Optimization of growth conditions and fatty acid analysis for three freshwater diatom isolates

Diatoms are a group of highly abundant and diverse aquatic algae species. They contain high lipid content along with many bioactive compounds that can be exploited for biotechnological applications. Despite these attractive attributes, diatoms are underrepresented in production projects due to difficulties in their cultivation. To optimize the growth of three freshwater diatom isolates, Cyclotella sp., Synedra sp. and Navicula sp., an orthogonal assay on N, P, Si and Fe, as well as temperature and pH, was designed using traditional single‐factor tests. We also studied the effect of using nanosilica as an alternate Si source on growth and found that the diatom isolates studied achieved their highest growth rates under different combinations of nutrient and environmental conditions. Silica had the greatest influence on growth, followed by phosphate and iron. The optimized growth conditions for Synedra sp. were N: 30 mg L^?1, P: 3 mg L^?1, Si: 14.8 mg L^?1, Fe: 0.448 mg L^?1, temperature 25°C and pH 8. For Navicula sp.: N: 20 mg L^?1, P: 2.5 mg L^?1, Si: 19.7 mg L^?1, Fe: 0.112 mg L^?1, temperature 30°C and pH 7.5–8. For Cyclotella sp.: N: 20 mg L^?1, P: 2.5 mg L^?1, Si: 19.7 mg L^?1, Fe: 0.448 mg L^?1, temperature 30°C and pH 7.5–8. Nano silica negatively affected growth in Navicula sp. and Cyclotella sp., but no such effect was observed in Synedra sp. Fatty acid profiling showed C16:0, C16:1(n ? 7), C18:0 and C20:5(n ? 3) as major fatty acids, with no significant differences in fatty acid methyl ester profiles between traditional and modified media. This work gives us a new insight into the growth requirements of freshwater diatom species, which are less studied than marine species.     

Effect of nitrate on lipid production by T. suecica, M. contortum, and C. minutissima

Microalgae are an alternative and sustainable source of lipids that can be used as a feedstock for biodiesel production. Nitrate is a good nitrogen source for many microalgae and affects biomass and lipid yields of microalgae. In this study, the effect of nitrate on cell growth and lipid production and composition in Monoraphidium contortum, Tetraselmis suecica, and Chlorella minutissima was investigated. Nitrate affected the production of biomass and the production and composition of lipids of the three microalgae tested. Increasing the nitrate concentration in the culture medium resulted in increased biomass production and higher biomass productivity. Furthermore, increasing the nitrate concentration resulted in a reduction in lipid content and productivity in M. contortum; however, the opposite effect was observed in T. suecica and C. minutissima cultures. C. minutissima and M. contortum lipids contain high levels of oleic acid, with values ranging from 26 to 45.7% and 36.4 to 40.1%, respectively. The data suggest that because of its high lipid productivity (13.79 mg L^?1 d^?1) and high oleic acid productivity (3.78 mg L^?1 d^?1), Chlorella minutissima is a potential candidate for the production of high quality biodiesel.     

Culture of microalgae Chlorella minutissima for biodiesel feedstock production

Microalgae are among the most promising of non‐food based biomass fuel feedstock alternatives. Algal biofuels production is challenged by limited oil content, growth rate, and economical cultivation. To develop the optimum cultivation conditions for increasing biofuels feedstock production, the effect of light source, light intensity, photoperiod, and nitrogen starvation on the growth rate, cell density, and lipid content of Chlorella minutissima were studied. The fatty acid content and composition of Chlorella minutissima were also investigated under the above conditions. Fluorescent lights were more effective than red or white light‐emitting diodes for algal growth. Increasing light intensity resulted in more rapid algal growth, while increasing the period of light also significantly increased biomass productivity. Our results showed that the lipid and triacylglycerol content were increased under N starvation conditions. Thus, a two‐phase strategy with an initial nutrient‐sufficient reactor followed by a nutrient deprivation strategy could likely balance the desire for rapid and high biomass generation (124 mg/L) with a high oil content (50%) of Chlorella minutissima to maximize the total amount of oil produced for biodiesel production. Moreover, methyl palmitate (C16:0), methyl oleate (C18:1), methyl linoleate (C18:2), and methyl linolenate (C18:3) are the major components of Chlorella minutissima derived FAME, and choice of light source, intensity, and N starvation impacted the FAME composition of Chlorella minutissima. The optimized cultivation conditions resulted in higher growth rate, cell density, and oil content, making Chlorella minutissima a potentially suitable organism for biodiesel feedstock production. Biotechnol. Bioeng. 2011;108: 2280–2287. © 2011 Wiley Periodicals, Inc.     

