Co2 Fixation

58 © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 1:58–71 (2011); DOI: 10.1002/ghg3

Review

Correspondence to: Angela Dibenedetto, Department of Chemistry and CIRCC, University of Bari, Campus Universitario 70126 Bari, Italy.

E-mail: [email protected]

Received October 21, 2010; revised December 21, 2010; accepted December 21, 2010

Published online at Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ghg3.006

The potential of aquatic biomass for CO2-enhanced fi xation and energy productionAngela Dibenedetto, University of Bari, Italy

Abstract: In this review, the use of micro- and macroalgae to fi x CO2 and produce energy is discussed. The fi xation of CO2 into aquatic biomass is an option which has recently come under intensive investi-gation as it can be utilized to stimulate the growth of seaweed or microalgae. Aquatic biomass has long been cultivated and used at industrial level as a source of chemicals (agar, alginate, carragenans, and fucerellans) or as food for humans or animal feed. Recent interest in its use as a source of biofuels is due to the need to shift from fi rst-generation biofuels (biodiesel and bioethanol produced from edible biomass) to non-food sources that may grow without the use of arable land. Aquatic biomass can be grown in salty water or fresh wastewater (municipalities or process water) or else in bioreactors to produce different fuels such as bio-oil, biodiesel, bioalcohol, biohydrogen. Biogas can be produced from residual biomass after liquid fuel extraction. Microalgae are attracting much attention as they are photosynthetic renewable resources, with high lipid content and faster growth rate than terrestrial plants; they can grow in saline waters which are not suited for agriculture. While the lipid content of microalgae on a dry cellular weight basis usually varies between 20 and 40%, a lipid content as high as 85% has been reported for selected microalgal strains. They can be easily manipulated through physical stress or genetic engineering. They can also produce bioethanol. The barrier to their exploita-tion is the high cost (up to 5000 US$/t) of growth and processing. Seaweeds produce less biofuel per t-dry weight, but their growing and processing costs are much lower. In perspective, aquatic biomass can become an interesting and ubiquitous source of energy.© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd.

Keywords: biofuels; biomethanol; bio-oil; carbon dioxide fi xation into aquatic biomass; macroalgae; microalgae

Introduction

It is now clear that the progressively increasing emission of CO2 into the atmosphere may aff ect the climate; overwhelming scientifi c evidence supports

this. Burning coal, natural gas, and oil is the main origin of CO2 that behaves as a trapping agent for heat radiating from the Earth’s surface.

Th e need to reduce atmospheric CO2 is documented by several global and local agreements1 which have pushed the identifi cation and development of new technologies for CO2 separation, capture, and seques-tration in geologic formations or chemical, biological, and technological utilization. Th e utilization approach is of great interest as CO2 may be converted into valuable commercial products, contributing, thus, to

Review: The potential of aquatic biomass for CO2 fi xation A Dibenedetto

59© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 1:58–71 (2011); DOI: 10.1002/ghg3

the recycling of carbon and reducing the extraction of fuels.

An attractive way to reduce the amount of CO2 emitted could be its utilization as either technological fl uid or Cn- or renewable C1-building block. Recov-ered CO2 can be used for producing chemicals, plastics, and other useful products: numerous reviews and books have been published on various aspects of CO2 utilization.2–8

In fact, CO2 is considered to be a green, or environ-mentally benign, solvent and a cheap, non-toxic, non-fl ammable, and naturally abundant source of carbon.9,10

As a technological fl uid, CO2 has been suggested as a sustainable alternative for organic solvents in various chemical processes.11,12 It can be also used in enhanced oil recovery (EOR), enhanced gas recovery (EGR), or as a cleaning agent for textiles, a refrigerant, a food conservative and a preservative. CO2 is also used as a chemical in the production of urea (95 Mt/y), salicylic acid (70 kt/y),13,14 monomer for polymers (e.g. polycarbonates), syngas, and fuels (dry reforming of methane, methanol). Its mineralization by carbon-ation of silicates is under investigation.

In addition to such applications, the enhanced fi xation of CO2 into biomass is a challenging option under intensive investigation. High concentrations of CO2 can be used to stimulate the wild growth of seaweed and microalgae off shore or in cultures onshore and also to promote their growth in fresh wastewater (municipal effl uents or selected process waters). In this review, the biological fi xation of CO2 into aquatic biomass is discussed.

