Shunni Zhu

Metabolic changes of starch and lipid triggered by nitrogen starvation in the microalga Chlorella zofingiensis.

The aim of this research was to study the metabolic changes of starch and lipid biosynthesis in the microalga Chlorella zofingiensis under nitrogen starvation in comparison to nitrogen abundant condition. C. zonfingiensis showed a rapid growth and kept stable chlorophyll content when grown in nitrogen-replete medium, while a severe inhibition of cell growth and a sharp degradation of chlorophyll occurred under nitrogen depletion. Nitrogen-replete C. zonfingiensis cells possessed basal levels of starch and lipid. Upon nitrogen starvation, both starch and lipid increased greatly within cells, but starch synthesis preceded lipid accumulation. After 2 days of stress condition, starch was partially degraded, possibly to support lipid synthesis. It was speculated that starch accumulation acted as a quick response to environmental stress, whereas lipid served as long-term energy storage. Additionally, C. zonfingiensis tends to lower the degree of unsaturation in response to nitrogen starvation which is desirable for biodiesel production.

Cultivation of Chlorella zofingiensis in bench-scale outdoor ponds by regulation of pH using dairy wastewater in winter, South China

Cultivation of Chlorella zofingiensis and nutrients removal in dairy wastewater were investigated in bench-scale outdoor ponds in winter, South China. The impacts of the two types of pH regulations, 5 ≈ 6% CO(2) and acetic acid (HAc) on this process were studied. After 6 days cultivation, the removal rates of total nitrogen (TN) and orthophosphate (PO(4)(3-)) using CO(2) regulation were better than those using HAc. The removal rates of PO(4)(3-) and TN were 97.5% and 51.7%, respectively using CO(2) regulation; 79.6% (TN) and 42.0% (PO(4)(3-)) were obtained using HAc regulation. Higher biomass, protein, sugar content, and stable pH control were found using CO(2) regulation. However, significantly higher lipid content (31.8%) was observed using HAc regulation. The dominant differences of fatty acids were the content of C18:1 and C18:3. The growth characteristics and environmental conditions especially during the typical logarithmic phase were also analyzed.

Kinetic study of hydrolysis of xylan and agricultural wastes with hot liquid water.

We investigated the kinetics of hot liquid water (HLW) hydrolysis over a 60-min period using a self-designed setup. The reaction was performed within the range 160-220 degrees C, under reaction conditions of 4.0 MPa, a 1:20 solid:liquid ratio (g/mL), at 500 rpm stirring speed. Xylan was chosen as a model compound for hemicelluloses, and two kinds of agricultural wastes-rice straw and palm shell-were used as typical feedstocks representative of herbaceous and woody biomasses, respectively. The hydrolysis reactions for the three kinds of materials followed a first-order sequential kinetic model, and the hydrolysis activation energies were 65.58 kJ/mol for xylan, 68.76 kJ/mol for rice straw, and 95.19 kJ/mol for palm shell. The activation energies of sugar degradation were 147.21 kJ/mol for xylan, 47.08 kJ/mol for rice straw and 79.74 kJ/mol for palm shell. These differences may be due to differences in the composition and construction of the three kinds of materials. In order to reduce the decomposition of sugars, the hydrolysis time of biomasses such as rice straw and palm shell should be strictly controlled.

Luxury uptake of phosphorus changes the accumulation of starch and lipid in Chlorella sp. under nitrogen depletion

The aim of this research was to study the effect of phosphorus supply on starch and lipid production under nitrogen starvation using Chlorella sp. as a model. High phosphate level had marginal effect on cell density but increased biomass growth. Massive phosphorus was assimilated quickly and mainly stored in the form of polyphosphate. The algal cells ceased phosphorus uptake when intracellular phosphorus reached a certain level. 5mM phosphate in the culture rendered a 16.7% decrease of starch synthesis and a 22.4% increase of lipid synthesis relative to low phosphate (0.17 mM). It is plausible that phosphate can regulate carbon partitioning between starch and lipid synthesis pathway by influencing ADP-glucose pyrophosphorylase activity. Moreover, high phosphate concentration enhanced the abundance of oleic acid, improving oil quality for biodiesel production. It is a promising cultivation strategy by integration of phosphorus removal from wastewater with biodiesel production for this alga.

Characterization of lipid and fatty acids composition of Chlorella zofingiensis in response to nitrogen starvation.

Cellular biochemical composition of the microalga Chlorella zofingiensis was studied under favorable and nitrogen starvation conditions, with special emphasis on lipid classes and fatty acids distribution. When algal cells were grown in nitrogen-free medium (N stress), the increase in the contents of lipid and carbohydrate while a decrease in protein content was detected. Glycolipids were the major lipid fraction (50.7% of total lipids) under control condition, while neutral lipids increased to be predominant (86.7% of total lipids) under N stress condition. Triacylglycerol (TAG) content in N stressed cells was 27.3% dw, which was over three times higher than that obtained under control condition. Within neutral lipids fraction, monounsaturated fatty acids (MUFA) were the main group (40.6%) upon N stress, in which oleic acid was the most representative fatty acids (34.5%). Contrarily, glycolipids and phospholipids showed a higher percentage of polyunsaturated fatty acids (PUFA). Lipid quality assessment indicated the potential of this alga as a biodiesel feedstock when its neutral lipids were a principal lipid fraction. The results demonstrate that the neutral lipids content is key to determine the suitability of the microalga for biodiesel, and the stress cultivation is essential for lipid quality.

Growth and lipid accumulation characteristics of Scenedesmus obliquus in semi-continuous cultivation outdoors for biodiesel feedstock production.

