Two-stage hydrolysis of invasive algal feedstock for ethanol fermentation

Life Cycle Assessment of a large commercial kelp farm in Shandong, China

The environmental benefits of seaweed cultivation have gained a lot of attention, both in policy strategies and by private companies. Sustainability evaluations of seaweed farming have however focused on a very small part of global production of seaweed - on European cultivations at research and pilot-scales although Asia stands for 99 % of global production with China alone producing 60 %. In this study, we use Life Cycle Assessment (LCA) to evaluate the environmental performance of a 400-hectare Chinese kelp farm with a yearly harvest of 60,000 tons. Primary data from the farm was used to assess impacts up until harvest for the functional unit of 1 ton of fresh-weight kelp. Included in the LCA were impact on climate change, acidification terrestrial and marine eutrophication, and use of land water and energy. In addition, we calculated nutrient uptake. Further, we extracted inventory data of four published LCA studies of farmed kelp and recalculated environmental impacts, applying the same background data and method choices with the aim to compare the effects of scale and cultivation system. The results of the hotspot analysis showed that the plastic ropes and buoys dominated impacts on climate change, freshwater and marine eutrophication, and energy consumption. Consequently, the most effective improvement action was recycling after use. The yearly harvest of the Chinese farm was 1000-4000 times larger than previously evaluated farms compared. Results suggest that streamlined and mature production in the large-scale Chinese kelp farm led to lower electricity and fuel consumption compared to small-scale production, thus placing the Chinese farm with a climate impact of 57.5 kg CO2 eq. per ton fresh-weight kelp on the lower end when comparing the carbon footprint. There was a large variation in carbon footprints, which implies that the kelp cultivation sector has considerable room for optimization.

Mechanism and kinetic model of microalgal enzymatic hydrolysis for prospective bioethanol conversion

Tetraselmis chuii is a potential microalgae that is in consideration for producing bioethanol owing to its large content of carbohydrates. The glucose production from T. chuii through an enzymatic process with cellulase and xylanase (pretreatment process) and α-amylase and glucoamylase (saccharification process) was studied. The mechanism of the enzymatic process was developed and the kinetic models were then evaluated. For the pretreatment process, enzymes with 30% concentration reacted at 30 °C for 40 min resulted in 35.9% glucose yield. For the saccharification process, the highest glucose yield of 90.03% was obtained using simultaneous α-amylase (0.0006%) and glucoamylase (0.01%) enzymes at 55 °C and for 40 min. The kinetic models fitted well with the experimental data. The model also revealed that the saccharification process performed better than the pretreatment process with a higher kinetic constant and lower activation energy. The proposed kinetic model plays an important role in implementing processes at a larger scale.

Microalgae to Bioenergy: Optimization of Aurantiochytrium sp. Saccharification

Microalgae are a promising feedstock for bioethanol production, essentially due to their high growth rates and absence of lignin. Hydrolysis-where the monosaccharides are released for further fermentation-is considered a critical step, and its optimization is advised for each raw material. The present study focuses on the thermal acid hydrolysis (with sulfuric acid) of Aurantiochytrium sp. through a response surface methodology (RSM), studying the effect of acid concentration, hydrolysis time and biomass/acid ratio on both sugar concentration of the hydrolysate and biomass conversion yield. Preliminary studies allowed to establish the range of the variables to be optimized. The obtained models predicted a maximum sugar concentration (18.05 g/L; R² = 0.990) after 90 min of hydrolysis, using 15% (w/v) biomass/acid ratio and sulfuric acid at 3.5% (v/v), whereas the maximum conversion yield (12.86 g/100 g; R² = 0.876) was obtained using 9.3% (w/v) biomass/acid ratio, maintaining the other parameters. Model outputs indicate that the biomass/acid ratio and time are the most influential parameters on the sugar concentration and yield models, respectively. The study allowed to obtain a predictive model that is very well adjusted to the experimental data to find the best saccharification conditions for the Aurantiochytrium sp. microalgae.

Anaerobic Degradation of the Invasive Weed Solidago canadensis L. (goldenrod) and Copper Immobilization by a Community of Sulfate-Reducing and Methane-Producing Bacteria