Biomass, in general, represents an environmentally and economically viable alternative to fossil fuels, moving a step toward the ‘zero emission’ option. As a matter of fact, the forecast is that biomass may contribute to the global energy balance with a share of more than 10% by 2050.15 Such expansion of the market would be possible if new biomass for energy were specifi cally grown and used in addition to the limited amount of terrestrial or residual biomass used today as an energy source. However, aquatic biomass may represent a convenient solution; it has a higher (from three to four times) growth-rate with respect to terrestrial plants, due to greater photosynthetic effi ciency. Microalgae have been extensively studied so far.16,17 More recently, marine macroalgae have been considered with increasing attention.18 CO2 recovered from either power plants or industrial fl ue gases could

be distributed into the algae culture under opportune conditions, implementing an enhanced recycle of carbon.

The interest in the exploitation of aquatic biomassAquatic biomass includes macroalgae, microalgae, and emergents (plants). Th ey can grow both in saltwater or freshwater. Macroalgae, commonly known as ‘seaweed’, are multicellular organisms. Th ey are oft en fast growing and can reach sizes of up to 60 m in length. Seaweed is mainly utilized for the production of human food, animal feed, and the extraction of hydrocolloids.

Microalgae are microscopic organisms. Diatoms are the dominant life form in phytoplankton and prob-ably represent the largest group of biomass producers on Earth. Green algae are especially abundant in freshwater. Th e golden algae are similar to diatoms and produce oils and carbohydrates. Emergents are plants that grow partially submerged in bogs and marshes.

Natural populations of seaweed have been used since the beginning of civilization for food, feed, and fertilizers; they were then cultivated and used at industrial level also as a source of agar, alginate, carrageenans, and fucerellans. Th e main producers are China, the Philippines, North and South Korea, Japan, and Indonesia with a world production of seaweed of several Mt/y.

Th e use of algae for energy production became a topic of discussion as recently as the 1970s aft er which several projects were funded to determine the techni-cal and economic feasibility of production of energy from marine biomass, i.e. macroalgae.19

Th e interest in the exploitation of aquatic biomass for energy production has grown considerably world-wide as highlighted by large industrial investment20 in such areas and by the huge number of international conferences and workshops organized and aimed at defi ning the real potential of aquatic biomass for the production of fuels. Th is enthusiasm is justifi ed by the larger productivity per hectare of aquatic biomass with respect to the terrestrial biomass (the productiv-ity of macroalgae under most performant conditions ranges from 150 to 600 tfw (tfresh weight) ha−1 y−1, compared with the typical value for sugarcane that ranges from 70 to 170 tfw ha−1 y−1). Th e higher amount of oil produced per ton of dry weight (the lipid

A Dibenedetto Review: The potential of aquatic biomass for CO2 fi xation


content of microalgae may largely vary with the species, typical values in the range of 20–40% of dry weight with exceptional records of 75–80%) and by the fact that aquatic biomass may be grown in saltwa-ter and freshwater, off ering a large choice of potential sites where it can be cultured.

Nevertheless, to enlarge commercialization of biofuels from aquatic biomass, several barriers and technical challenges must be overcome. In particular, building large ponds (of several hectares) involves large investments in terms of capital and operational (cultivation, harvesting, work-up) costs. Particular attention should be devoted to start algal strains that may produce for years.

MicroalgaeMicroalgae are currently cultivated commercially as feed for fi sh around the world in several dozen small- to medium-scale production systems, produc-ing from a few tens of tons to several hundreds of tons of biomass annually. Th e main algae genera currently cultivated photosynthetically (e.g. with light) for various nutritional products are Spirulina, Chlorella, Dunaliella, and Haematococcus (Table 1).

Microalgae can be grown in open ponds or in photobioreactors. Th e culture in open ponds is more

economically favorable,27 (Table 2; photobioreactors are much more expensive to build than open ponds) but raises the issue of land cost and water availability, appropriate climatic conditions, nutrients cost, and production. In the open pond option, other cultiva-tion aspects should be taken into consideration, such as maintenance of long-term growth of the desired algae strain without interference by competitors, grazers, or pathogens. Th ese results also indicate how much the overall production cost depends on the reactor (open or closed) cost.28

By using open-pond systems, nutrients can be provided through run-off water from nearby living areas or by channelling the water from wastewater treatment plants. Some source of waste CO2 could be effi ciently bubbled into the ponds and captured by the algae (Fig. 1). Th e water is moved by paddle wheels or rotating structures (raceway systems), and some

Micro-algae Characteristics

Spirulina is a multicellular, fi lamentous blue-green algae. Various commercial Spirulina production plants are currently in operation.22–25 Growth rate: 30 g/m2·day dry weight. Temperature: Optimum between 35–37 °C. Very tolerable to pH change.25

Chlorella sp. is a unicellular organism that can be found in almost any water environment (fresh water and marine). Growth rate: 26 g/m2·day dry weight. Temperature: 35–37 °C (depending on species). pH: Depends on species.