Cultivation of Chlorella pyrenoidosa in soybean processing wastewater

Chlorella pyrenoidosa was cultivated in soybean processing wastewater (SPW) in batch and fed-batch cultures without a supply of additional nutrients. The alga was able to remove 77.8 ± 5.7%, 88.8 ± 1.0%, 89.1 ± 0.6% and 70.3 ± 11.4% of soluble chemical oxygen demand (SCOD_Cr), total nitrogen (TN), NH₄⁺-N and total phosphate (TP), respectively, after 120 h in fed-batch culture. C. pyrenoidosa attained an average biomass productivity of 0.64 g L⁻¹ d⁻¹, an average lipid content of 37.00 ± 9.34%, and a high lipid productivity of 0.40 g L⁻¹ d⁻¹. Therefore, cultivation of C. pyrenoidosa in SPW could yield cleaner water and useful biomass.     

Rheological properties of <Emphasis Type="Italic">Chlorella pyrenoidosa</Emphasis> culture grown heterotrophically in a fermentor

Rheological properties of Chlorella pyrenoidosa culture grown heterotrophically in a 14 L fermentor were investigated. It was found that the fluid viscosity was rather low and remained almost unchanged during the cultivation, implying that no (or very few) viscous substances were excreted into the medium. Investigation of the condensed suspension of C. pyrenoidosa showed that for biomass concentration under 150 g.L⁻¹, the suspension of C. pyrenoidosa exhibited Newtonian behavior, and the fluid viscosity was rather low (about 40 mPa·s) and increased very slowly with the increase in cell concentration. With further increase in biomass concentration however, the fluid rheological behavior changed to non-Newtonian, and the fluid viscosity increased rapidly with the increase in cell concentration. From the viewpoint of rheology, C. pyrenoidosa is an excellent organism for high-cell-density culture, and there will be no rheological problems at cell densities under 150 g.L⁻¹.     

Manipulation of nitrogen levels and mode of cultivation are viable methods to improve the lipid,fatty acids,phytochemical content,and bioactivities in Chlorella minutissima

The effect of nitrogen (N: low = 2% N or moderate = 10% N) levels and cultivation (photoautotrophic or mixotrophic) modes on the biochemicals in Chlorella minutissima was evaluated using a mass culture system. Moderate N and mixotrophic cultures had higher biomass, protein, carbohydrate content and photosynthetic pigments than the low N and photoautotrophic treatments. In contrast, lipid and fatty acid content of the low N and photoautotrophic treatments were higher than in the moderate N and mixotrophic cultures. More phytochemicals were accumulated in moderate N and mixotrophic cultures which corresponded to better antioxidant capacity in the extracts. The most potent (0.7 mg · mL^?1) acetylcholinesterase inhibitory activity was displayed by moderate N and mixotrophic treatment. Approximately 60% of the extracts exhibited a noteworthy antimicrobial activity regardless of the N levels and cultivation modes. Thus, moderate N level enhanced the phytochemicals and biological activities of C. minutissima cultured under a mixotrophic system.     

