Characterization of the Growth,Chlorophyll Content and Lipid Accumulation in a Marine Microalgae Dunaliella tertiolecta under Different Nitrogen to Phosphorus Ratios

Microalgal lipids are regarded as main future feedstock of biofuels for its higher efficiency of accumulation and sus- tainable production. In order to investigate the effect of various nitrogen to phosphorus ratios on cells growth, chlorophyll content and accumulation of lipids in Dunaliella tertiolecta, experiments were carried out in modified microalgal medium with inorganic nitrogen （nitrate-nitrogen） or organic nitrogen （urea-nitrogen） as the sole nitrogen source at initial N：P ratios ranging from 1：1 to 32：1. The favorable N：P of 16：1 in the nitrate-N or urea-N medium yielded the maximum cell density and specific growth rate. Decrease in chlorophyll content were observed at the N：P of 4：1 in both nitrate-N and urea-N cultures. It was also observed that the maximum lipids concentration was obtained at the N：P of 4：1 in both nitrate and urea nutrient medium. The lipid productivity and lipid content of cultures in the urea-N medium at the N：P of 4： lwere markedly higher than those from cultures with other N：P ratios （p〈 0.05）. The results of this work illustrate the possibility that higher ratios of nitrogen to phosphorus have enhancing effect on cells growth of D. tertiolecta. Conversely, higher lipid accumulation is associated with a decrease in chlorophyll content under lower ratios of nitro- gen to phosphorus. The results confirm the hypothesis of this study that a larger metabolic flux has been channeled to lipid accumu- lation in D. tertiolecta cells when the ratios of nitrogen to phosphorus drop below a critical level.

Influence of the concentration ratio of nitrogen to phosphorus on the growth and interspecies competition of two red tide algae

The growth and interspecies competition of two red tide algal species Thalassiosira pseudonana Hasle et Heimdal and Gymnodinium sp. were studied under different concentration ratios of nitrogen to phosphorus, and the algal hatch culture experiments were conducted. The physiological and biochemical indexes were measured periodically, including the maximum comparing growth rate, relative growth rate, average double time and chlorophyll a concentration. The results showed that when the concentration ratio of nitrogen to phosphorus was 16： 1, the maximum comparing growth rate, relative growth rate and chlorophyll a concentration of Thalassiosira pseudonana all reached the highest,and average double time was the shortest. This implied that the optimal concentration ratio of nitrogen to phosphorus of Thalassiosira pseudonana is 16： 1. When the concentration ratio of nitrogen to phosphorus was 6：1, the maximum comparing growth rate, relative growth rate and the chlorophyll a concentration of Gymnodinium sp. reached the highest, and average double time was the shortest, so the optimal concentration ratio of nitrogen to phosphorus of Gymnodinium sp. is 6： 1. From the growth curves as indicated both in the cell density and the chlorophyll a concentration, it is suggested that the influence of concentration ratio of nitrogen to phosphorus on the chlorophyll a concentration and the cell density are almost the same. Different concentration ratios of nitrogen to phosphorus had weak influence on community succession and the competition between the two algae. Gymnodinium sp. may use the phosphorus in vivo for growth, so it is important to pay attention to the concealment of phosphorus, in order to avoid the outbreak of red tide. On the basis of the importance of nitrogen and phosphorus and the ratio of their concentration, the possible outbreak mechanism of red tide of the two algae was also discussed.

Controlling cyanobacterial blooms by managing nutrient ratio and limitation in a large hypereutrophic lake: Lake Taihu, China

Excessive nitrogen（N） and phosphorus（P） loading of aquatic ecosystems is a leading cause of eutrophication and harmful algal blooms worldwide, and reducing nutrient levels in water has been a primary management objective. To provide a rational protection strategy and predict future trends of eutrophication in eutrophic lakes, we need to understand the relationships between nutrient ratios and nutrient limitations. We conducted a set of outdoor bioassays at the shore of Lake Taihu. It showed that N only additions induced phytoplankton growth but adding only P did not. Combined N plus P additions promoted higher phytoplankton biomass than N only additions, which suggested that both N and P were deficient for maximum phytoplankton growth in this lake（TN：TP = 18.9）. When nutrients are present at less than 7.75–13.95 mg/L TN and 0.41–0.74 mg/L TP, the deficiency of either N or P or both limits the growth of phytoplankton. N limitation then takes place when the TN：TP ratio is less than 21.5–24.7（TDN：TDP was 34.2–44.3）, and P limitation occurs above this. Therefore, according to this ratio, controlling N when N limitation exists and controlling P when P deficiency is present will prevent algal blooms effectively in the short term. But for the long term, a persistent dual nutrient（N and P） management strategy is necessary.