Dunaliella is a type of halophile microalgae especially found in sea salt fi elds. Growth rate: 1.65 g/m2·day dry weight. Temperature and pH: Depends on species.

Haematococcus pluvialis is a freshwater species of Chlorophyta. It is usually found in temperate regions around the world.26 Growth rate: 9–13 g/m2·day dry weight.

Table 1. Main strains of microalgae currently cultivated.

Table 2. Capital construction costs for three different algal production systems.21

Production systems Costs ($/ha)

Open pond 76 000

Raceways 161 000

Enclosed tubes 348 000



mixing can be accomplished by appropriately de-signed guides.

Th ere are two major types of ponds for mass cultiva-tion of microalgae: horizontal ponds and sloped cultivations ponds. Among horizontal ponds, the most preferred are the raceways ponds (Fig. 2a); circular ponds (Fig. 2b) are very expensive as they require high-energy consumption and it is diffi cult to obtain turbulence in the center of the pond. Raceway ponds, usually lined with plastic or cement, are about 20–35 cm deep to ensure adequate exposure to sunlight. Paddlewheels provide motive force and keep the algae suspended in the water. Th e ponds are supplied with water and nutrients, and mature algae are continuously removed at one end.

Th e slope cultivation pond is designed to create a turbulent fl ow while the algal culture passes through a sloping enclosure. Th e main disadvantage of this method is the cost involved.30

Methods to cultivate algae have been developed over the years. Recent developments in algae growth technology include vertical reactors31 and bag reac-tors32 made of polythene mounted on metal frames, reducing the land required for cultivation.

Using such bioreactors, microalgae can grow under light-irradiation and temperature-controlled condi-tions, with an enhanced fi xation of CO2 that is bubbled through the culture medium. Algae receive sunlight either directly through the transparent container walls or via light fi bers or tubes that chan-nel the light from sunlight collectors.

A number of systems with horizontal and vertical tubes, bags, or plates are made of either glass or transparent plastic exposed to the sun either in the free air or in greenhouses (Fig. 3).

Th e production of microalgae in open ponds de-pends on the climatic conditions. Solar irradiation and temperature are the most important factors aff ecting the farming process and its productivity. Th ese two parameters drive the growing period and, thus, the economics of the process. Th e availability of land and water are key factors for developing open-pond cultures. Semi-desert fl at lands unsuitable for tourism, industry, agriculture, or municipal develop-ment were also selected if, in such areas, biomass cultivation is strongly aff ected by the supply of CO2 and water. In fact, either CO2 or water becomes a limiting factor. In an open-pond system, the loss of water is greater than in closed tubular cultivation or bag cultivation methods. Th e water can be saline groundwater or local industrial water, draining from agricultural areas and recycled aft er harvesting algae. CO2 sources for algae growth can be from CO2 pipelines, fl ue gases from power plants, or any other source rich in CO2.

Nutrients (N- and P-compounds, micronutrients) represent one of the major costs of algal growth. Th e use of wastewater (sewage, fi sheries, and some indus-trial waters) rich in N- and P-nutrients is an economic option with a double benefi t represented by recovery and utilization of useful inorganic compounds, and the production of clean water that, fi nally, can be

Figure 1. Raceway system. Source: Lookback Biodiesel from Algae.29

Figure 2. a) Raceway pond; b) circular pond.



reused or discharged into natural basins. Should nutrients be added to water, the grown biomass will not produce a zero-emission fuel as the production of nutrients has a large associated emission of CO2. Th erefore, the use of wastewater rich in N- and P-compounds is a must in such technology. Th e direct use of fl ue gases as CO2 providers requires that algae should be resistant to the pollutants that are usually present in the fl ue-gas stream, namely nitrogen- and sulfur-oxides. Studies have shown that 150 ppm of NO2 and 200 ppm of SO2 do not aff ect the growth of some algal species.33

