Immobilized microalgae in biotechnology

Here we present a brief account of current data on immobilization of oxygenic phototrophic microorganisms—cyanobacteria and eukaryotic microalgae—in natural and artificial experimental systems. We emphasize that immobilization e.g. in biofilms is a basic, widespread in nature strategy ensuring the survival of microorganisms. Accordingly, the artificially immobilized microalgal cells might be considered as a special group of biomimetic materials. Special attention is paid to the effect(s) of different immobilization on the physiology of microalgal cells and their stress tolerance as well as productivity of microalgal cultures. A comparison of the advantages and drawbacks of different immobilization techniques and cell carriers is presented. The review concludes with outlook on the possibilities of using of the immobilized phototrophic cells in biotechnology. Specific areas include (but not limited to) the biomass and metabolites production and harvesting, removal of heavy metals, biocapture of nutrients from wastewater and destroying of organic pollutants are explored.     

Wechselwirkungen zwischen seltenen Erdmetallen und Mikroorganismen

In recent years there have been a lot of works on the accumulation and removal of heavy metals from aqueous solutions and industrial waste waters by different kinds of microorganisms. A variety of microorganisms are known to be tolerant to mercury, copper, chromium, silver, cadmium etc., or to have a high accumulation capacity for these elements. But nothing is known about the interactions between microorganisms and REE*-ions in aqueous solutions. The objectives of our experiments were to determine the ability of various microorganisms to accumulate REE and to determine what possible effects, in terms of growth, different REE-ion concentrations may have on the bacteria cultures. Experiments on the accumulation of La and Pr by a freeze dried bacteria mixed culture, carried out with model solutions, showed that the accumulation process is completed few minutes after contact. The uptake is independent on pH-value and temperature and amounts up to 63 mg REE/g biomass in both cases. A gram⁺ bacterium spec., isolated from an industrial waste water, accumulates large quantities of REE in dependence on the pre-treatment of the cells, e.g. in the case of resting or freeze dried cells up to 100 mg/g biomass and in the case of heat treated cells up to 40 mg/l.     

Recycling and reuse of spent microalgal biomass for sustainable biofuels

The use of microalgal biomass (MAB) for biofuel production has been recognized since long. Despite distinct advantages of algal biofuels, however, their sustainability and economic viability is still doubtful. Overall process cost and low energy recovery need to be significantly improved. The use of MAB, after extracting primary fuels in the form of hydrogen, methane, biodiesel and bioethanol, can be one promising route. This algal biomass, collectively termed as spent microalgal biomass (SMAB), contains even up to 70% of its initial energy level and also retains nutrients including proteins, carbohydrates, and lipids. Potential application routes include diet for animals and fish, the removal of heavy metals and dyes from wastewater, and the production of bioenergy (e.g., biofuels and electricity). Unlike whole algae biomass whose applications are relatively well documented, SMAB has been studied only to limited degree. Therefore, this work gives a brief overview of various ways of SMAB applications. An insight into current status, barriers and future prospects on SMAB research is provided. The feasibility of each application is evaluated on the basis of its energy recovery, economic viability, and future perspectives are provided.     

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.     

絮凝微生物采收微藻的作用机制研究进展

微藻广泛分布于自然界,其易培养,生长快且应用价值高,普遍用于生物燃料、医学原料、优质食品源及畜牧养殖业等。近年来,通过对光生物反应器改造设计、高产藻株筛选、代谢通路基因改造等方法实现微藻产量的提高,而在微藻处理的下游过程的研究与创新不足,特别是微藻采收已经成为其产业发展的瓶颈。本文综述了絮凝法在微藻采收中的作用,重点讨论了絮凝微生物在微藻采收中的作用,并对絮凝微生物对微藻的絮凝机制进行广泛探讨,为絮凝微生物采收微藻提供理论依据。     

Microalgal responses to physico-chemical variability in the small temporarily open/closed Seteni Estuary,South Africa