The weed Solidago canadensis L. poses a global threat to the environment as it spreads uncontrollably on roadsides, in forests, fields, meadows, and farmland. Goldenrod emits toxic substances that suppress other plants on the site, displacing wild ones. Thus, goldenrod conquers huge areas very quickly. The use of herbicides and mechanical methods does not solve the problem of the spontaneous spread of goldenrod. On the other hand, many scientists consider goldenrod as a valuable source of biologically active substances: flavonoids, phenolic compounds, vitamins, etc. In this study, we consider Solidago plants as a promising, free (cheap), and renewable substrate for the production of methane gas. The goal of the study was to identify the main patterns of degradation of the Solidago canadensis L. plant by methane-producing and sulfate-reducing bacteria with methane gas production and simultaneous detoxification of toxic copper. The composition of the gas phase was monitored by gas chromatography. The pH and redox potential parameters were determined potentiometrically; metal concentrations were measured by photometry. The concentration of flavonoids, sugars and phenolic compounds in plant biomass was determined according to well-known protocols. As a result of the study, high efficiencies of methane degradation in the Solidago plant and copper detoxification were obtained. Methane yield has reached the value of 68.2 L kg^-1 TS of Solidago canadensis L. biomass. The degradation coefficient (K_d) was also high at 21.4. The Cu(II) was effectively immobilized by methanogens and sulfate reducers during the goldenrod degradation at the initial concentrations of 500 mg L^-1. Thus, a new method of beneficial application of invasive plants was presented. The result confirms the possibility of using methanogenic microorganisms to produce methane gas from invasive weeds and detoxification of toxic metals.

Integrated glycolysis and pyrolysis process for multiple utilization and cadmium collection of hyperaccumulator Sedum alfredii

Phytoremediation is a cost-effective and environmentally-friendly method to treat cadmium (Cd) contaminated soils, however, there is still a lack of safe disposal methods of harvested hyperaccumulators. In this study, by integrating glycolysis and pyrolysis, we investigated the possibility of bioproduct production and Cd collection from the hyperaccumulator Sedum alfredii. By means of acid-alkali pretreatment, the degree of cellulose polymerization was reduced by 36.24% while the surface accessibility was increased by 115.80%, resulting in a bioethanol yield of 9.29%. Meanwhile, 99.22% of total Cd of biomass could be reclaimed by collecting H₂SO₄-pretreatment waste. The saccharification residue was subsequently modified by NaOH-pretreatment-filtrate and converted into biochar at 500 °C which possessed a maximum Cd²⁺ sorption capacity of 60.52 mg g^-1 based on the Langmuir model. Furthermore, sustainability analysis indicated that the economic input of this process is acceptable when considering its good environmental benefits. Taken together, our study provides a strategy for simultaneous bioethanol and biochar production during Cd collection from the hyperaccumulator S. alfredii, which could be a promising alternative for the suitable treatment of metal-enriched plants.

Valorization of sorghum distillery residue to produce bioethanol for pollution mitigation and circular economy

This research aims to study the wet torrefaction (WT) and saccharification of sorghum distillery residue (SDR) towards hydrochar and bioethanol production. The experiments are designed by Box-Behnken design from response surface methodology where the operating conditions include sulfuric acid concentration (0, 0.01, and 0.02 M), amyloglucosidase concentration (36, 51, and 66 IU), and saccharification time (120, 180, and 240 min). Compared to conventional dry torrefaction, the hydrochar yield is between 13.24 and 14.73%, which is much lower than dry torrefaction biochar (yield >50%). The calorific value of the raw SDR is 17.15 MJ/kg, which is significantly enhanced to 22.36-23.37 MJ/kg after WT. When the sulfuric acid concentration increases from 0 to 0.02 M, the glucose concentration in the product increases from 5.59 g/L to 13.05 g/L. The prediction of analysis of variance suggests that the best combination to maximum glucose production is 0.02 M H₂SO₄, 66 IU enzyme concentration, and 120 min saccharification time, and the glucose concentration is 30.85 g/L. The maximum bioethanol concentration of 19.21 g/L is obtained, which is higher than those from wheat straw (18.1 g/L) and sweet sorghum residue (16.2 g/L). A large amount of SDR is generated in the kaoliang liquor production process, which may cause environmental problems if it is not appropriately treated. This study fulfills SDR valorization for hydrochar and bioenergy to lower environmental pollution and even achieve a circular economy.

Emerging technologies for conversion of sustainable algal biomass into value-added products: A state-of-the-art review

Concerns regarding high energy demand and gradual depletion of fossil fuels have attracted the desire of seeking renewable and sustainable alternatives. Similar to but better than the first- and second-generation biomass, algae derived third-generation biorefinery aims to generate value-added products by microbial cell factories and has a great potential due to its abundant, carbohydrate-rich and lignin-lacking properties. However, it is crucial to establish an efficient process with higher competitiveness over the current petroleum industry to effectively utilize algal resources. In this review, we summarize the recent technological advances in maximizing the bioavailability of different algal resources. Following an overview of approaches to enhancing the hydrolytic efficiency, we review prominent opportunities involved in microbial conversion into various value-added products including alcohols, organic acids, biogas and other potential industrial products, and also provide key challenges and trends for future insights into developing biorefineries of marine biomass.

