Increased pigment and lipid content,lipid variety,and cell and population size of the microalgae Chlorella spp. when co-immobilized in alginate beads with the microalgae-growth-promoting bacterium Azospirillum brasilense

Three strains of the freshwater microalgae used for wastewater treatment, Chlorella vulgaris and Chlorella sorokiniana co-immobilized separately in alginate beads with the microalgae-growth-promoting bacterium Azospirillum brasilense Cd, resulted in significant changes in microalgal-population size, cell size, cell cytology, pigment, lipid content, and the variety of fatty acids produced in comparison with microalgae immobilized in alginate without the bacterium. Cells of C. vulgaris UTEX 2714 did not change in size, but the population size within the beads significantly increased. On the other hand, C. vulgaris UTEX 395 cells grew 62% larger, but their numbers did not increase. The population of C. sorokiniana UTEX 1602 increased, but not their cell size. The content of pigments chlorophyll a and b, lutein, and violoaxanthin increased in all microalgal species. The lipid content also significantly increased in all three strains, and the number of different fatty acids in the microalgae increased from four to eight. This study indicates that the microalgae-growth-promoting bacterium induced significant changes in the metabolism of the microalgae.     

Increased growth of the microalga Chlorella vulgaris when coimmobilized and cocultured in alginate beads with the plant-growth-promoting bacterium Azospirillum brasilense

Coimmobilization of the freshwater microalga Chlorella vulgaris and the plant-growth-promoting bacterium Azospirillum brasilense in small alginate beads resulted in a significantly increased growth of the microalga. Dry and fresh weight, total number of cells, size of the microalgal clusters (colonies) within the bead, number of microalgal cells per cluster, and the levels of microalgal pigments significantly increased. Light microscopy revealed that both microorganisms colonized the same cavities inside the beads, though the microalgae tended to concentrate in the more aerated periphery while the bacteria colonized the entire bead. The effect of indole-3-acetic acid addition to microalgal culture prior to immobilization of microorganisms in alginate beads partially imitated the effect of A. brasilense. We propose that coimmobilization of microalgae and plant-growth-promoting bacteria is an effective means of increasing microalgal populations within confined environments.     

微藻在废水处理和生物质回收再利用方面的研究进展

随着经济的发展和人口的增加,环境污染和水资源短缺已经成为不可避免的全球性问题。基于微藻的废水处理技术不仅可以净化废水、解决环境污染问题,还可以利用废水中的营养元素合成生物质,现如今这种技术已经受到越来越多的关注。为了进一步提高废水处理效果、降低废水处理成本,有必要了解微藻去除废水中营养物质和污染物的机理,开发下游低成本收获技术,提升微藻高价值副产物的生产。本文综述了微藻去除碳、氮、磷、重金属、抗生素和有机物的机理和影响因素,总结了微藻的不同收获方式和微藻生物质在各个领域的应用。最后,分析了不同微藻共培养体系和微藻固定化技术的优缺点,并展望了微藻废水处理技术未来的发展方向。     

Association with an ammonium-excreting bacterium allows diazotrophic culture of oil-rich eukaryotic microalgae

Concerns regarding the depletion of the world's reserves of oil and global climate change have promoted an intensification of research and development toward the production of biofuels and other alternative sources of energy during the last years. There is currently much interest in developing the technology for third-generation biofuels from microalgal biomass mainly because of its potential for high yields and reduced land use changes in comparison with biofuels derived from plant feedstocks. Regardless of the nature of the feedstock, the use of fertilizers, especially nitrogen, entails a potential economic and environmental drawback for the sustainability of biofuel production. In this work, we have studied the possibility of nitrogen biofertilization by diazotrophic bacteria applied to cultured microalgae as a promising feedstock for next-generation biofuels. We have obtained an Azotobacter vinelandii mutant strain that accumulates several times more ammonium in culture medium than wild-type cells. The ammonium excreted by the mutant cells is bioavailable to promote the growth of nondiazotrophic microalgae. Moreover, this synthetic symbiosis was able to produce an oil-rich microalgal biomass using both carbon and nitrogen from the air. This work provides a proof of concept that artificial symbiosis may be considered an alternative strategy for the low-N-intensive cultivation of microalgae for the sustainable production of next-generation biofuels and other bioproducts.     