The Seteni Estuary is a small temporarily open/closed estuary (TOCE) in South Africa under the influence of agricultural practices. While the general significance of microalgae to estuarine production is widely recognised, the factors regulating microalgal biomass in these heterogeneous systems are less well understood, particularly when man-induced pressures are superimposed on natural variability. This study investigated microalgal responses to physico-chemical variability in the nutrient-enriched Seteni Estuary in 2008–2009. Microphytobenthic biomass ranged from 1.1 to 91.7 mg Chl a m^-2, while phytoplankton biomass varied between 0.22 and 18 mg Chl a m^-3. Despite the high dissolved inorganic nitrogen (DIN) concentrations recorded, salinity and rainfall were identified as the main environmental drivers, highlighting the importance of allochthonous inputs. While this system appears to function as a typical TOCE, the relatively low microalgal biomass recorded may be an artifact of several factors such as phosphorus limitation, herbicide treatment, etc., acting singly or together to depress biomass levels. However, should the balance be upset, the most probable result would be a dramatic increase in microalgal biomass, to the point where harmful algal blooms may ensue. The system therefore needs to be closely monitored to prevent further degradation.     

Characterization of Desmodesmus pleiomorphus isolated from a heavy metal-contaminated site: biosorption of zinc

Microalgae have been proven efficient biological vectors for heavy metal uptake. In order to further study their biosorption potential, a strain of Desmodesmus pleiomorphus (L) was isolated from a strongly contaminated industrial site in Portugal. Under different initial Zn²⁺ concentrations, metal removal by that strain reached a maximum of 360 mg Zn/g biomass after 7 days, at 30 mg Zn/l, after an initial rapid phase of uptake. Comparative studies were carried out using a strain of the same microalgal species that is commercially available (ACOI 561): when exposed to 30 mg Zn/l, it could remove only 81.8 mg Zn/g biomass. Biosorption experiments using inactivated biomass of the isolated strain reached a maximum Zn²⁺ uptake of 103.7 mg/g. Metal removal at various initial pH values was studied as well; higher removal was obtained at pH 5.0. The microalga strain L, isolated from the contaminated site, exhibited a much higher removal capacity than the commercial strain, and the living biomass yielded higher levels of metal removal than its inactivated form.     

Modelling growth of,and removal of Zn and Hg by a wild microalgal consortium

Microorganisms isolated from sites contaminated with heavy metals usually possess a higher removal capacity than strains from regular cultures. Heavy metal-containing soil samples from an industrial dumpsite in Northern Portugal were accordingly collected; following enrichment under metal stress, a consortium of wild microalgae was obtained. Their ability to grow in the presence of, and their capacity to recover heavy metals was comprehensively studied; the datasets thus generated were fitted to by a combined model of biomass growth and metal uptake, derived from first principles. After exposure to 15 and 25 mg/L Zn²⁺ for 6 days, the microalgal consortium reached similar, or higher cell density than the control; however, under 50 and 65 mg/L Zn²⁺, 71% to 84% inhibition was observed. Growth in the presence of Hg²⁺ was significantly inhibited, even at a concentration as low as 25 μg/L, and 90% inhibition was observed above 100 μg/L. The maximum amount of Zn²⁺ removed was 21.3 mg/L, upon exposure to 25 mg/L for 6 day, whereas the maximum removal of Hg²⁺ was 335 μg/L, upon 6 day in the presence of 350 μg/L. The aforementioned mechanistic model was built upon Monod assumptions (including heavy metal inhibition), coupled with Leudeking–Piret relationships between the rates of biomass growth and metal removal. The overall fits were good under all experimental conditions tested, thus conveying a useful tool for rational optimisation of microalga-mediated bioremediation.     

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.     

Bioakkumulation und Biosorption zur Detoxifikation und Entfernung von Metallen

Growing and resting cells of microorganisms are able to accumulate metal ions. These reactions are based on a storage within the cells as well as sorption at the cell wall. The intracellular storage takes place preferably by growing cells. The stored amounts of metals by resting cells depend on the concentration of metal ions in the aqueous phase, the pH-value, and, in some cases on the temperature. Maximum concentrations for some metals and strains of microorganisms are given. The representation of the uptake reaction as an adsorption process using a FREUNDLICH -Isotherm demonstrates straight lines with different clopes. The removal of the metals from the biomass by a desorption reaction is possible in some cases, other possibilities are a reducing process or the formation of an ash of the biomss containing compounds of metals. These reactions may be used simultaneously for detoxification and removal of metals from waste water.     