Investigation of biomass concentration,lipid production,and cellulose content in Chlorella vulgaris cultures using response surface methodology

The microalgae Chlorella vulgaris produce lipids that after extraction from cells can be converted into biodiesel. However, these lipids cannot be efficiently extracted from cells due to the presence of the microalgae cell wall, which acts as a barrier for lipid removal when traditional extraction methods are employed. Therefore, a microalgae system with high lipid productivity and thinner cell walls could be more suitable for lipid production from microalgae. This study addresses the effect of culture conditions, specifically carbon dioxide and sodium nitrate concentrations, on biomass concentration and the ratio of lipid productivity/cellulose content. Optimization of culture conditions was done by response surface methodology. The empirical model for biomass concentration (R² = 96.0%) led to a predicted maximum of 1123.2 mg dw L^?1 when carbon dioxide and sodium nitrate concentrations were 2.33% (v/v) and 5.77 mM, respectively. For lipid productivity/cellulose content ratio (R² = 95.2%) the maximum predicted value was 0.46 (mg lipid L^?1 day^?1)(mg cellulose mg biomass^?1)^?1 when carbon dioxide concentration was 4.02% (v/v) and sodium nitrate concentration was 3.21 mM. A common optimum point for both variables (biomass concentration and lipid productivity/cellulose content ratio) was also found, predicting a biomass concentration of 1119.7 mg dw L^?1 and lipid productivity/cellulose content ratio of 0.44 (mg lipid L^?1 day^?1)(mg cellulose mg biomass^?1)^?1 for culture conditions of 3.77% (v/v) carbon dioxide and 4.01 mM sodium nitrate. The models were experimentally validated and results supported their accuracy. This study shows that it is possible to improve lipid productivity/cellulose content by manipulation of culture conditions, which may be applicable to any scale of bioreactors. Biotechnol. Bioeng. 2013; 110: 2114–2122. © 2013 Wiley Periodicals, Inc.     

Growth model for raceway pond cultivation of Desmodesmus sp. MCC34 isolated from a local water body

Biofuel production by microalgae has the advantage of higher biomass productivity over land crops. The selection of potential microalgae depends on the growth in outdoor mass cultivation during different seasons, which can be predicted by a mathematical model. Here, freshwater green algae were isolated from a local water body in Pilani, Rajasthan, India (geographical coordinates: 28°22′N 75°36′E) and characterized by microscopy and ribosomal RNA analysis. The strain was submitted to the Indian Agricultural Research Institute's microbial culture collection (IARI, India) and identified as Desmodesmus sp. MCC34. This strain, along with a fresh water green algae (Chlorella minutissima), two marine green algae species (Dunaliella salina and Dunaliella tertiolecta) and two nitrogen fixing cyanobacteria (Nostoc muscorum and Anabaena doliolum), were screened for lipid productivity and growth kinetics under culture room and raceway pond conditions. Desmodesmus sp. MCC34 showed the highest specific growth rate (0.26 day^?1), biomass production (1.9 g L^?1) and lipid productivity (103 mg L^?1 day^?1). The optimal temperature and saturating light intensity for maximal growth of Desmodesmus sp. MCC34 were 35 °C and 75 μmol m^?2 s^?1 with molar extinction coefficient of 0.22 m² g^?1, respectively. Desmodesmus sp. MCC34 was then subjected to outdoor cultivation in a 20‐m long raceway pond for 18 days during March and November 2013. The areal biomass productivity and volumetric biomass productivity were 13946.23 kg ha^?1 year^?1 and 56.94 mg L^? 1day^?1 during the month of March, decreasing to 6262.28 kg ha^?1 year^?1 and 25.57 mg L^? 1day^?1 during the month of November. A mathematical model was constructed to explain the relationship between biomass production and growth parameters such as temperature, light intensity and nutrient concentration. The productivity values predicted with the proposed model correspond well with the experimental data, suggesting the validity of the model.     

