Microalgal Biorefinery Concepts’ Developments for Biofuel and Bioproducts: Current Perspective and Bottlenecks

Microalgae have received much interest as a biofuel feedstock. However, the economic feasibility of biofuel production from microalgae does not satisfy capital investors. Apart from the biofuels, it is necessary to produce high-value co-products from microalgae fraction to satisfy the economic aspects of microalgae biorefinery. In addition, microalgae-based wastewater treatment is considered as an alternative for the conventional wastewater treatment in terms of energy consumption, which is suitable for microalgae biorefinery approaches. The energy consumption of a microalgae wastewater treatment system (0.2 kW/h/m³) was reduced 10 times when compared to the conventional wastewater treatment system (to 2 kW/h/m³). Microalgae are rich in various biomolecules such as carbohydrates, proteins, lipids, pigments, vitamins, and antioxidants; all these valuable products can be utilized by nutritional, pharmaceutical, and cosmetic industries. There are several bottlenecks associated with microalgae biorefinery. Hence, it is essential to promote the sustainability of microalgal biorefinery with innovative ideas to produce biofuel with high-value products. This review attempted to bring out the trends and promising solutions to realize microalgal production of multiple products at an industrial scale. New perspectives and current challenges are discussed for the development of algal biorefinery concepts.

The effects of Microalgae Biomass Co-Substrate on Biogas Production from the Common Agricultural Biogas Plants Feedstock

The aim of this study was to determine the effects on methane production of the addition of microalgae biomass of Arthrospira platensis and Platymonas subcordiformis to the common feedstock used in agricultural biogas plants (cattle manure, maize silage). Anaerobic biodegradability tests were carried out using respirometric reactors operated at an initial organic loading rate of 5.0 kg volatile solids (VS)/m3, temperature of 35°C, and a retention time of 20 days. A systematic increase in the biogas production efficiency was found, where the ratio of microalgae biomass in the feedstock increased from 0% to 40% (%VS). Higher microalgae biomass ratio did not have a significant impact on improving the efficiency of biogas production, and the biogas production remained at a level comparable with 40% share of microalgae biomass in the feedstock. This was probably related to the carbon to nitrogen (C/N) ratio decrease in the mixture of substrates. The use of Platymonas subcordiformis ensured higher biogas production, with the maximum value of 1058.8 ± 25.2 L/kg VS. The highest content of methane, at an average concentration of 65.6% in the biogas produced, was observed in setups with Arthrospira plantensis biomass added at a concentration of between 20%–40% to the feedstock mixture.

Biogas production coupled to repeat microalgae cultivation using a closed nutrient loop

Anaerobic digestion is an established technology to produce renewable energy as methane-rich biogas for which microalgae are a suitable substrate. Besides biogas production, anaerobic digestion of microalgae generates an effluent rich in nutrients, so-called digestate, that can be used as a growth medium for microalgal cultures, with the potential for a closed nutrient loop and sustainable bioenergy facility. In this study, the methane potential and nutrient mobilization of the microalga Scenedemus dimorphus was evaluated under continuous conditions. The suitability of using the digestate as culture medium was also evaluated. The results show that S. dimorphus is a suitable substrate for anaerobic digestion with an average methane yield of 199 mL g^-1 VS. The low level of phosphorus in digestate did not limit algae growth when used as culture medium. The potential of liquid digestate as a superior culture medium rather than inorganic medium was demonstrated.

Microbial communities mediating algal detritus turnover under anaerobic conditions

BACKGROUND

Algae encompass a wide array of photosynthetic organisms that are ubiquitously distributed in aquatic and terrestrial habitats. Algal species often bloom in aquatic ecosystems, providing a significant autochthonous carbon input to the deeper anoxic layers in stratified water bodies. In addition, various algal species have been touted as promising candidates for anaerobic biogas production from biomass. Surprisingly, in spite of its ecological and economic relevance, the microbial community involved in algal detritus turnover under anaerobic conditions remains largely unexplored.