Purification and characterization of a noble thermostable algal starch liquefying alpha-amylase from Aeribacillus pallidus BTPS-2 isolated from geothermal spring of Nepal

A thermophilic strain, Aeribacillus pallidus BTPS-2 was isolated from Bhurung geothermal spring of Nepal. The 16 s rRNA sequence showed 99.8 % similarity with the type strain Aeribacillus pallidus DSM 3670. The morphological, physiological and biochemical properties were similar to the type strain. Alpha-amylase from A. pallidus BTPS-2 was purified to 19-fold purification by DEAE-Cellulose ion exchange chromatography. The K_m value of amylase on starch was 0.51 ± 0.05 mg/mL. The optimum pH and temperature were 7.0 and 70 °C. SDS-PAGE analysis showed a single band at 100 kDa. The half-life of the enzyme at 80 °C was 2.81 h. The enzyme showed an inhibitory effect in the presence of Fe²⁺, Pb²⁺, Sn²⁺ and Hg²⁺ at 10 mM concentrations. TLC analysis showed that the enzyme is a liquifying alpha-amylase. The enzyme reduced the viscosity of algal biomass suspension up to 74.2 ± 0.17 % which was more efficient than Bacillus amyloliquefaciens alpha-amylase (80.5 ± 0.2 %).

Algae as potential feedstock for the production of biofuels and value-added products: Opportunities and challenges

The current review explores the potential application of algal biomass for the production of biofuels and bio-based products. The variety of processes and pathways through which bio-valorization of algal biomass can be performed are described in this review. Various lipid extraction techniques from algal biomass along with transesterification reactions for biodiesel production are briefly discussed. Processes such as the pretreatment and saccharification of algal biomass, fermentation, gasification, pyrolysis, hydrothermal liquefaction, and anaerobic digestion for the production of biohydrogen, bio-oils, biomethane, biochar (BC), and various bio-based products are reviewed in detail. The biorefinery model and its collaborative approach with various processes are highlighted for the production of eco-friendly, sustainable, and cost-effective biofuels and value-added products. The authors also discuss opportunities and challenges related to bio-valorization of algal biomass and use their own perspective regarding the processes involved in production and the feasibility to make algal research a reality for the production of biofuels and bio-based products in a sustainable manner.

Co-production of biodiesel and bioethanol using psychrophilic microalga Chlamydomonas sp. KNM0029C isolated from Arctic sea ice

BACKGROUND

Biofuels, generated using microalgae as sustainable energy, have received a lot of attention. Microalgae can be cultivated at low cost with CO₂ and solar energy without competition from edible crops. Psychrophilic microalgae can be a suitable feedstock to produce biofuels without the environmental constraints of low temperatures, because they can grow below 10 °C. However, there is a lack of efficient strategies using psychrophilic microalgae to produce biodiesel and bioethanol. Therefore, the current study aimed to optimize the production of biodiesel and bioethanol from Arctic Chlamydomonas sp. KNM0029C at low temperatures.

RESULTS

After incubation in a 20-L photobioreactor, fatty acid methyl ester (FAME) was extracted using modified FAME extraction methods, producing a maximum yield of 0.16-g FAME/g KNM0029C. Residual biomass was pretreated for bioethanol production, and the yields from different methods were compared. The highest bioethanol yield (0.22-g/g residual biomass) was obtained by pretreatment with enzyme (amyloglucosidase) after sonication. Approximately 300-mg biofuel was obtained, including 156-mg FAME biodiesel and 144-mg bioethanol per g dried cells, representing the highest recorded yield from psychrophilic microalgae.

CONCLUSIONS

This is the first to attempt at utilizing biomass from psychrophilic Arctic microalga Chlamydomonas sp. KNM0029C for the co-production of bioethanol and biodiesel, and it yielded the highest values among reported studies using psychrophilic organisms. These results can be used as a source for the efficient biofuel production using polar microalgae.

Bioethanol production from agarophyte red seaweed, Gelidium elegans, using a novel sample preparation method for analysing bioethanol content by gas chromatography

In this study, Gelidium elegans is investigated for ethanol production. A combination of factors including different temperatures, acid concentration and incubation time was evaluated to determine the suitable saccharification conditions. The combination of 2.5% (w/v) H₂SO₄ at 120 °C for 40 min was selected for hydrolysis of the seaweed biomass, followed by purification, and fermentation to yield ethanol. The galactose and glucose were dominant reducing sugars in the G. elegans hydrolysate and under optimum condition of dilute acid hydrolysis, 39.42% of reducing sugars was produced and fermentation resulted in ethanol concentration of 13.27 ± 0.47 g/L. A modified method was evaluated for sample preparation for gas chromatography (GC) analysis of the ethanol content. A solvent mixture of acetonitrile and iso-butanol precipitated dissolved organic residues and reduced water content in GC samples at least by 90%. Results showed that this method could be successfully used for bioethanol production from seaweed.