Heterotrophic production of lutein by selected Chlorella strains

Seven Chlorella strains representing three species obtained from culture collections and research laboratories were screened for their potential of heterotrophic production of lutein on two different media (Basal and Kuhl) containing glucose. While both media supported good growth and lutein formation of the seven strains in darkness, higher biomass concentrations and lutein content were achieved on Basal medium. Chlorella protothecoides CS-41 was chosen from the seven strains for further investigation due to its higher productivities of both biomass and lutein. The maximal biomass concentration and lutein content of C. protothecoides cultivated heterotrophically with 9 g L^-1 glucose in a 3.7-L fermentor were respectively 4.6 g dry cells L^-1 and 4.60 mg lutein g^-1 dry cells on Basal medium, and 4.0 g dry cells L^-1 and 4.36 mg lutein g^-1 dry cells on Kuhl medium. The heterotrophic cultivation process was scaled up successfully to 30 L using a fermentor, in which the Basal medium containing 36 g L^-1 glucose was used; the maximal biomass concentration of 16.4 g dry cells L^-1, specific growth rate of 0.92 d^-1,lutein content of 4.85 mg lutein g^-1 dry cells,growth yield of 0.47 g dry cells g^-1 glucose and lutein yield of 1.93 mg lutein g^-1 glucose were respectively achieved. This revised version was published online in August 2006 with corrections to the Cover Date.     

Monitoring microalgal growth of Chlorella minutissima with a new all solid-state contact nitrate selective sensor

As third generation feedstock, microalgae are microorganisms that can grow only in the optimum conditions. There are parameters including the concentration of macro and microelements in nutrient solution, pH, temperature and light intensity that have significant impact on microalgal growth. In recent years, various sensing devices have been developed for sensitive measurement of these parameters during microalgal growth. In this study, a new potentiometric nitrate selective sensor was developed to indicate the nitrate uptake of microalgae and the effect of nitrate nutrient on microalgal growth, specifically, and this sensor was successfully applied to determine nitrate concentration in medium during microalgal growth. Moreover, the effects of nitrate, carbonate and phosphate concentration in the growth medium on biomass production of Chlorella minutissima were investigated by using Box–Behnken design method, and optimum conditions were determined for the highest biomass production of microalgae. As a result of the experiments, it was seen that the highest C. minutissima production was achieved using the medium consist of 2.63 g/L NaNO₃, 0.35 g/L Na₂CO₃ and 0.4 g/L KH₂PO_4. Statistically, it was observed that there was a proportional relationship between the microalgae production and investigated parameters such as carbon, nitrogen and phosphate amounts of culture mediums. The electrode showed a wide linear range between 1.0 × 10⁻¹ and 5.0 × 10⁻⁵ M with a detection limit of the 5 × 10⁻⁶ M and the response time was found as 10 s. The results showed that developed nitrate selective sensor could be successfully applied for continuous measurement of nitrate in microalgal productions at reduced cost.     

METHYLAMINE UPTAKE IN THE GREEN ALGA CHLORELLA PYRENOIDOSA1

Methylamine uptake in nitrogen-starved Chlorella pyrenoidosa Beij. follows Michaelis-Menten kinetics: maximum uptake is about 1.6 nmol μl^?1· cells · min^?1, half-saturation occurs at 4 μM methylamine, and the slope in the range where uptake is proportional to concentration is 0.4 nmol μl^?1· min^?1·μM^?1. In cells grown in the presence of a non-limiting nitrogen concentration, methylamine uptake is directly proportional to concentration up to at least 0.5 mM, and the slope is 1/500 that for starved cells. Similar uptake kinetics have been reported for Penicillium chrysogenum and attributed to an inducible “ammonium permease.” Apparently, a similar permease occurs in algae.     

Biodegradation of p-nitrophenol by microalgae

A study was made on the use of a mixed microalgal consortium to degrade p-nitrophenol. The consortium was obtained from a microbial community in a waste container fed with the remains and by-products of medium culture containing substituted aromatic pollutants (nitrophenols, chlorophenols, fluorobenzene). After selective enrichment with p-nitrophenol (p-NP), followed by an antibiotic treatment, an axenic microalgal consortium was recovered, which was able to degrade p-nitrophenol. At a concentration of 50 mg L^–1, total degradation occurred within 5 days. Two species, Chlorella vulgaris var. vulgaris f. minuscula and Coenochloris pyrenoidosa, were isolated from the microalgal consortium. The species were able to accomplish p-NP biodegradation when cultured separately, although Coenochloris pyrenoidosa was more efficient, achieving the same degradation rate as the original axenic microalgal consortium. When Coenochloris pyrenoidosa was associated with Chlorella vulgaris in a 3:1 ratio, complete removal of the nitro-aromatic compound occurred within three days. This is apparently the first report on the degradation of a nitro-aromatic compound by microalgae.     