RESULTS

Here, we characterized the microbial communities mediating the degradation of Chlorella vulgaris (Chlorophyta), Chara sp. strain IWP1 (Charophyceae), and kelp Ascophyllum nodosum (phylum Phaeophyceae), using sediments from an anaerobic spring (Zodlteone spring, OK; ZDT), sludge from a secondary digester in a local wastewater treatment plant (Stillwater, OK; WWT), and deeper anoxic layers from a seasonally stratified lake (Grand Lake O' the Cherokees, OK; GL) as inoculum sources. Within all enrichments, the majority of algal biomass was metabolized within 13-16 weeks, and the process was accompanied by an increase in cell numbers and a decrease in community diversity. Community surveys based on the V4 region of the 16S rRNA gene identified different lineages belonging to the phyla Bacteroidetes, Proteobacteria (alpha, delta, gamma, and epsilon classes), Spirochaetes, and Firmicutes that were selectively abundant under various substrate and inoculum conditions. Within all kelp enrichments, the microbial communities structures at the conclusion of the experiment were highly similar regardless of the enrichment source, and were dominated by the genus Clostridium, or family Veillonellaceae within the Firmicutes. In all other enrichments the final microbial community was dependent on the inoculum source, rather than the type of algae utilized as substrate. Lineages enriched included the uncultured groups VadinBC27 and WCHB1-69 within the Bacteroidetes, genus Spirochaeta and the uncultured group SHA-4 within Spirochaetes, Ruminococcaceae, Lachnospiraceae, Yongiibacter, Geosporobacter, and Acidaminobacter within the Firmicutes, and genera Kluyvera, Pantoea, Edwardsiella and Aeromonas, and Buttiauxella within the Gamma-Proteobaceteria order Enterobacteriales.

CONCLUSIONS

Our results represent the first systematic survey of microbial communities mediating turnover of algal biomass under anaerobic conditions, and highlights the diversity of lineages putatively involved in the degradation process.

A Review on the Valorization of Macroalgal Wastes for Biomethane Production

The increased use of terrestrial crops for biofuel production and the associated environmental, social and ethical issues have led to a search for alternative biomass materials. Terrestrial crops offer excellent biogas recovery, but compete directly with food production, requiring farmland, fresh water and fertilizers. Using marine macroalgae for the production of biogas circumvents these problems. Their potential lies in their chemical composition, their global abundance and knowledge of their growth requirements and occurrence patterns. Such a biomass industry should focus on the use of residual and waste biomass to avoid competition with the biomass requirements of the seaweed food industry, which has occurred in the case of terrestrial biomass. Overabundant seaweeds represent unutilized biomass in shallow water, beach and coastal areas. These eutrophication processes damage marine ecosystems and impair local tourism; this biomass could serve as biogas feedstock material. Residues from biomass processing in the seaweed industry are also of interest. This is a rapidly growing industry with algae now used in the comestible, pharmaceutical and cosmetic sectors. The simultaneous production of combustible biomethane and disposal of undesirable biomass in a synergistic waste management system is a concept with environmental and resource-conserving advantages.

Metagenome from a Spirulina digesting biogas reactor: analysis via binning of contigs and classification of short reads

BACKGROUND

Anaerobic digestion is a biological process in which a consortium of microorganisms transforms a complex substrate into methane and carbon dioxide. A good understanding of the interactions between the populations that form this consortium can contribute to a successful anaerobic digestion of the substrate. In this study we combine the analysis of the biogas production in a laboratory anaerobic digester fed with the microalgae Spirulina, a protein rich substrate, with the analysis of the metagenome of the consortium responsible for digestion, obtained by high-throughput DNA sequencing. The obtained metagenome was also compared with a metagenome from a full scale biogas plant fed with cellulose rich material.

RESULTS

The optimal organic loading rate for the anaerobic digestion of Spirulina was determined to be 4.0 g Spirulina L(-1) day(-1) with a specific biogas production of 350 mL biogas g Spirulina (-1) with a methane content of 68 %. Firmicutes dominated the microbial consortium at 38 % abundance followed by Bacteroidetes, Chloroflexi and Thermotogae. Euryarchaeota represented 3.5 % of the total abundance. The most abundant organism (14.9 %) was related to Tissierella, a bacterium known to use proteinaceous substrates for growth. Methanomicrobiales and Methanosarcinales dominated the archaeal community. Compared to the full scale cellulose-fed digesters, Pfam domains related to protein degradation were more frequently detected and Pfam domains related to cellulose degradation were less frequent in our sample.