Mechano-Enzymatic Deconstruction with a New Enzymatic Cocktail to Enhance Enzymatic Hydrolysis and Bioethanol Fermentation of Two Macroalgae Species

The aim of this study was to explore the efficiency of a mechano-enzymatic deconstruction of two macroalgae species for sugars and bioethanol production, by using a new enzymatic cocktail (Haliatase) and two types of milling modes (vibro-ball: VBM and centrifugal milling: CM). By increasing the enzymatic concentration from 3.4 to 30 g/L, the total sugars released after 72 h of hydrolysis increased (from 6.7 to 13.1 g/100 g TS and from 7.95 to 10.8 g/100 g TS for the green algae U. lactuca and the red algae G. sesquipedale, respectively). Conversely, total sugars released from G. sesquipedale increased (up to 126% and 129% after VBM and CM, respectively). The best bioethanol yield (6 g_eth/100 g TS) was reached after 72 h of fermentation of U. lactuca and no increase was obtained after centrifugal milling. The latter led to an enhancement of the ethanol yield of G. sesquipedale (from 2 to 4 g/100 g TS).

Enzymatic saccharification of seaweeds into fermentable sugars by xylanase from marine Bacillus sp. strain BT21

Enzymatic hydrolysis of seaweed biomass was studied using xylanase produced from marine bacteria Bacillus sp. strain BT21 through solid-state fermentation of wheat bran. Three types of seaweeds, Ahnfeltia plicata, Padina tetrastromatica and Ulva lactuca, were selected as representatives of red, brown, and green seaweeds, respectively. Seaweed biomass was pretreated with hot water. The efficiency of pretreated biomass to release reducing sugar by the action of xylanase as well as the type of monosaccharide released during enzyme saccharification of seaweed biomass was studied. It was seen that pretreated biomass of seaweed A. plicata, U. lactuca, and P. tetrastroma, at 121 °C for 45 min, followed by incubation with 50 IU xylanase released reducing sugars of 233 ± 5.3, 100 ± 6.1 and 73.3 ± 4.1 µg/mg of seaweed biomass, respectively. Gas chromatography analysis illustrated the release of xylose, glucose, and mannose during the treatment process. Hot water pre-treatment process enhanced enzymatic conversion of biomass into sugars. This study revealed the important role of xylanase in saccharification of seaweed, a promising feedstock for third-generation bioethanol production.

Evaluation of various solvent systems for lipid extraction from wet microalgal biomass and its effects on primary metabolites of lipid-extracted biomass

Microalgae have tremendous potential to grow rapidly, synthesize, and accumulate lipids, proteins, and carbohydrates. The effects of solvent extraction of lipids on other metabolites such as proteins and carbohydrates in lipid-extracted algal (LEA) biomass are crucial aspects of algal biorefinery approach. An effective and economically feasible algae-based oil industry will depend on the selection of suitable solvent/s for lipid extraction, which has minimal effect on metabolites in lipid-extracted algae. In current study, six solvent systems were employed to extract lipids from dry and wet biomass of Scenedesmus obliquus. To explore the biorefinery concept, dichloromethane/methanol (2:1 v/v) was a suitable solvent for dry biomass; it gave 18.75% lipids (dry cell weight) in whole algal biomass, 32.79% proteins, and 24.73% carbohydrates in LEA biomass. In the case of wet biomass, in order to exploit all three metabolites, isopropanol/hexane (2:1 v/v) is an appropriate solvent system which gave 7.8% lipids (dry cell weight) in whole algal biomass, 20.97% proteins, and 22.87% carbohydrates in LEA biomass. Graphical abstract: Lipid extraction from wet microalgal biomass and biorefianry approach.

Production and purification of amylolytic enzymes for saccharification of microalgal biomass

The aim of this study was the production of amylolytic enzymes by solid state or submerged fermentations (SSF or SF, respectively), followed by purification using chemical process or microfiltration and immobilization of purified enzymes in a polyurethane support. The free and immobilized enzymes obtained were used to evaluate enzymatic hydrolysis of the polysaccharides of Spirulina. Microfiltration of the crude extracts resulted in an increase in their specific activity and thermal stability at 40°C and 50°C for 24h, as compared to extracts obtained by SSF and SF. Immobilization of polyurethane purified enzyme produced yields of 332% and 205% for the enzymes obtained by SF and SSF, respectively. Free or immobilized enzymes favor the generation of fermentable sugar, being the application of the purified and immobilized enzymes in the hydrolysis of microalgal polysaccharides considered a promising alternative towards development of the bioethanol production process from microalgal biomass.

Marine Enzymes and Microorganisms for Bioethanol Production

Bioethanol is a potential alternative fuel to fossil fuels. Bioethanol as a fuel has several economic and environmental benefits. Though bioethanol is produced using starch and sugarcane juice, these materials are in conflict with food availability. To avoid food-fuel conflict, the second-generation bioethanol production by utilizing nonfood lignocellulosic materials has been extensively investigated. However, due to the complexity of lignocellulose architecture, the process is complicated and not economically competitive. The cultivation of lignocellulosic energy crops indirectly affects the food supplies by extensive land use. Marine algae have attracted attention to replace the lignocellulosic feedstock for bioethanol production, since the algae grow fast, do not use land, avoid food-fuel conflict and have several varieties to suit the cultivation environment. The composition of algae is not as complex as lignocellulose due to the absence of lignin, which renders easy hydrolysis of polysaccharides to fermentable sugars. Marine organisms also produce cold-active enzymes for hydrolysis of starch, cellulose, and algal polysaccharides, which can be employed in bioethanol process. Marine microoorganisms are also capable of fermenting sugars under high salt environment. Therefore, marine biocatalysts are promising for development of efficient processes for bioethanol production.

