect of water exchange rate on the growth, physiological and biochemical parameters and nutrients absorption ability of seaweed is rarely studied, and whether there is interactions between the effect of water exchange rate and nutrient concentration on the seaweed growth has not been reported. Based on a review of the effects of nutrient and water flow on seaweed from physiological and biochemical aspects, this study investigate the physiological and biochemical responses of two common seaweed grow in Zhejiang Province Sargassum fusiforme and Ulva lactuca-to different levels of nutrient concentration, water exchange rate and their interactions, which will serve as theoretical basis for the eutrophication bioremediation of coastal water by seaweeds. The main conclusions are as follows:1. The nutrient concentration, water exchange rate and their interactions have influence on the specific growth rate (SGR) of Sargassum fusiforme. The pattern of varies in the SGR of Sargassum fusiforme is consistent under different water exchange rates. The growth of Sargassum fusiforme is simulated under the medium nutrient concentration, and inhibited under the high nutrient concentration. As the water exchange rate increases, SGR kept increasing under the mediun nutrient concentration, and showed a trend first rise and then decline under the high nutrient concentration. The nutrient supply has a significant impact on soluble protein (SP) content. In addition, the water exchange rate caused significant changes in the soluble carbohydrate (SC) content.2. The growth of Ulva lactuca is mainly affected independently by the nutrient levels and water exchange rate. The water exchange rate has no observed effect on the SGR of Ulva lactuca. However, in general the SGR arrived 5.93 ± 0.82%d-1 under the water exchange rate of 100 vol/d, which was significantly higher than the SGR (in average 3.85 ± 0.21% d-1) under other rates (i.e.0,25,50,200 vol/d). The SGR increases linearly as the nutrient concentration enriched. The SGR arrived 6.31 ± 0.38% d-1 and 8.00 ± 0.51% d-1 under the medium and high nutrient concentration respectively. Although Ulva lactuca under the high nutrient concentration maintained a high growth rate, the thalli was bleaching when the water exchange rate was as high as 200 vol/d. With the increase of the nutrient concentration, the Carotenoid (Car) content showed an increasing trend, the Chlorophyll a (Chl a) and the SP contents shows a trend from rising to decline. The SC content showed an opposite trend comparing to the Chl a and SP. The nutrient concentration significantly influenced the contents of Chl a, Car, SP, SC in Ulva lactuca.3. Comparing the physiological and biochemical responses of Sargassum fusiforme and Ulva lactuca, the SGR of Ulva lactuca (4.23 ± 0.24% d-1) were higher than Sargassum fusiforme (2.42 ± 0.15% d-1). The high nutrient concentration has an inhibited the growth of Sargassum fusiforme, but not for Ulva lactuca, so Ulva lactuca has survival advantage in eutrophic environment. Nutrient can strengthen their photosynthetic reaction of Sargassum fusiforme and Ulva lactuca to promote the growth, but the mechanisms of enhancement were different. In details, nutrient has a significant influence on the Chl a and Car contents of Ulva lactuca, but not for the Sargassum fusiforme, Moreover, nutrient significantly differed the SP content of Sargassum fusiforme, and SP and SC contents of Ulva lactuca. The water exchange rate significantly changed the SC content of Sargassum fusiforme, illustrating that CO2 increased by water exchange has influenced the synthesis of SC, however the influence is not obvious under the high nutrient supply.4. From the small scale test on the growth of Sargassum fusiforme and Ulva lactuca under various water flow rate, we find that the SGR of Ulva lactuca is higher than Sargassum fusiforme with the water flow rate from 0.1 m/s to 0.3 m/s under the typical eutrophic Chinese coastal waters. The water exchange rate has a significant influence on the growth of Sargassum fusiforme and Ulva lactuca, but the water flow rate hasn’t. Although Sargassum fusiforme would be inhibited under the high nutrient concentration, the water exchange rate and water flow rate will relieve the toxic to a ceatain degree. Enhancing the water flow rate can prevent the bieaching of Ulva lactuca, but the overall growth rate of Ulva lactuca is significant lower than those under corresponding water exchange rates.