CONCLUSIONS

The results presented in this study suggest that Spirulina is a suitable substrate for the production of biogas. The proteinaceous substrate appeared to have a selective impact on the bacterial community that performed anaerobic digestion. A direct influence of the substrate on the selection of specific methanogenic populations was not observed.

Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs

Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.

A comprehensive substance flow analysis of a municipal wastewater and sludge treatment plant

The fate of total organic carbon, 32 elements (Al, Ag, As, Ba, Be, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Sb, Se, Sn, Sr, Ti, V, and Zn) and 4 groups of organic pollutants (linear alkylbenzene sulfonates, bis(2-ethylhexyl)phthalate, polychlorinated biphenyl and polycyclic aromatic hydrocarbons) in a conventional wastewater treatment plant were assessed. Mass balances showed reasonable closures for most of the elements. However, gaseous emissions were accompanied by large uncertainties and show the limitation of mass balance based substance flow analysis. Based on the assessment, it is evident that both inorganic and organic elements accumulated in the sewage sludge, with the exception of elements that are highly soluble or degradable by wastewater and sludge treatment processes. The majority of metals and metalloids were further accumulated in the incineration ash, while the organic pollutants were effectively destroyed by both biological and thermal processes. Side streams from the sludge treatment process (dewatering and incineration) back to the wastewater treatment represented less than 1% of the total volume entering the wastewater treatment processes, but represented significant substance flows. In contrast, the contribution by spent water from the flue gas treatment process was almost negligible. Screening of human and eco-toxicity by applying the consensus-based environmental impact assessment method USEtox addressing 15 inorganic constituents showed that removal of inorganic constituents by the wastewater treatment plant reduced the toxic impact potential by 87-92%.

Anaerobic digestion of the microalga Spirulina at extreme alkaline conditions: biogas production, metagenome, and metatranscriptome

A haloalkaline anaerobic microbial community obtained from soda lake sediments was used to inoculate anaerobic reactors for the production of methane rich biogas. The microalga Spirulina was successfully digested by the haloalkaline microbial consortium at alkaline conditions (pH 10, 2.0 M Na⁺). Continuous biogas production was observed and the obtained biogas was rich in methane, up to 96%. Alkaline medium acted as a CO₂ scrubber which resulted in low amounts of CO₂ and no traces of H₂S in the produced biogas. A hydraulic retention time (HRT) of 15 days and 0.25 g Spirulina L⁻¹ day⁻¹ organic loading rate (OLR) were identified as the optimal operational parameters. Metagenomic and metatranscriptomic analysis showed that the hydrolysis of the supplied substrate was mainly carried out by Bacteroidetes of the “ML635J-40 aquatic group” while the hydrogenotrophic pathway was the main producer of methane in a methanogenic community dominated by Methanocalculus.

Metagenome changes in the mesophilic biogas-producing community during fermentation of the green alga Scenedesmus obliquus

A microalgal biomass offers a potential alternative to the maize silage commonly used in biogas technology. In this study, photoautotrophically grown Scenedesmus obliquus was used as biogas substrate. This microalga has a low C/N ratio of 8.5 relative to the optimum 20-30. A significant increase in the ammonium ion content was not observed. The methane content of the biogas generated from Sc. obliquus proved to be higher than that from maize silage, but the specific biogas yield was lower. Semi-continuous steady biogas production lasted for 2 months. Because of the thick cell wall of Sc. obliquus, the biomass-degrading microorganisms require additional time to digest its biomass. The methane concentration in the biogas was also high, in co-digestion (i.e., 52-56%) as in alga-fed anaerobic digestion (i.e., 55-62%). These results may be related to the relative predominance of the order Clostridiales in co-digestion and to the more balanced C/N ratio of the mixed algal-maize biomass. Predominance of the order Methanosarcinales was observed in the domain Archaea, which supported the diversity of metabolic pathways in the process.

Exploitation of algal-bacterial associations in a two-stage biohydrogen and biogas generation process

BACKGROUND

The growing concern regarding the use of agricultural land for the production of biomass for food/feed or energy is dictating the search for alternative biomass sources. Photosynthetic microorganisms grown on marginal or deserted land present a promising alternative to the cultivation of energy plants and thereby may dampen the 'food or fuel' dispute. Microalgae offer diverse utilization routes.