Chemical analysis and biorefinery of red algae Kappaphycus alvarezii for efficient production of glucose from residue of carrageenan extraction process

BACKGROUND

Biorefineries serve to efficiently utilize biomass and their by-products. Algal biorefineries are designed to generate bioproducts for commercial use. Due to the high carbohydrate content of algal biomass, biorefinery to generate biofuels, such as bioethanol, is of great interest. Carrageenan is a predominant polysaccharide hydrocolloid found in red macroalgae and is widely used in food, cosmetics, and pharmaceuticals. In this study, we report the biorefinery of carrageenan derived from processing of experimental strains of the red macroalgae Kappaphycus alvarezii. Specifically, the chemical composition and enzymatic hydrolysis of the residue produced from carrageenan extraction were evaluated to determine the conditions for efficient generation of carbohydrate bioproducts.

RESULTS

The productivity and growth rates of K. alvarezii strains were assessed along with the chemical composition (total carbohydrates, ash, sulfate groups, proteins, insoluble aromatics, galacturonic acid, and lipids) of each strain. Two strains, brown and red, were selected based on their high growth rates and productivity and were treated with 6 % KOH for extraction of carrageenan. The yields of biomass from treatment with 6 % KOH solution of the brown and red strains were 89.3 and 89.5 %, respectively. The yields of carrageenan and its residue were 63.5 and 23 %, respectively, for the brown strain and 60 and 27.8 %, respectively, for the red strain. The residues from the brown and red strains were assessed to detect any potential bioproducts. The galactan, ash, protein, insoluble aromatics, and sulfate groups of the residue were reduced to comparable extents for the two strains. However, KOH treatment did not reduce the content of glucan in the residue from either strain. Glucose was produced by enzymatic hydrolysis for 72 h using both strains. The glucan conversion was 100 % for both strains, and the concentrations of glucose from the brown and red strains were 13.7 and 11.5 g L(-1), respectively. The present results highlight the efficiency of generating a key bioproduct from carrageenan residue.

CONCLUSIONS

This study demonstrates the potential for glucose production using carrageenan residue. Thus, the biorefinery of K. alvarezii can be exploited not only to produce carrageenan, but also to generate glucose for future use in biofuel production.

Use of wavelength-selective optical light filters for enhanced microalgal growth in different algal cultivation systems

This work is to use thin film nano-materials as light filters to selectively transmit certain wavelengths from natural sunlight to algal culture. A red light filter (620-710 nm) and blue filter (450-495 nm) were evaluated. Algae were grown in flasks, flat panel reactors, and rotating algal biofilm (RAB) system. It was found that the light filters did not improve algal growth in flask cultures, probably due to the additional reflection of light by the glass wall of the flasks. However, the light filters significantly (P<0.05) improved biomass yield (13-34%) in flat panel reactors and biomass productivity (70-100%) in RAB system, depending on the growth mode and lighter filters. Such improvements may be due to the eliminating the ultra-violet (UV) damaging the cellular structure. The biomass compositions did not change significantly among different light-filter cultures (P>0.05). The research shows a great potential of using light filters to improve microalgal growth.

Yearlong evaluation of performance and durability of a pilot-scale Revolving Algal Biofilm (RAB) cultivation system

Current algal cultivation has been mainly performed in open ponds or photobioreactors in which algal cells are suspended and harvested through flocculation and centrifugation. A unique attachment based Revolving Algal Biofilm (RAB) cultivation system was recently developed for easy biomass harvest with enhanced biomass productivity. The objective of this research was to evaluate the performance (durability, algal growth, and the geometry) of the RAB system at pilot-scale. A yearlong test of the RAB system was successfully conducted at a greenhouse facility at Boone, Iowa, USA. The RAB resulted in an average of 302% increase in biomass productivity compared to a standard raceway pond, with a maximum biomass productivity (ash free) of 18.9 g/m(2)-day being achieved. The RAB with a vertical configuration generated higher productivity than the triangular RAB. Collectively, the research shows that the RAB as an efficient algal culture system has great potential for being deployed at commercial scale.

Perspectives on engineering strategies for improving biofuel production from microalgae--a critical review

Although the potential for biofuel production from microalgae via photosynthesis has been intensively investigated, information on the selection of a suitable operation strategy for microalgae-based biofuel production is lacking. Many published reports describe competitive strains and optimal culture conditions for use in biofuel production; however, the major impediment to further improvements is the absence of effective engineering strategies for microalgae cultivation and biofuel production. This comprehensive review discusses recent advances in understanding the effects of major environmental stresses and the characteristics of various engineering operation strategies on the production of biofuels (mainly biodiesel and bioethanol) using microalgae. The performances of microalgae-based biofuel-producing systems under various environmental stresses (i.e., irradiance, temperature, pH, nitrogen depletion, and salinity) and cultivation strategies (i.e., fed-batch, semi-continuous, continuous, two-stage, and salinity-gradient) are compared. The reasons for variations in performance and the underlying theories of the various production strategies are also critically discussed. The aim of this review is to provide useful information to facilitate development of innovative and feasible operation technologies for effectively increasing the commercial viability of microalgae-based biofuel production.