It must be noted that the resistance to NOx and SOy is not a common feature of all algal species, and this may represent a limitation to the direct use of fl ue gases. Another point that demands clarifi cation is the optimal concentration of CO2 in the culture, as CO2 addition lowers the pH of the medium. Although the response to the concentration of CO2 may be diff erent for the various algal species, operating at pH close to 6 may, in general, strongly aff ect algal growth. However, one of the key points in culturing microalgae, or algae in general, is to generate the optimal concentration of CO2 in the gas and liquid phase. CO2 can be supplied into the algal suspension in the form of fi ne bubbles. Th e drawback of this methodology is the residence time in the pond which is not suffi cient to allow the CO2 to be dissolved.23 A lot of CO2 is lost to the atmosphere and only 13–20% of CO2 is used.

A diff erent method of supplying CO2 is the gas exchanger; this consists of a plastic frame, which is covered by transparent sheeting and immersed in the suspension. CO2 is fed into the unit and the ex-changer fl oated on the surface. CO2 needs to be in a concentrated form and 25–60% of it is suspended and used.23 Although it is a most eff ective method, it presents as a drawback the need to use very concen-trated and pure CO2 which is trapped under the

transparent plastic frame; in this way, very little CO2 is lost into the atmosphere.

It has been experimentally calculated that for every 1 g of algal biomass produced, 1.8–2 g of CO2 is utilized (this is on the assumption that algae biomass consists of ~50% carbon). Th erefore, for a 6000 m2 pond (single algal pond), a total amount of 180 kg algal biomass (considering an algal growth rate of 30 g/m2·day) will be produced per day which uses 324–360 kg CO2 per day. If one assumes that a 500 MW coal-fi red power plant produces 9 × 106 kg CO2/day, the total amount of CO2 used per day per 6000 m2 pond is 0.0036% of total CO2.

Th ese values depend on the growth rate of the microalgae and on the pond system used.34,35 Th e growth rate is dependent on the temperature and the season (high growth rate in the summer and low growth rate in the winter). It must be concluded that although the amount of CO2 utilized is not very high, a very valuable product is obtained in high yields.

Microalgae may easily adapt to the culture condi-tions36,37 if the several parameters which infl uence the rate of growth and cell composition of micro-organisms are kept under strict control in order to guarantee a constant quality of the biomass, a para-meter particularly important for biomass exploitation.

Another factor which infl uences the growth of microalgae is the irradiation. Both in ponds and in bioreactors, light availability is of paramount impor-tance. Shadow or short light-cycles may cause a slow-down of growth; conversely intense light (as may occur in desert areas or bioreactors) does not guarantee fast growth as it may modify the cell functions.38,39

Tropical or semi-tropical areas are the most practi-cal locations for algal culture systems.22 Before starting to build a culture system, it is necessary to consider several aspects including the evaporation

Figure 3. a) Horizontal glass tubes; b) Vertical Algae Reactor; c) Water supported fl exible fi lms; d) Plastic bag.



rate which may represent a problem in dry tropical areas (where the evaporation rate is higher than the precipitation rate, a high evaporation rate increases salt concentration and pumping costs due to water loss).36 Th e precipitation rate can cause dilution and a loss of nutrients and algal biomass. Humidity must also be considered as with low relative humidity, high rates of evaporation occur that can have a cooling eff ect on the medium,30 while with high relative humidity and no winds an increase of temperature in the medium may occur (even up to 40 °C). Th e availability of water is another consideration as a location must be chosen where there is a constant and abundant supply of water for the mass culture pond systems.