RESULTS

A two-stage energetic utilization, using a natural mixed population of algae (Chlamydomonas sp. and Scenedesmus sp.) and mutualistic bacteria (primarily Rhizobium sp.), was tested for coupled biohydrogen and biogas production. The microalgal-bacterial biomass generated hydrogen without sulfur deprivation. Algal hydrogen production in the mixed population started earlier but lasted for a shorter period relative to the benchmark approach. The residual biomass after hydrogen production was used for biogas generation and was compared with the biogas production from maize silage. The gas evolved from the microbial biomass was enriched in methane, but the specific gas production was lower than that of maize silage. Sustainable biogas production from the microbial biomass proceeded without noticeable difficulties in continuously stirred fed-batch laboratory-size reactors for an extended period of time. Co-fermentation of the microbial biomass and maize silage improved the biogas production: The metagenomic results indicated that pronounced changes took place in the domain Bacteria, primarily due to the introduction of a considerable bacterial biomass into the system with the substrate; this effect was partially compensated in the case of co-fermentation. The bacteria living in syntrophy with the algae apparently persisted in the anaerobic reactor and predominated in the bacterial population. The Archaea community remained virtually unaffected by the changes in the substrate biomass composition.

CONCLUSION

Through elimination of cost- and labor-demanding sulfur deprivation, sustainable biohydrogen production can be carried out by using microalgae and their mutualistic bacterial partners. The beneficial effect of the mutualistic mixed bacteria in O2 quenching is that the spent algal-bacterial biomass can be further exploited for biogas production. Anaerobic fermentation of the microbial biomass depends on the composition of the biogas-producing microbial community. Co-fermentation of the mixed microbial biomass with maize silage improved the biogas productivity.

Microalgae to biofuels: life cycle impacts of methane production of anaerobically digested lipid extracted algae

This study presents experimental measurements of the biochemical methane production for whole and lipid extracted Nannochloropsis salina. Results show whole microalgae produced 430 cm(3)-CH4 g-volatile solids(-1) (g-VS) (σ=60), 3 times more methane than was produced by the LEA, 140 cm(3)-CH4 g-VS(-1) (σ=30). Results illustrate current anaerobic modeling efforts in microalgae to biofuel assessments are not reflecting the impact of lipid removal. On a systems level, the overestimation of methane production is shown to positively skew the environmental impact of the microalgae to biofuels process. Discussion focuses on a comparison results to those of previous anaerobic digestion studies and quantifies the corresponding change in greenhouse gas emissions of the microalgae to biofuels process based on results from this study.

Long-term anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment

This paper describes the anaerobic digestion of microalgal biomass from high rate ponds in continuous anaerobic reactors for biogas production. With hydraulic retention times (HRT) of 15 and 20 days, the volatile solids (VS) removal did not exceed 30%, and the methane production rate ranged between 0.12 and 0.14 L CH4/L day. To improve process performance, microwave irradiation at 900 W for 3 min (specific energy ∼70,000 kJ/kg VS) was applied as a pretreatment step. The VS removal increased to 40 and 45% at 15 and 20 days HRT, respectively. Consequently, the methane production rate increased to 0.16 and 0.20 L CH4/L day at 15 and 20 days HRT, respectively. Microscopic analysis confirmed cell wall damage, although generally without lysis, after irradiating microalgal biomass. However, the energy consumption was much higher than the extra energy production of the process. Indeed, microalgal biomass should not only be thickened but also dewatered if microwave irradiation was to be applied as a pretreatment to anaerobic digestion for biogas production.