Modeling of lutein production by heterotrophic <Emphasis Type="Italic">Chlorella</Emphasis> in batch and fed-batch cultures

Chlorella is a promising alternative resource of lutein (xanthophyll) production as it can be cultivated heterotrophically in fermentors. In this paper, a kinetic model for lutein production by heterotrophic Chlorella pyrenoidosa was developed based on batch cultivations in 250-ml flasks and a 19-l fermentor. The model was validated by experimental data from two fed-batch cultivations performed in the same fermentor. The dynamic behavior of lutein production by C. pyrenoidosa with various concentrations of glucose and nitrogen was analyzed based on the kinetic model. Model-based analyses suggested that glucose concentrations between 5 and 24 g/l and nitrogen concentrations between 0.7 and 12 g/l during the cultivation were favorable for lutein production by heterotrophic C. pyrenoidosa. It also showed that fed-batch cultivations are more suitable for efficient production of lutein than batch ones. The results obtained in this study may contribute to commercial lutein production by heterotrophic Chlorella.     

Heterotrophic production of Chlorella sp. TISTR 8990—biomass growth and composition under various production conditions

The green microalga Chlorella sp. TISTR 8990 was grown heterotrophically in the dark using various concentrations of a basal glucose medium with a carbon‐to‐nitrogen mass ratio of 29:1. The final biomass concentration and the rate of growth were highest in the fivefold concentrated basal glucose medium (25 g L^?1 glucose, 2.5 g L^?1 KNO₃) in batch operations. Improving oxygen transfer in the culture by increasing the agitation rate and decreasing the culture volume in 500‐mL shake flasks improved growth and glucose utilization. A maximum biomass concentration of nearly 12 g L^?1 was obtained within 4 days at 300 rpm, 30°C, with a glucose utilization of nearly 76% in batch culture. The total fatty acid (TFA) content of the biomass and the TFA productivity were 102 mg g^?1 and 305 mg L^?1 day^?1, respectively. A repeated fed‐batch culture with four cycles of feeding with the fivefold concentrated medium in a 3‐L bioreactor was evaluated for biomass production. The total culture period was 11 days. A maximum biomass concentration of nearly 26 g L^?1 was obtained with a TFA productivity of 223 mg L^?1 day^?1. The final biomass contained (w/w) 13.5% lipids, 20.8% protein and 17.2% starch. Of the fatty acids produced, 52% (w/w) were saturated, 41% were monounsaturated and 7% were polyunsaturated (PUFA). A low content of PUFA in TFA feedstock is required for producing high quality biodiesel. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1589–1600, 2017     

Carbon dioxide biofixation by Chlorella vulgaris at different CO2 concentrations and light intensities

In order to develop an effective CO₂ mitigation process using microalgae for potential industrial application, the growth and physiological activity of Chlorella vulgaris in photobioreactor cultures were studied. C. vulgaris was grown at two CO₂ concentrations (2 and 13% of CO₂ v/v) and at three incident light intensities (50, 120 and 180 μmol m^?2 s^?1) for 9 days. The measured specific growth rate was similar under all conditions tested but an increase in light intensity and CO₂ concentration affected the biomass and cell concentrations. Although carbon limitation was observed at 2% CO₂, similar cellular composition was measured in both conditions. Light limitation induced a net change in the growth behavior of C. vulgaris. Nitrogen limitation seemed to decrease the nitrogen quota of the cells and rise the intracellular carbon:nitrogen ratio. Exopolysaccharide production per cell appeared to be affected by light intensity. In order to avoid underestimation of the CO₂ biofixation rate of the microalgae, exopolysaccharide production was taken into account. The maximum CO₂ removal rate (0.98 g CO₂ L^?1 d^?1) and the highest biomass concentration (4.14 g DW L^?1) were determined at 13% (v/v) CO₂ and 180 μmol m^?2 s^?1. Our results show that C. vulgaris has a real potential for industrial CO₂ remediation.     