The effect of physicochemical conditions and nutrient sources on maximizing the growth and lipid productivity of green microalgae

Biodiesel is a renewable fuel produced mostly from edible and non‐edible vegetables, by transesterification of neutral lipids (triacylglycerols). However, vegetable oil‐based biodiesel production competes with food crops for arable land, increasing food prices and leading to biodiversity loss. The production of biodiesel from oleaginous microorganisms – particularly microalgae – has attracted attention due to the higher lipid productivity of these organisms, when compared with vegetables. Several environmental factors – including light, temperature, pH and the presence of nutrients (particularly nitrogen, phosphorus and iron) – influence directly the ability of microalgae to produce and store triacylglycerols and other lipids, and also modulate microalgal growth. Although some environmental factors affect several species in a similar manner, differential responses between species are frequent, highlighting the importance of identifying optimal cultivation conditions for each species, to balance growth and lipid productivity for biodiesel production. Here, we reviewed the particular influence of the physicochemical and nutritional factors on the growth and lipid productivity of different green oleaginous microalgae species.     

Optimization of pH induced flocculation of marine and freshwater microalgae via central composite design

Microalgae harvesting via pH induced flocculation along with utilization of recovered medium after flocculation is one of the most economical methods for separating the microalgal biomass in order to reduce the dewatering cost. In this study, optimization of marine and freshwater microalgae flocculation by pH adjustment was investigated via central composite design methodology. One molar of KOH and NaOH solutions were used to increase the pH level of the microalgal culture. Increasing pH value of the medium provided the highest flocculation efficiency up to 92.63 and 86.18% with pH adjusted to 10.5 with KOH and NaOH solutions for marine microalgae Nannochloropsis oculata and freshwater microalgae Chlorella minutissima, respectively. Also, it was revealed that microalgae cells were still alive after flocculation process and their biochemical composition was not changed, and flocculated medium can be used again for the next microalgal production. According to the results, it can be said that this method is cheap and effective, simple to operate and provides the utilization of flocculated medium again.     

Effects of bacterial communities on biofuel-producing microalgae: stimulation,inhibition and harvesting

Despite the great interest in microalgae as a potential source of biofuel to substitute for fossil fuels, little information is available on the effects of bacterial symbionts in mass algal cultivation systems. The bacterial communities associated with microalgae are a crucial factor in the process of microalgal biomass and lipid production and may stimulate or inhibit growth of biofuel-producing microalgae. In addition, we discuss here the potential use of bacteria to harvest biofuel-producing microalgae. We propose that aggregation of microalgae by bacteria to achieve >90% reductions in volume followed by centrifugation could be an economic approach for harvesting of biofuel-producing microalgae. Our aims in this review are to promote understanding of the effects of bacterial communities on microalgae and draw attention to the importance of this topic in the microalgal biofuel field.     

Screening of the culture media with different concentrations of nutrients for cultivation of the microalgae associated with the invertebrates of the White Sea

The growth and biomass accumulation of three microalgal strains of Desmodesmus (Scenedesmaceae, Chlorophyceae), 1Рm66В, 2Cl66E, and 3Dp86Е-1, isolated from the White Sea benthic invertebrates were studied under conditions of batch culture in different standard media (BG-11, Prat, Goldberg, Gromov, Tamiya, artificial seawater) and modified media. The culture condition, biomass accumulation, and uptake of nitrate and phosphate were recorded. A significant alkalization of the culture medium up to pH 10 has been observed during a vigorous growth of the microalgae. The most significant biomass accumulation has been recorded in BG-11 (in complete or modified medium with addition of artificial seawater), Tamiya, and Prat media. Addition of seawater did not affect the growth of Desmodesmus sp. in the nitrate-containing media, although that maintained growth of the microalgae in the nitrogen-lacking media without cell aggregation. The BG-11 medium appears suitable for isolation and cultivation of both symbiotic and free-living microalgae by all the tested features. The Prat medium is the best for maintaining the microalgal strains in living collection.     