Effects of thermal pretreatment on anaerobic digestion of Nannochloropsis salina biomass

The marine microalga Nannochloropsis salina was investigated as feedstock for anaerobic digestion under batch and semi-continuous conditions for the first time. Biodegradability and methane yield were low under both digestion conditions. Thermal pretreatment prior to anaerobic digestion significantly increased the methane yield from 0.2 to 0.57 m(3) kg VS(-1) under batch conditions and from 0.13 to 0.27 m(3) kg VS(-1) in semi-continuous digestion. Still, the methane yield was limited with semi-continuous feeding due to volatile fatty acid (VFA) accumulation in the digester caused by high ammonium and salt concentrations in the feedstock. Despite VFA accumulation adaption of the microorganisms to the changing conditions and high buffer capacity resulted in steady methane production. A first energy balance considering the required heat for thermal pretreatment revealed significant benefit from the pretreatment. Conversely, the high energy demand for dewatering algal cultures is one major bottleneck for industrial-scale processing of microalgae.

Addressing the challenges for sustainable production of algal biofuels: II. Harvesting and conversion to biofuels

In order to ensure the sustainability of algal biofuel production, a number of issues need to be addressed. Previously, we reviewed some of the questions in this area involving algal species and the important challenges of nutrient supply and how these might be met. Here, we take up issues involving harvesting and the conversion ofbiomass to biofuels. Advances in both these areas are required if these third-generation fuels are to have a sufficiently high net energy ratio and a sustainable footprint. A variety of harvesting technologies are under investigation and recent studies in this area are presented and discussed. A number of different energy uses are available for algal biomass, each with their own advantages as well as challenges in terms of efficiencies and yields. Recent advances in these areas are presented and some of the especially promising conversion processes are highlighted.

Enhanced methane production from Taihu Lake blue algae by anaerobic co-digestion with corn straw in continuous feed digesters

Anaerobic digestion of Taihu blue algae was tested in laboratory scale, continuous feed digesters (hydraulic retention time 10 days) at 35°C and various organic loading rates (OLR). The methane production and biomass digestion performed well at OLR below 4.00 gVSL(-1)d(-1) but deteriorated as OLR increased due to the increased ammonia concentration, causing inhibition mainly to acetate and propionate degradation. Supplementing corn straw as co-feedstock significantly improved the digestion performance. The optimal C/N ratio for the co-digestion was 20:1 at OLR of 6.00 gVSL(-1) d(-1). Methane yield of 234 mL CH4 gVS(-1) and methane productivity of 1404 mL CH4 L(-1) d(-1) were achieved with solid removal of 63%. Compared with the algae alone, the methane productivity was increased by 46% with less accumulation of ammonia and fatty acids. The reactor rate-limiting step was acetate and propionate degradation.

Characterization of a Methanogenic Community within an Algal Fed Anaerobic Digester

The microbial diversity and metabolic potential of a methanogenic consortium residing in a 3785-liter anaerobic digester, fed with wastewater algae, was analyzed using 454 pyrosequencing technology. DNA was extracted from anaerobic sludge material and used in metagenomic analysis through PCR amplification of the methyl-coenzyme M reductase α subunit (mcrA) gene using primer sets ML, MCR, and ME. The majority of annotated mcrA sequences were assigned taxonomically to the genera Methanosaeta in the order Methanosarcinales. Methanogens from the genus Methanosaeta are obligate acetotrophs, suggesting this genus plays a dominant role in methane production from the analyzed fermentation sample. Numerous analyzed sequences within the algae fed anaerobic digester were unclassified and could not be assigned taxonomically. Relative amplicon frequencies were determined for each primer set to determine the utility of each in pyrosequencing. Primer sets ML and MCR performed better quantitatively (representing the large majority of analyzed sequences) than primer set ME. However, each of these primer sets was shown to provide a quantitatively unique community structure, and thus they are of equal importance in mcrA metagenomic analysis.

Biogas productivity by co-digesting Taihu blue algae with corn straw as an external carbon source

A batch anaerobic test was conducted to evaluate the effects of adding high carbon content of corn straw to the digestion of Taihu blue algae to attain an optimal C/N ratio for higher methane yield. The addition of corn straw in algae at a C/N ratio of 20/1 increased methane yield by 61.69% at 325 mL g(-1)VS(-1) (compared with 201 mL g(-1) VS(-1) of algae digestion alone), followed by C/N ratios of 16/1 and 25/1, all operated at 20 g VSL(-1) and 35 °C. The results suggest the optimal C/N ratio for co-digestion of algae with corn straw is 20/1. The findings could offer options for efficient methane production and waste treatment.