Galactose Consuming Microbes for Ethanol Production from Seaweed

Gracilaria sp. is one of macroalgae that contain high amount of sugar especially galactose. This galactose can be fermented into bioethanol by galactose-consuming microbes similar to the widely known ethanol production by yeast S.cerevisae. Hence the main objective of this study is to isolate galactose consuming microbes which capable of fermenting galactose into bioethanol. For this purpose, microbes-containing sample from seaweed culture were grown on galactose agar and tested for their survival as well as ethanol production capability. Seven isolated microbes belong to fungi and bacteria species were tested for their capability in fermenting galactose to produce ethanol. The concentrations of ethanol that have been produced by isolated microbes were analyzed by dichromate method whereas the consumption of galactose was determined by Dinitrosalicyclic acid (DNS) method. It was found that ethanol production resulted from fermentation by S1, S2, S3 and S4 were 0.80% (w/v), 0.74% (w/v), 0.81% (w/v) and 0.85% (w/v) respectively. Identification of these strains, as well as optimization of their ethanol fermentation are undergoing in our laboratory.

Enzymatic hydrolysis and production of bioethanol from common macrophytic green alga Ulva fasciata Delile

The green seaweed Ulva which proliferates fast and occurs abundantly worldwide was used as a feedstock for production of ethanol following enzymatic hydrolysis. Among the different cellulases investigated for efficient saccharification, cellulase 22119 showed the highest conversion efficiency of biomass into reducing sugars than Viscozyme L, Cellulase 22086 and 22128. Pre-heat treatment of biomass in aqueous medium at 120°C for 1h followed by incubation in 2% (v/v) enzyme for 36 h at 45°C gave a maximum yield of sugar 206.82±14.96 mg/g. The fermentation of hydrolysate gave ethanol yield of 0.45 g/g reducing sugar accounting for 88.2% conversion efficiency. These values are substantially higher than those of reported so far for both agarophytes and carrageenophytes. It was also confirmed that enzyme can be used twice without compromising on the saccharification efficiency. The findings of this study reveal that Ulva can be a potential feedstock for bioethanol production.

Development of a rotating algal biofilm growth system for attached microalgae growth with in situ biomass harvest

This work aimed to develop a rotating algal biofilm (RAB) cultivation system that can be widely adopted by microalgae producers for easy biomass harvest. Algal cells were grown on the surface of a material rotating between nutrient-rich liquid and CO2-rich gaseous phase. Scrapping biomass from the attached surface avoided the expensive harvest operations such as centrifugation. Among various attachment materials, cotton sheet resulted in best algal growth, durability, and cost effectiveness. A lab-scale RAB system was further optimized with harvest frequency, rotation speed, and CO2 levels. The algal biomass from the RAB system had a similar water content as that in centrifuged biomass. An open pond raceway retrofitted with a pilot-scale RAB system resulted in a much higher biomass productivity when compared to a control open pond. Collectively, the research shows that the RAB system is an efficient algal culture system for easy biomass harvest with enhanced biomass productivity.

Bioprocess development on microalgae-based CO2 fixation and bioethanol production using Scenedesmus obliquus CNW-N

A two-stage cultivation strategy was applied to achieve greater CO2 fixation and carbohydrate productivity with an indigenous microalga Scenedesmus obliquus CNW-N, which was first cultivated using a nutrient-rich medium to promote cell growth, and was then switched to a nutrient-deficient condition to trigger carbohydrate accumulation. The optimal biomass productivity, carbohydrate productivity, and CO2 fixation rate were 681.4, 352.9, and 1192.5 mg L(-1) d(-1), respectively, with a 51.8% carbohydrate content (based on dry weight). This performance is better than the results in most related studies. The microalgal carbohydrate was mainly composed of glucose, which accounts for nearly 80% of total sugars. Dilute acid hydrolysis with 2% H2SO4 can saccharify the wet microalgal biomass effectively, achieving a glucose yield of 96-98%. Using the acidic hydrolysate of the microalga as feedstock, the separate hydrolysis and fermentation (SHF) process gave an ethanol concentration of 8.55 g L(-1), representing a theoretical yield of nearly 99.8%.