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.     

Possibilities of bioconversion of agricultural waste with the use of microalgae

A new methodology of biological treatment and conversion of farm waste (manure and wash water) with the use of intensively cultivated phototrophic microorganisms (microalgae) is reviewed. Criteria for selection of microalgae and peculiarities of their intensive cultivation for efficient removal of biogenic elements from and destruction of the organic components of the wastes as well as the possibilities of cost-effective utilization of the resulting microalgal biomass are considered. Advantages and drawbacks of the new methodology are compared with those of conventional anaerobic techniques. Special attention is paid to the integrated technologies combining the aerobic conversion methods with microalgal post-treatment.     

苏云金芽胞杆菌治理镍污染的方法的建立

利用微生物治理重金属污染已经成为一个研究的热点,并被视为将最终替代传统的物理、化学等处理方式的一种方法.但由于一些微生物存在安全性、繁殖速度慢等问题而造成了处理效果不佳.因此,以安全性高、繁殖速度快的苏云金芽胞杆菌(Bacillus thuringiensis,简称Bt)为研究载体,寻找最适Bt的镍污染处理方法对于提高...     

Removal of heavy metals using a brewer's yeast strain of Saccharomyces cerevisiae: the flocculation as a separation process

In this work, a brewer's yeast strain was used to remove heavy metals from a synthetic effluent. The solid-liquid separation process was carried out using the flocculation ability of the strain. The yeast strain was able to sediment in the presence of Cu2+, Ni2+, Zn2+, Cd2+ and Cr3+, which evidences that the flocculation can be used as a cheap and natural separation process for an enlarged range of industrial effluents. For a biomass concentration higher than 0.5 g/l, more than 95% of the cells were settled after 5 min; this fact shows that the auto-aggregation of yeast biomass is a rapid and efficient separation process. Cells inactivated at 45 degrees C maintain the sedimentation characteristics, while cells inactivated at 80 degrees C lose partially (40%) the flocculation. The passage of metal-loaded effluent through a series of sequential batches allowed, after the second batch, the reduction of the Ni2+ concentration in solution for values below the legal limit of discharge of wastewater in natural waters (2mg/l); this procedure corresponds to a removal of 91%. A subsequent batch had a marginal effect on Ni2+ removal (96%). Together, the results obtained suggest that the use of brewing flocculent biomass looks a promising alternative in the bioremediation of metal-loaded industrial effluents since the removal of the heavy metals and cell separation are simultaneously achieved.     

Bioflocculation of microalgae and bacteria combined with flue gas to improve sewage treatment

Although microalgae are promising for a cradle-to-cradle design approach of sewage treatment, their application is hampered by high harvesting costs and low C:N ratios of sewage. Therefore, the potential of microalgal bacterial flocs (MaB-flocs) was investigated for the secondary treatment of sewage supplemented with different flue gas flow rates (FGFRs) from a coal power plant. Effluent (N, P, turbidity and pH) and off gas discharge levels (NO(x), SO(x)) met the European discharge limits with a hydraulic retention time of only 0.67 days and an FGFR of 0.6Lh(-1) (0.0025 vvm). This FGFR provided sufficient carbon and resulted in removal efficiencies of 48 ± 7% CO(2), 87 ± 5% NO(x) and 99 ± 1% SO(2). MaB-flocs settled fast reaching up to a density of 19 g VSSL(-1). High biomass productivities (0.18 gL(-1)day(-1)) were obtained under a low light intensity. This successful reactor performance indicates the large potential for the industrial application of MaB-flocs for flue gas sparged sewage treatment.     