Materials flow modeling of nutrient recycling in biodiesel production from microalgae

Biodiesel production based on microalgae as feedstock is associated with a high demand of nutrients, respectively nitrogen and phosphorus. The production of 1l biodiesel requires between 0.23 and 1.55 kg nitrogen and 29-145 g of phosphorus depending of the cultivation conditions for microalgae. The supply of nutrients can be expected to severely limit the extent to which the production of biofuels from microalgae can be sustainably expanded. The nutrient demand can be reduced if the nutrients in the residual algae biomass after oil extraction are reused for algae cultivation. This modeling work illustrates that for the investigated process chains and scenarios the nutrient recycling rates are in the range from 30% to 90% for nitrogen and from 48% to 93% for phosphorus. The highest recycling values can be achieved by hydrothermal gasification of the oil-free residues.

Renewable energy from Cyanobacteria: energy production optimization by metabolic pathway engineering

The need to develop and improve sustainable energy resources is of eminent importance due to the finite nature of our fossil fuels. This review paper deals with a third generation renewable energy resource which does not compete with our food resources, cyanobacteria. We discuss the current state of the art in developing different types of bioenergy (ethanol, biodiesel, hydrogen, etc.) from cyanobacteria. The major important biochemical pathways in cyanobacteria are highlighted, and the possibility to influence these pathways to improve the production of specific types of energy forms the major part of this review.

Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris

The main goal of this present study is to investigate the feasibility of coupling algae production (Chlorella vulgaris) to an anaerobic digestion unit. An intermediate settling device was integrated in order to adapt the feed-flow concentration and the flow rate. Digestion of C. vulgaris was studied under 16 and 28 days hydraulic retention times (HRT), with a corresponding organic loading rate of 1g(COD)L(-1). Increasing the HRT achieved 51% COD removal with a methane production measured at 240 mL g(VSS)(-1). Performing different HRTs and dynamic monitoring during degradation highlighted differential hydrolysis of microalgae compartments. However, 50% of the biomass did not undergo anaerobic digestion, even under long retention times. This points out the interest for further studies on pre-treatment performances and more generally speaking on the need for intensifying microalgae biomass digestion.

Revival of the biological sunlight-to-biogas energy conversion system

In the quest for renewable resources, algae are increasingly receiving attention. Their high growth rate, high CO(2) fixation and their lack of requirement for fertile soil surface represent several advantages as compared to conventional (energy) crops. Through their ability to store large amounts of oils, they qualify as a source for biodiesel. Algal biomass, however, can also be used as such, namely as a substrate for anaerobic digestion. In the present research, we investigated the use of algae for energy generation in a stand-alone, closed-loop system. The system encompasses an algal growth unit for biomass production, an anaerobic digestion unit to convert the biomass to biogas and a microbial fuel cell to polish the effluent of the digester. Nutrients set free during digestion can accordingly be returned to the algal growth unit for a sustained algal growth. Hence, a system is presented that continuously transforms solar energy into energy-rich biogas and electricity. Algal productivities of 24-30 ton VS ha(-1) year(-1) were reached, while 0.5 N m(3) biogas could be produced kg(-1) algal VS. The system described resulted in a power plant with a potential capacity of about 9 kW ha(-1) of solar algal panel, with prospects of 23 kW ha(-1).

Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable

The potential of microalgae as a source of biofuels and as a technological solution for CO2 fixation is subject to intense academic and industrial research. In the perspective of setting up massive cultures, the management of large quantities of residual biomass and the high amounts of fertilizers must be considered. Anaerobic digestion is a key process that can solve this waste issue as well as the economical and energetic balance of such a promising technology. Indeed, the conversion of algal biomass after lipid extraction into methane is a process that can recover more energy than the energy from the cell lipids. Three main bottlenecks are identified to digest microalgae. First, the biodegradability of microalgae can be low depending on both the biochemical composition and the nature of the cell wall. Then, the high cellular protein content results in ammonia release which can lead to potential toxicity. Finally, the presence of sodium for marine species can also affect the digester performance. Physico-chemical pretreatment, co-digestion, or control of gross composition are strategies that can significantly and efficiently increase the conversion yield of the algal organic matter into methane. When the cell lipid content does not exceed 40%, anaerobic digestion of the whole biomass appears to be the optimal strategy on an energy balance basis, for the energetic recovery of cell biomass. Lastly, the ability of these CO2 consuming microalgae to purify biogas and concentrate methane is discussed.