Engineering strategies for improving the CO2 fixation and carbohydrate productivity of Scenedesmus obliquus CNW-N used for bioethanol fermentation

Engineering strategies were applied to improve the cell growth, CO2 fixation ability, and carbohydrate productivity of a Scenedesmus obliquus CNW-N isolate. The resulting carbohydrate-rich microalgal biomass was subsequently utilized as feedstock for ethanol fermentation. The microalga was cultivated with 2.5% CO2 in a photobioreactor on different operation modes. Semi-batch operations with 50% replacement of culture medium resulted in the highest CO2 fixation rate (1546.7 mg L(-1) d(-1)), carbohydrate productivity (467.6 mg L(-1) d(-1)), and bioethanol yield (0.202 g/g biomass). This performance is better than most reported values in the literature. The microalgal biomass can accumulate nearly 50% carbohydrates, as glucose accounted for nearly 80% of the total carbohydrate content. This glucose-predominant carbohydrate composition of the microalga is well suited for fermentative bioethanol production. Therefore, using the proposed carbohydrate-rich microalgal biomass both as the carbon sink and as the feedstock provides a feasible alternative to current carbon-reduction and bioethanol-production strategies.

Pretreatment of the macroalgae Chaetomorpha linum for the production of bioethanol--comparison of five pretreatment technologies

A qualified estimate for pretreatment of the macroalgae Chaetomorpha linum for ethanol production was given, based on the experience of pretreatment of land-based biomass. C. linum was subjected to hydrothermal pretreatment (HTT), wet oxidation (WO), steam explosion (STEX), plasma-assisted pretreatment (PAP) and ball milling (BM), to determine effects of the pretreatment methods on the conversion of C. linum into ethanol by simultaneous saccharification and fermentation (SSF). WO and BM showed the highest ethanol yield of 44 g ethanol/100g glucan, which was close to the theoretical ethanol yield of 57 g ethanol/100g glucan. A 64% higher ethanol yield, based on raw material, was reached after pretreatment with WO and BM compared with unpretreated C. linum, however 50% of the biomass was lost during WO. Results indicated that the right combination of pretreatment and marine macroalgae, containing high amounts of glucan and cleaned from salts, enhanced the ethanol yield significantly.

Bioethanol production from the macroalgae Sargassum spp

Macroalgae, an abundant and carbon-neutral renewable resource, with several species rich in carbohydrates are suitable for bioethanol production. This study focused on the pretreatment, enzyme saccharification and fermentation of Sargassum spp., a brown macroalgae for bioethanol production. The optimal acid pretreatment condition achieved in terms of glucose and reducing sugar yields was 3.4-4.6% (w/v) H2SO4 concentration, 115°C and 1.50h. The pretreated biomass was hydrolyzed with cellulase enzyme system supplemented with β-glucosidase. After fermentation by Saccharomyces cerevisiae at 40°C, pH of 4.5 for 48 h, the ethanol conversion rate of the enzyme hydrolysate reached 89%, which was markedly higher than the theoretical yield of 51% based on glucose as substrate. Since all the glucose was consumed during fermentation, other sugar sources might be present in the hydrolysate. The macroalgae, Sargassum spp., showed significant potential as a renewable feedstock for the production of bioethanol.

Bioethanol production from Gracilaria verrucosa, a red alga, in a biorefinery approach

In this study, Gracilaria verrucosa, red seaweed has been used for production of agar and bioethanol. The algae harvested at various time durations resulted in extraction of ~27-33% agar. The leftover pulp was found to contain ~62-68% holocellulose, which on enzymatic hydrolysis yielded 0.87 g sugars/g cellulose. The enzymatic hydrolysate on fermentation with Saccharomyces cerevisiae produced ethanol with an ethanol yield of 0.43 g/g sugars. The mass balance evaluation of the complete process demonstrates that developing biorefinery approach for exploiting Gracilaria verrucosa, a red alga, could be commercially viable.

Bioethanol production using carbohydrate-rich microalgae biomass as feedstock

This study aimed to evaluate the potential of using a carbohydrate-rich microalga Chlorella vulgaris FSP-E as feedstock for bioethanol production via various hydrolysis strategies and fermentation processes. Enzymatic hydrolysis of C. vulgaris FSP-E biomass (containing 51% carbohydrate per dry weight) gave a glucose yield of 90.4% (or 0.461 g (g biomass)(-1)). The SHF and SSF processes converted the enzymatic microalgae hydrolysate into ethanol with a 79.9% and 92.3% theoretical yield, respectively. Dilute acidic hydrolysis with 1% sulfuric acid was also very effective in saccharifying C. vulgaris FSP-E biomass, achieving a glucose yield of nearly 93.6% from the microalgal carbohydrates at a starting biomass concentration of 50 g L(-1). Using the acidic hydrolysate of C. vulgaris FSP-E biomass as feedstock, the SHF process produced ethanol at a concentration of 11.7 g L(-1) and an 87.6% theoretical yield. These findings indicate the feasibility of using carbohydrate-producing microalgae as feedstock for fermentative bioethanol production.