微生物与重金属间的相互作用及其应用研究

从多方面阐述了微生物与重金属二者间相互作用,指出微生物在生长代谢过程中能淋滤、吸收和转化重金属,对重金属有一定的抗性和解毒作用;但是,一定浓度的重金属对微生物过程及其种群具有较大的毒性。影响微生物在环境介质中的活动,矿业工程生产工艺已充分利用微生物能淋滤,吸收和转化重金属等特性来处理低品位难浸矿石,环境保护领域也积极利用微生物对重金属的抗性和解毒作用来实现工业废弃物的处理以及被重金属污染土壤的修复。利用微生物的生物量及其活性可以评价环境中不同介质的重金属污染水平。     

Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review

Microalgae have the ability to mitigate CO₂ emission and produce oil with a high productivity, thereby having the potential for applications in producing the third-generation of biofuels. The key technologies for producing microalgal biofuels include identification of preferable culture conditions for high oil productivity, development of effective and economical microalgae cultivation systems, as well as separation and harvesting of microalgal biomass and oil. This review presents recent advances in microalgal cultivation, photobioreactor design, and harvesting technologies with a focus on microalgal oil (mainly triglycerides) production. The effects of different microalgal metabolisms (i.e., phototrophic, heterotrophic, mixotrophic, and photoheterotrophic growth), cultivation systems (emphasizing the effect of light sources), and biomass harvesting methods (chemical/physical methods) on microalgal biomass and oil production are compared and critically discussed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.     

Phytoremediation of mine tailings in temperate and arid environments

Phytoremediation is an emerging technology for the remediation of mine tailings, a global problem for which conventional remediation technologies are costly. There are two approaches to phytoremediation of mine tailings, phytoextraction and phytostabilization. Phytoextraction involves translocation of heavy metals from mine tailings to the plant shoot biomass followed by plant harvest, while phytostabilization focuses on establishing a vegetative cap that does not shoot accumulate metals but rather immobilizes metals within the tailings. Phytoextraction is currently limited by low rates of metal removal which is a combination of low biomass production and insufficiently high metal uptake into plant tissue. Phytostabilization is currently limited by a lack of knowledge of the minimum amendments required (e.g., compost, irrigation) to support long-term plant establishment. This review addresses both strategies within the context of two specific climate types: temperate and arid. In temperate environments, mine tailings are a source of metal leachates and acid mine drainage that contaminate nearby waterways. Mine tailings in arid regions are subject to eolian dispersion and water erosion. Examples of phytoremediation within each of these environments are discussed. Current research suggests that phytoextraction, due to high implementation costs and long time frames, will be limited to sites that have high land values and for which metal removal is required. Phytostabilization, due to lower costs and easier implementation, will be a more commonly used approach. Complete restoration of mining sites is an unlikely outcome for either approach.     

Metal biosorption-flotation. Application to cadmium removal

Biosorption, using suspended non-living biomass, and flotation (for consequent solid/liquid separation of the metal-loaded biomass) have been studied in the laboratory as a possible combined process, for the removal of toxic metals (i.e. cadmium) from dilute aqueous solutions. The various parameters of the process were investigated in depth, including re-use of biosorbent. A filter aid (contained in the biomass industrial waste used) was found not really to interfere. -potential measurements of the aforementioned system were also carried out. Promising results were obtained during continuous-flow experiments. A flotation residence time of 4 min was achieved. Metal removal and suspended biomass recovery were generally over 95%.     

Biofuels from microalgae

Microalgae are a diverse group of prokaryotic and eukaryotic photosynthetic microorganisms that grow rapidly due to their simple structure. They can potentially be employed for the production of biofuels in an economically effective and environmentally sustainable manner. Microalgae have been investigated for the production of a number of different biofuels including biodiesel, bio-oil, bio-syngas, and bio-hydrogen. The production of these biofuels can be coupled with flue gas CO2 mitigation, wastewater treatment, and the production of high-value chemicals. Microalgal farming can also be carried out with seawater using marine microalgal species as the producers. Developments in microalgal cultivation and downstream processing (e.g., harvesting, drying, and thermochemical processing) are expected to further enhance the cost-effectiveness of the biofuel from microalgae strategy.