自养和兼养条件下蛋白核小球藻昼夜节律的响应

【目的】研究自养和兼养两种培养方式对蛋白核小球藻(Chlorella pyrenoidosa)生长、细胞分裂和生化组分积累的影响,探讨人工培养蛋白核小球藻的昼夜节律响应机制和优化技术。【方法】小球藻自养培养采用BG11培养基,兼养培养基在BG11培养基中添加4种不同浓度(1、5、10、20 g/L)的葡萄糖,培养周期为10 d。血球板计数法测定藻细胞浓度,干重法测定藻细胞生物量。显微观察藻细胞大小和分裂情况。脂染色法测定小球藻总脂的含量,藻细胞的叶绿素、蛋白和淀粉分别采用甲醇、氢氧化钠、硝酸钙浸提后通过紫外分光光度法定量测定。【结果】葡萄糖兼养培养对蛋白核小球藻具有显著的促生长效应,最适浓度为10 g/L。10 d收获时,兼养组(10 g/L葡萄糖)藻细胞浓度和干重分别是自养组的2.57倍和6.73倍。分析一昼夜中的藻细胞增殖规律可知,第2天和第5天时自养组中增殖的新生子细胞约有76.00%在黑暗期分裂产生,而兼养组中第2天和第5天光照期的新细胞增殖量占比分别达到40.90%和67.50%。一昼夜内藻细胞大小的迁移动态监测表明,第2天自养组藻细胞的体积变化静息期为8 h,兼养组只有4 h;第5天两组藻细胞大小迁移动态的昼夜节律明显,但兼养组黑暗结束后较大细胞(D6μm)占比显著高于自养组。第8天时,兼养组藻细胞已处于稳定期,总脂和蛋白含量均显著高于自养组,藻细胞总脂和色素含量在一昼夜中相对稳定,但蛋白和淀粉含量分别在光照8 h和12 h左右达到峰值。从第2天开始,对兼养组细胞每天进行2 h光延长,收获时藻细胞浓度和干重分别比对照组提高13%和11%。【结论】葡萄糖兼养培养能大幅提高蛋白核小球藻的生物量。蛋白核小球藻生长增殖与生化组分积累均受昼夜节律调控,自养条件下藻细胞以光照期生长黑暗期增殖为主。兼养培养提高藻细胞生物量的机制在于缩短藻细胞生长静息期,在昼夜节律中加速藻细胞生长并显著提高通过细胞周期检查点的细胞比例,光照期效应尤其明显。藻细胞蛋白和淀粉含量昼夜节律明显,最佳收获时间分别在光照8 h和12 h后。     

CO2 utilization in the production of biomass and biocompounds by three different microalgae

The atmospheric CO₂ increase is considered the main cause of global warming. Microalgae are photosynthetic microorganisms that can help in CO₂ mitigation and at the same time produce value‐added compounds. In this study, Scenedesmus obliquus , Chlorella vulgaris , and Chlorella protothecoides were cultivated under 0.035 (air), 5 and 10% (v/v) of CO₂ concentrations in air to evaluate the performance of the microalgae in terms of kinetic growth parameters, theoretical CO₂ biofixation rate, and biomass composition. Among the microalgae studied, S. obliquus presented the highest values of specific growth rate (μ = 1.28 d^?1), maximum productivities (P _max = 0.28 g L^?1d^?1), and theoretical CO₂ biofixation rates (0.56 g L^?1d^?1) at 10% CO₂. The highest oil content was found at 5% CO₂, and the fatty acid profile was not influenced by the concentration of CO₂ in the inflow gas mixture and was in compliance with EN 14214, being suitable for biodiesel purposes. The impact of the CO₂ on S. obliquus cells’ viability/cell membrane integrity evaluated by the in‐line flow cytometry is quite innovative and fast, and revealed that 86.4% of the cells were damaged/permeabilized in cultures without the addition of CO₂.     