In an effort to identify suitable microalgal species for biodiesel production, seven species were isolated from various habitats and their growth characteristics were compared. The results demonstrated that a green alga Scenedesmus obliquus could grow more rapidly and synthesize more lipids than other six microalgal strains. S. obliquus grew well both indoors and outdoors, and reached higher μmax indoors than that outdoors. However, the cells achieved higher dry weight (4.36 g L(-1)), lipid content (49.6%) and productivity (183 mg L(-1) day(-1)) outdoors than in indoor cultures. During the 61 days semi-continuous cultivation outdoors, high biomass productivities (450-550 mg L(-1) day(-1)) and μmax (1.05-1.44 day(-1)) were obtained. The cells could also achieve high lipid productivities (151-193 mg L(-1) day(-1)). These results indicated that S. obliquus was promising for lipids production in semi-continuous cultivation outdoors.

Biomass measurement of microalgae cultivated under photoautotrophic conditions for biofuels.

Linear regression equations for dry cell weight versus optical density (OD680) for 16 strains of microalgae were developed. Carbon contents of the green algae and diatoms were between 40.56–49.62% and 25.74–41.7%, respectively. The ventilation (especially the addition of CO2) and high light intensity significantly improved the microalgae growth rates and biomass amounts. Dry cell weight of eight strains up to 2.119–3.452 g·L−1 were achieved when ambient air containing 5% CO2 (v/v) and high light intensity were applied. The microalgal lipids contained fatty acid methyl esters mainly with C16:0, C18:1, C18:2, and C18:3, which were considered suitable for producing biodiesel.

Process Analysis of Alkaline Flocculation Harvesting for Chaetoceros muelleri and Scenedesmus quadricauda

Alkaline flocculation could be an attractive microalgae harvesting method, because it is low-cost, low energy consumption, and non-toxic to microalgal cells, and the high pH effectively sterilizes the microalgal biomass as well as the process water. In this paper, the mechanism of alkaline flocculation process and the difference between the two strains marine diatom Chaetoceros muelleri #862 and freshwater algae Scenedesmus quadricauda #507 were analyzed. The particle size of C. muelleri #862 cells increased linearly with the increasing pH value which increased dramatically by nearly fivefolds after alkaline flocculation. When pH was below 10.5, the absolute value of zeta potential on the surface of C. muelleri #862 increased rapidly, and also the conductivity of the solution declined quickly. The alkaline flocculation of C. muelleri #862 was mainly attributed to the production of Mg(OH)2 rather than calcium phosphate or calcium hydroxide. For the strain S. quadricauda #507, the cells still existed integrally as normal cells after alkaline flocculation. Firstly, with the increasing of pH in the solution, the absolute value of zeta potential had not declined. When pH was around 10.8, the absolute value of zeta potential had declined abruptly and the solution conductivity was increasing all the time. The cell wall of S. quadricauda #507 with the high content of cellulose Iα and low crystallization degree was good for the next step of saccharification or oil extraction.

Water-saving analysis on an effective water reuse system in biodiesel feedstock production based on Chlorella zofingiensis fed-batch cultivation

The micralgae-based biofuel obtained from dairy wastewater (DWW) is considered a promising source of energy. However, this process consumes water due to the concentration of wastewater being normally too high for some micoralgae cultivation, and dilution is always needed. In this work, the cultivation of microalgae has been examined in non-recirculated water (NR) and recirculated water systems (R). The growth of Chlorella zofingiensis and the nutrient removal of DWW have been recorded. The comparison indicates the R had a little more advantage in biomass and lipid output (1.55, 0.22 g, respectively) than the NR (1.51, 0.20 g, respectively). However, the total chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) removals of the R were lower than those of the NR system during the culture. The highest removal of total COD, TKN, and TP were 85.05%, 93.64%, and 98.45%, respectively. Furthermore, no significant difference has been observed in the higher heating value and lipid content of the biomass of the R and NR. The results show the R can save 30% of the total water input during the culture. All above results indicate the R system has great potential in industry.

Culture of four microalgal strains for bioenergy production and nutrient removal in the meliorative municipal wastewater.

ABSTRACT Microalgae have been considered as the most promising sources of alternative bioenergy. For the purpose of saving costs, the present work focused on the potential use of microalgae in the meliorative municipal wastewater, which contains 90% municipal wastewater and 10% dairy wash wastewater. Four microalgal species, Palmellococcus miniatus, Neochloris oleoabundans, Scenedesmus quadricanda #507, and Chlorella zofingiensis, were cultured in pure municipal wastewater and meliorative municipal wastewater, respectively, for 5 days. Their biomass accumulation and removal rates of nitrogen and phosphate were measured. Results showed that the growth rates of Neochloris oleoabundans, Palmellococcus miniatus, and Chlorella zofingiensis in meliorative municipal wastewater (>0.8 g·L–1·d–1) were significantly higher than that in municipal wastewater (2.6 g·L–1·d–1), while there was no significant difference between the growth rates of Scenedesmus quadricanda #507 in meliorative municipal wastewater and in municipal wastewater. Neochloris oleoabundans exhibited the highest growth rate (0.86 g·L–1·d–1) and relatively high nutrient removal capacity. Scenedesmus quadricanda #507 had the highest P removal rate of over 94%. The four species have a similar N removal rate at about 90%. The results showed that the highest average removal rate of N and P were about 23.1 mg·L–1·d–1 and 7.1 mg·L–1·d–1. Furthermore, the content of lipid or carbohydrates increased and a different profile of fatty acids were found compared to those in municipal wastewater. Cellular components changes of microalgae in meliorative municipal wastewater were favorable as raw materials for bioethanol and biodiesel production. Cultivation with meliorative municipal wastewater is a win-win culture mode that facilitates the biomass production, lipid and carbohydrate accumulation, and wastewater purification.