Integration of microalgae cultivation with industrial waste remediation for biofuel and bioenergy production: opportunities and limitations

There is currently a renewed interest in developing microalgae as a source of renewable energy and fuel. Microalgae hold great potential as a source of biomass for the production of energy and fungible liquid transportation fuels. However, the technologies required for large-scale cultivation, processing, and conversion of microalgal biomass to energy products are underdeveloped. Microalgae offer several advantages over traditional 'first-generation' biofuels crops like corn: these include superior biomass productivity, the ability to grow on poor-quality land unsuitable for agriculture, and the potential for sustainable growth by extracting macro- and micronutrients from wastewater and industrial flue-stack emissions. Integrating microalgal cultivation with municipal wastewater treatment and industrial CO(2) emissions from coal-fired power plants is a potential strategy to produce large quantities of biomass, and represents an opportunity to develop, test, and optimize the necessary technologies to make microalgal biofuels more cost-effective and efficient. However, many constraints on the eventual deployment of this technology must be taken into consideration and mitigating strategies developed before large scale microalgal cultivation can become a reality. As a strategy for CO(2) biomitigation from industrial point source emitters, microalgal cultivation can be limited by the availability of land, light, and other nutrients like N and P. Effective removal of N and P from municipal wastewater is limited by the processing capacity of available microalgal cultivation systems. Strategies to mitigate against the constraints are discussed.     

Effects of shear stress on microalgae – A review

Cultivation of microalgae requires consideration of shear stress, which is generated by operations such as mixing, circulation, aeration and pumping that are designed to facilitate mass and heat transfer as well as light distribution in cultures. Excessive shear stress can cause increased cell mortality, decreased growth rate and cell viability, or even cell lysis. This review examines the sources of shear stress in different cultivation systems, shear stress tolerance of different microalgal species and the physiological factors and environmental conditions that may affect shear sensitivity, and potential approaches to mitigate the detrimental effects of shear stress. In general, green algae have the greatest tolerance to shear stress, followed by cyanobacteria, haptophytes, red algae, and diatoms, with dinoflagellates comprising the most shear-sensitive species. The shear-sensitivity of microalgae is determined primarily by cell wall strength, cell morphology and the presence of flagella. Turbulence, eddy size, and viscosity are the most prominent parameters affecting shear stress to microalgal cells during cultivation.     

微藻废水生物处理技术研究进展

微藻因生长速率快、细胞脂质含量高及具有生物隔离二氧化碳能力,已作为新一代生物质能源受到广泛关注.然而,投入大量淡水资源并需在生长期间持续提供营养物质已成为规模化培育微藻的主要障碍.将微藻培育系统与废水处理相结合是经济可行的污水资源化方案.基于微藻生长期间对氮磷等营养物质的利用机制,本文综述了微藻在各类废水生物处理过程中的应用情况,着重分析了其对废水中有机与无机化合物、重金属以及病原体的去除或抑制能力.同时,考察了废水初始营养物浓度、光照、温度、pH与盐度以及气体交换量等环境因素对微藻生长代谢的影响.此外,结合微藻规模化应用所面临的问题,对微藻废水处理技术的应用前景及发展方向进行了展望,旨在为水生态系统的建设与管理提供参考.     

Heavy metal sorption by microalgae

Viable microalgae are known to be able to accumulate heavy metals (bioaccumulation). Against a background of the increasing environmental risks caused by heavy metals, the microalgae Chlorella vulgaris and Spirulina platensis and their potential for the biological removal of heavy metals from aqueous solutions were taken as an example for investigation. Small-scale cultivation tests (50 1) with Cd-resistant cells of Chlorella vulgaris have shown that approx. 40% of the added 10 mg Cd/l was removed from the solution within seven days. At this heavy metal concentration sensitive cells died. Non-viable microalgae are able to eliminate heavy metal ions in a short time by biosorption in uncomplicated systems, without any toxicity problems. Compared with original biomasses, the sorption capacity of microalgal by-products changes only insignificantly. Their low price makes them economical.     