Polysaccharide hydrolysis with engineered Escherichia coli for the production of biocommodities

Escherichia coli can ferment a broad range of sugars, including pentoses, hexoses, uronic acids, and polyols. These features make E. coli a suitable microorganism for the development of biocatalysts to be used in the production of biocommodities and biofuels by metabolic engineering. E. coli cannot directly ferment polysaccharides because it does not produce and secrete the necessary saccharolytic enzymes; however, there are many genetic tools that can be used to confer this ability on this prokaryote. The construction of saccharolytic E. coli strains will reduce costs and simplify the production process because the saccharification and fermentation can be conducted in a single reactor with a reduced concentration or absence of additional external saccharolytic enzymes. Recent advances in metabolic engineering, surface display, and excretion of hydrolytic enzymes provide a framework for developing E. coli strains for the so-called consolidated bioprocessing. This review presents the different strategies toward the development of E. coli strains that have the ability to display and secrete saccharolytic enzymes to hydrolyze different sugar-polymeric substrates and reduce the loading of saccharolytic enzymes.

Anaerobic conversion of microalgal biomass to sustainable energy carriers--a review

This review discusses anaerobic production of methane, hydrogen, ethanol, butanol and electricity from microalgal biomass. The amenability of microalgal biomass to these bioenergy conversion processes is compared with other aquatic and terrestrial biomass sources. The highest energy yields (kJ g(-1) dry wt. microalgal biomass) reported in the literature have been 14.8 as ethanol, 14.4 as methane, 6.6 as butanol and 1.2 as hydrogen. The highest power density reported from microalgal biomass in microbial fuel cells has been 980 mW m(-2). Sequential production of different energy carriers increases attainable energy yields, but also increases investment and maintenance costs. Microalgal biomass is a promising feedstock for anaerobic energy conversion processes, especially for methanogenic digestion and ethanol fermentation. The reviewed studies have mainly been based on laboratory scale experiments and thus scale-up of anaerobic utilization of microalgal biomass for production of energy carriers is now timely and required for cost-effectiveness comparisons.

Characterization and optimization of carbohydrate production from an indigenous microalga Chlorella vulgaris FSP-E

In this study, three indigenous microalgae isolates were examined for their ability to produce carbohydrates. Among them, Chlorella vulgaris FSP-E displayed relatively high cell growth rate and carbohydrate content. The carbohydrate productivity of C. vulgaris FSP-E was further improved by using engineering strategies. The results show that using an appropriate light intensity and inoculum size could effectively promote cell growth and carbohydrate productivity. Nitrogen starvation triggered the accumulation of carbohydrates in the microalga, achieving a carbohydrate content of 51.3% after 4-day starvation. Under the optimal conditions, the highest biomass and carbohydrate productivity were 1.437 and 0.631 g L(-1) d(-1), respectively. This performance is better than that reported in most related studies. Since glucose accounted for nearly 93% of the carbohydrates accumulated in C. vulgaris FSP-E, the microalga is an excellent feedstock for bioethanol fermentation.

Strategies for the production of high concentrations of bioethanol from seaweeds: production of high concentrations of bioethanol from seaweeds

Bioethanol has attracted attention as an alternative to petroleum-derived fuel. Seaweeds have been proposed as some of the most promising raw materials for bioethanol production because they have several advantages over lignocellulosic biomass. However, because seaweeds contain low contents of glucans, i.e., polysaccharides composed of glucose, the conversion of only the glucans from seaweed is not sufficient to produce high concentrations of ethanol. Therefore, it is also necessary to produce ethanol from other specific carbohydrate components of seaweeds, including sulfated polysaccharides, mannitol, alginate, agar and carrageenan. This review summarizes the current state of research on the production of ethanol from seaweed carbohydrates for which the conversion of carbohydrates to sugars is a key step and makes comparisons with the production of ethanol from lignocellulosic biomass. This review provides valuable information necessary for the production of high concentrations of ethanol from seaweeds.

Biochemical features and bioethanol production of microalgae from coastal waters of Pearl River Delta

This study describes identification, cultivation, monitoring of carbohydrate accumulation and bioethanol production from microalgal strains from the coastal waters of Pearl River Delta. Eighteen identified strains belong to the families Chlorellaceae, Scotiellocystoidaceae, Neochloridaceae, Selenastraceae and Scenedesmaceae. Of isolated strains Mychonastes afer PKUAC 9 and Scenedesmus abundans PKUAC 12 were selected for further biomass and ethanol production analysis. Comparison of three cultivation modes (stationary, shaken and aerated) resulted in the highest biomass productivity obtained for aerated cultures that yielded 0.09 g and 0.11 g dry weight per day per litre of medium for M. afer PKUAC 9 and S. abundans PKUAC 12, respectively. Carbohydrate accumulation monitored by FTIR showed that early stationary phase is optimal for biomass harvest. Microalgal biomass was successfully used as a carbohydrate feedstock for fermentative bioethanol production. S. abundans PKUAC 12 was superior feedstock for bioethanol production when pre-treated with the combination of dilute acid treatment and cellulase.