Effect of carbon dioxide on the rheological behavior of submerged cultures of Chlorella minutissima in stirred tank reactors

The objective of this study was to quantify the effect of algal biomass concentration on the rheology of the algal culture broth. Batch cultivations of Chlorella minutissima were carried out with air and carbon dioxide in a stirred tank bioreactor with a working volume of 1.8 L. The apparent viscosity of the culture broth was significantly affected by the cell mass concentrations in the bioreactor. Culture broth containing 50 g/L cell mass from air fed was found to exhibit an apparent viscosity of 1.52 mPa.s. The apparent viscosity of the carbon‐dioxide‐fed cultivations was found to increase by 20% at a shear rate of 100 s^?1. The flow behavior of the system was adequately described by the Herschel–Bulkley model with a small yield stress.     

Productivity of Chlorella sorokiniana in a short light‐path (SLP) panel photobioreactor under high irradiance

Maximal productivity of a 14 mm light‐path panel photobioreactor under high irradiance was determined. Under continuous illumination of 2,100 µmol photons m^?2 s^?1 with red light emitting diodes (LEDs) the effect of dilution rate on photobioreactor productivity was studied. The light intensity used in this work is similar to the maximal irradiance on a horizontal surface at latitudes lower than 37°. Chlorella sorokiniana, a fast‐growing green microalga, was used as a reference strain in this study. The dilution rate was varied from 0.06 to 0.26 h^?1. The maximal productivity was reached at a dilution rate of 0.24 h^?1, with a value of 7.7 g dw m^?2 h^?1 (m² of illuminated photobioreactor surface) and a volumetric productivity of 0.5 g dw L^?1 h^?1. At this dilution rate the biomass concentration inside the reactor was 2.1 g L^?1 and the photosynthetic efficiency was 1.0 g dw mol photons. This biomass yield on light energy is high but still lower than the theoretical maximal yield of 1.8 g mol photons^?1 which must be related to photosaturation and thermal dissipation of absorbed light energy. Biotechnol. Bioeng. 2009; 104: 352–359 © 2009 Wiley Periodicals, Inc.     

定向驯化对异养蛋白核小球藻硒的耐受性及富集能力的影响

硒(Se)是生物体必不可少的微量元素,硒缺乏会导致人产生克山病、大骨节病等疾病,缺硒也会给畜牧业带来巨大损失。目前的补硒产品存在硒含量和生物利用度低、安全性差等问题,而通过小球藻培养可获得生物利用度高、安全性好的有机硒,因此是非常有应用前景的补硒产品。首先,为了获得对硒的耐受性和富集能力更强的藻种,研究通过定向驯化的方式逐步提高培养基中Na₂SeO₃浓度来驯化蛋白核小球藻,并对驯化时间和驯化过程中Na₂SeO₃的浓度梯度进行了优化。结果表明,驯化后的藻种对硒的耐受性和富集能力明显提高。在5L发酵罐中,驯化后的藻株可以耐受40mg/L的Na₂SeO₃,胞内有机硒合成速率提高了175.6%。之后,在5L发酵罐中进一步优化了硒的补加方式,在异养培养过程中分批补料添加40mg/LNa₂SeO₃时,最终获得的蛋白核小球藻细胞干重达106.4g/L,有机硒含量为1227mg/kg,有机硒合成速率为1.36mg/(L·h)。研究结果与已有蛋白核小球藻异养富硒文献报道的最高细胞密度75g/L和最高有机硒含量560mg/kg相比分别提高了41.9%和119.1%。上述结果表明,通过定向驯化的方法,可大大提高蛋白核小球藻对硒的耐受性和富集能力。