Progress in physicochemical parameters of microalgae cultivation for biofuel production

Microalgae have been exploited for biofuel generation in the current era due to its enormous energy content, fast cellular growth rate, inexpensive culture approaches, accumulation of inorganic compounds, and CO₂ sequestration. Currently, research is ongoing towards the advancement of the microalgae cultivation parameters to enhance the biomass yield. The main objective of this study was to delineate the progress of physicochemical parameters for microalgae cultivation such as gaseous transfer, mixing, light demand, temperature, pH, nutrients and the culture period. This review demonstrates the latest research trends on mass transfer coefficient of different microalgae culturing reactors, gas velocity optimization, light intensity, retention time, and radiance effects on microalgae cellular growth, temperature impact on chlorophyll production, and nutrient dosage ratios for cellulosic metabolism to avoid nutrient deprivation. Besides that, cultivation approaches for microalgae associated with mathematical modeling for different parameters, mechanisms of microalgal growth rate and doubling time have been elaborately described. Along with that, this review also documents potential lipid-carbohydrate-protein enriched microalgae candidates for biofuel, biomass productivity, and different cultivation conditions including open-pond cultivation, closed-loop cultivation, and photobioreactors. Various photobioreactor types, the microalgae strain, productivity, advantages, and limitations were tabulated. In line with microalgae cultivation, this study also outlines in detail numerous biofuels from microalgae.     

Bisource carbon dioxide feeding of the photosynthetic process of microalgae autotrophic cultivation

The mechanism of carbon dioxide saturation of alkaline nutrient medium for the purpose of autotrophic cultivation of microalgae is studied. Carbon dioxide desorption from aqueous solutions of sodium hydrocarbonate is investigated and the influence of chemical equilibria in the solutions on the cultivation of carbon dioxide supply is discussed. It is pointed out that these solutions can be considered to be carriers of the assimilable carbon source for the microalgae cultivation, most efficient at pH 8.5. A comparative estimation of the carbon dioxide and the hydrocarbonic ion as assimilable components of the chemical equilibria under study was done, leading to the conclusion that the more probable component in an alkaline medium autotrophic microalgae cultivation is carbon dioxide. A scheme of bisource feeding of the photosynthetic process, with carbon dioxide for the autotrophic microalgae cultivation in a alkaline nutrient medium, providing a stable and economical cultivation process, is proposed.     

Enhanced accumulation of starch and total carbohydrates in alginate-immobilized Chlorella spp. induced by Azospirillum brasilense: II. Heterotrophic conditions

The effect of the bacterium Azospirillum brasilense jointly immobilized with Chlorella vulgaris or C. sorokiniana in alginate beads on total carbohydrates and starch was studied under dark and heterotrophic conditions for 144h in synthetic growth medium supplemented with either d-glucose or Na-acetate as carbon sources. In all treatments, enhanced total carbohydrates and starch content per culture and per cell was obtained after 24h; only jointly immobilized C. vulgaris growing on d-glucose significantly increased total carbohydrates and starch content after 96h. Enhanced accumulation of carbohydrate and starch under jointly immobilized conditions was variable with time of sampling and substrate used. Similar results occurred when the microalgae was immobilized alone. In both microalgae growing on either carbon sources, the bacterium promoted accumulation of carbohydrates and starch; when the microalgae were immobilized alone, they used the carbon sources for cell multiplication. In jointly immobilized conditions with Chlorella spp., affinity to carbon source and volumetric productivity and yield were higher than when Chlorella spp. were immobilized alone; however, the growth rate was higher in microalgae immobilized alone. This study demonstrates that under heterotrophic conditions, A. brasilense promotes the accumulation of carbohydrates in two strains Chlorella spp. under certain time-substrate combinations, producing mainly starch. As such, this bacterium is a biological factor that can change the composition of compounds in microalgae in dark, heterotrophic conditions.     

Heterotrophic cultivation of microalgae for production of biodiesel

High cell density cultivation of microalgae via heterotrophic growth mechanism could effectively address the issues of low productivity and operational constraints presently affecting the solar driven biodiesel production. This paper reviews the progress made so far in the development of commercial-scale heterotrophic microalgae cultivation processes. The review also discusses on patentable concepts and innovations disclosed in the past four years with regards to new approaches to microalgal cultivation technique, improvisation on the process flow designs to economically produced biodiesel and genetic manipulation to confer desirable traits leading to much valued high lipid-bearing microalgae strains.     