Growth-inhibitory and metal-binding proteins in Chlorella vulgaris exposed to cadmium or zinc

Phytochelatins, with the general structure of (gamma-Glu-Cys)n-Gly (n=2-11), are usually recognized as being strongly induced by metals in microalgae and play an important role in the detoxification of heavy metals in environment. However, there have been few studies on metallothionein (MT) synthesis in Chlorella vulgaris (C. vulgaris) exposed to heavy metals. The present study describes the growth inhibition of C. vulgaris exposed to different concentrations of cadmium and zinc, and the induction of metal-binding MT-like proteins in the cells. The amounts of metal-binding proteins, induced in the alga exposed to different concentrations of Cd and Zn, were analyzed with a size-exclusion HPLC coupled to ICP-MS. After being purified with a gel filtration column (Sephadex G-75, 3.5cmx80cm) and a desalting column (G-25, 1.5cmx30cm), the isoforms and sub-isoforms of Zn-binding protein were characterized by a reverse phase-HPLC coupled to electrospray ionization and a triple quadrupole mass spectrometer (HPLC-ESI-MS/MS). In addition, the ultraviolet spectra of purified Zn-binding proteins were analyzed in media with different pH values. The results showed that the significant inhibitory effects (at p<0.05) on the cell growth were observed when excessive metals such as 80micromoll(-1) of Cd, and 60 and 80micromoll(-1) of Zn were added. The Cd/Zn-binding proteins induced in C. vulgaris exposed to Cd and Zn were referred to as Cd/Zn-MT-like proteins in which the mean molecular mass of the apo-MT-like was 6152Da. The induced Cd/Zn-MT-like proteins might be involved in the detoxification of heavy metals, such as cadmium and zinc, by the alga.

Anti-tumour-promoting glyceroglycolipids from the green alga, Chlorella vulgaris

Two new monogalactosyl diacylglycerols were isolated from the freshwater green alga, Chlorella vulgaris, as anti-tumour promoters, together with three monogalactosyl diacylglycerols and two digalactosyl diacylglycerols. The new monogalactosyl diacylglycerol containing (7Z,10Z)-hexadecadienoic acid showed a more potent inhibitory effect toward tumour promotion than the other glycerolipids isolated.

Algal biotechnology

The review gives a general outline of macro- and microalgal biotechnology. The main methods by which algae are cultivated and harvested are described. The first section deals with the environmental factors affecting mass culture and the principles governing the design and operation of mass cultivation systems. The second section gives the main current and potential uses of algae: in wastewater treatment, a source of food and feed, an energy source, and in the production of common and fine chemicals, such as polysaccharides, lipids, glycerol, pigments, and enzymes. Pharmaceutical uses of algae are described, and their potential as a source of novel biologically-active compounds is discussed. Future developments and the great potential of algae are considered.

Osmotic adjustment in Spirulina platensis

The filamentous cyanobacterium Spirulina platensis has been examined for salt tolerance and osmotic adjustment. Salinities up to 150% seawater had little effect on growth yield or photosynthetic O2 evolution; higher salinities were markedly inhibitory. Osmotic adjustment was achieved by the intracellular accumulation of the low-molecular-weight carbohydrate glucosyl-glycerol in response to increased external salinity: in fullstrength (100%) seawater glucosyl-glycerol accounted for approximately 5.0% of the dry weight of the cyanobacterium. Trehalose was also present, particularly in cells at low salt concentration, and in 50% seawater medium accounted for up to 1.0% of the dry weight of the cyanobacterium. For cells grown in 100% seawater the ratio of trehalose to glucosyl-glycerol varied with temperature: at 37°C trehalose comprised 31% (w/w) of the low-molecular-weight carbohydrates while at 20°C only 9% of the total was trehalose. When subjected to hypo-osmotic shock the intracellular concentration of glucosyl-glycerol decreased and this was mirrored by an increase in glycogen. An understanding of the osmotic adjustment of S. platensis has implications both for the mass culturing of this and other strains of Spirulina and possibly also for the quality of the harvested product.