Heterotrophic production of eicosapentaenoic acid by microalgae

Eicosapentaenoic acid (EPA) is an omega-3 polyunsaturated fatty acid that plays an important role in the regulation of biological functions and prevention and treatment of a number of human diseases such as heart and inflammatory diseases. As fish oil fails to meet the increasing demand for purified EPA, alternative sources are being sought. Microalgae contain large quantities of high-quality EPA and they are considered a potential source of this important fatty acid. Some microalgae can be grown heterotrophically on cheap organic substrate without light. This mode of cultivation can be well controlled and provides the possibility to maximize EPA production on a large scale. Numerous strategies have been investigated for commercial production of EPA by microalgae. These include screening of high EPA-yielding microalgal strains, improvement of strains by genetic manipulation, optimization of culture conditions, and development of efficient cultivation systems. This paper reviews recent advances in heterotrophic production of EPA by microalgae with an emphasis on the use of diatoms as producing organisms.     

Effect of ammonium load on the growth of attached microalgae in the northern Baltic Sea

The effect of ammonium discharge from a food factory on the growth of attached microalgae was monitored north of the Hanko peninsula, on the southwestern coast of Finland. The impact of the discharge was studied at twelve localities, at four stages of seasonal succession. The microalgae were sampled from glass slides exposed at 0.4 m depth for two weeks. The variables measured for the microalgal growth were chlorophylla, primary production and total organic carbon (TOC). These were compared with planktonic chlorophylla and nutrient concentrations. The growth of attached microalgae displayed a consistent pattern of spatial distribution. Depending on season, TOC and primary production values were 7 to 70 times higher and chlorophylla values up to 1000 times higher close to the effluent outlet than in undisturbed areas of the archipelago. The microalgal samples near the discharge were characterized by low TOC/chlorophylla and TOC/primary production ratios. The temporal consistency of microalgal distribution illustrates the advantages of using attached algal assemblages in monitoring programmes.     

微藻三酰甘油合成途径及其最新研究进展

三酰甘油(triacylglycerols,TAGs)是动物、植物、微生物和微藻细胞主要的储藏性脂类,它可应用于食品、轻工业和生物燃料等方面,是一种新型可再生能源——生物柴油生产的重要原料。与高等油料作物相比,微藻具有光合作用效率高、生长速度快、油脂产量高、不占用农业耕地和适应多种生长环境等优势,是一种潜在的新型生物柴油生产原料。然而,目前人们对有机体,尤其是微藻细胞内TAG合成与积累的分子机制及细胞的代谢调控机制还知之甚少。对TAG合成的一系列重要过程,包括脂肪酸的合成,TAG生物合成的主要途径和旁路途径,以及与TAG合成相关的关键酶和重要基因等进行了综述,特别对微藻细胞中与TAG合成相关的关键基因的最新研究进展进行了总结,旨在更好地了解油脂代谢的调控途径,为最大限度地供应生物柴油的生产原料提供理论基础。     

Integration of biological waste conversion and wastewater treatment plants by microalgae cultivation

In this study, growth performance and lipid content of two microalgae species Neochloris oleoabundans and Chlorella vulgaris are monitored by using three different types of sludge waste feedstocks obtained from the water treatment plants located in Bedonia, Borgotaro and Fornovo (Montagna2000 Spa, Province of Parma, Italy). The sludge waste is optimized in order to achieve microalgal growth media and dispose of the sewage sludge produced at the wastewater treatment facilities. Both photoautotrophic and heterotrophic growth conditions are applied to the microalgal cultivations. The growth parameters of microalgae strains such as cell concentration, growth rate, optical density, cell biovolume, photosynthetic pigments and lipid contents are monitored. The amounts of total dried lipid biomass, obtained by the biological conversion of the wet sludge waste, are determined. Lipid production of microalgal cells grown in the medium optimized from sludge waste from the Fornovo site provides the highest amount of microalgal lipid content for N. oleoabundans and C. vulgaris photoautotrophic cultivations, while sludge waste from the Bedonia site provides for N. oleoabundans heterotrophic cultivation.