MacroalgaeMacroalgae (seaweed) have quite diff erent properties than microalgae. Th eir use for energy production has received less attention so far. Th e big advantage of macroalgae is their high biomass productivity (faster growth in dry weight ha−1 y−1 than for most terrestrial crops). Th e productivity of natural basins is in the range 1–20 kg m−2 y−1 dry weight (10–150 tdw ha−1 y−1) for a 7–8 month culture. Either brown algae (Laminaria, Sargassum) or red algae (Palmaria Pal-mata, Pleonosporum spp, Porphyra tenera) have been used. Interestingly, macroalgae are very eff ective in nutrients (N, P) uptake from sewage and industrial wastewater. Th e estimated recovery capacity is 16 kg ha−1 d−1.40 To this end macroalgae have been used in Europe for cleaning municipal wastewater41,42 and in Europe and Japan for the treatment of fi shery effl uents42,43 and for recycling nutrients. Th e use of macroalgae for cleaning up effl uents from fi sheries has an economic value as macroalgae can reduce the concentration of nitrogen derivatives such as urea, amines, ammonia, nitrite, or nitrate to a level that is not toxic for fi sh allowing the reuse of water, thus reducing the cost of their growth. In Europe, macro-algae are grown in experimental fi elds and natural basins. Th ey may be cultivated in three dimensions rather than in two as on land. Macroalgae can be grown on nets or lines, and can be seeded onto thin lightweight lines suspended over a larger horizontal rope.44 Th e capacity of macroalgae as biofi lters or for nutrient uptake has been tested in the north-western Mediterranean Sea, along French coasts45 using Ulva lactuca or Enteromorpha intestinalis that adapted to

non-natural basins. Also in a colder climate, macroal-gae grow at an interesting rate. For example, in Den-mark, the Odjense Fiord produces ca. 10 kt per day of dry-weight-biomass equivalent to ca. 10 t per year per ha.

Although macroalgae can grow in both hemi-spheres, climatic factors may aff ect the productivity by reducing either the rate of growth or the growing season. Th e Mediterranean Sea has ideal climatic conditions for a long growing season, with good solar irradiation intensity and duration, and with a correct temperature. Moreover, along the coasts of several EU countries (Italy, Spain, France, Greece) fi sh ponds exist that may be the ideal location for algae ponds.

As reported earlier, a point that requires careful evaluation is the infl uence of CO2 concentration on algae growth. In fact, macroalgae may use either CO2 or HCO3

− as a source of carbon, making the pH requirements less strict. In fact, algae that use HCO3

−

would prefer basic water, while algae that use CO2 would grow better in more acidic media, where the free CO2 concentration is higher.

Th e photosynthesis of macroalgae is saturated at diff erent levels of CO2, ranging from 500 to 2000 ppm.46,47 Th is means that with CO2 concentra-tion up to fi ve times the atmospheric concentration, under the correct light conditions and nutrient supply macro algae may grow with the same or better perfor-mance than they show in natural environments.48

Concentrations of CO2 in the gas phase up to 5% (that means 150 times atmospheric concentration) have been used and have been shown to be acceptable for growing macroalgae such as Gracilaria bursapas-toris, Chaetomorpha linum and Pterocladiella capilla-cea.49 Macroalgae (Table 3) require less sophisticated techniques for growing: coastal farms are the most used techniques for macroalgae. Th e world market of seaweed is high. Its aquaculture production is around 11.3 million wet tonnes. China is the main producer (92% of the world seaweed supply).50 Brown seaweed represents 63.8% of the production, while red seaweed represents 36.0% and green seaweed 0.2%. Approxi-mately one million tonnes of wet seaweed are har-vested and extracted to produce about 55 000 tones of hydrocolloids, valued at almost US$ 600 million.51

Th e adaptation from wild conditions to pond culture is not straightforward. Th alli can be cut and used for starting a new culture. In principle, it is more suitable to cultivate macroalgae using natural climatic condi-tions, as the adaptation to diff erent climates may not



be easy. Th e knowledge of physiological conditions is essential for the defi nition of the best operative parameters (optimized growing conditions).52

Emergents or aquatic plants Aquatic plants, also known as hydrophytes, grow in ponds, shallow lakes, marshes, ditches, reservoirs, swamps, canals, and sewage lagoons. Less frequently, they also live in fl owing water, in streams, rivers, and springs. Such macroscopic aquatic fl ora includes aquatic angiosperms (fl owering plants), pteridophytes (ferns), and bryophytes. Th ey can be divided into four categories according to the habit of growth: fl oating unattached, fl oating attached, submersed, and emer-gent. Macrophytes play a key role in nutrient cycling to and from the sediments, and help stabilize river

and stream banks. Plants are oft en used for water phytodepuration as they effi ciently use N and P compounds present in wastewater; some species can also concentrate heavy metals.53 Such macrophytes can be spontaneous, but in some countries they are grown for several purposes, from water treatment to nutrition (human and animal) and the production of materials used in various sectors, including building. With respect to microalgae and macroalgae, plants may contain a larger content of cellulosic materials and require diff erent technologies for their treatment and use for energetic purposes.

Harvesting of aquatic biomassDiff erent kinds of aquatic biomass require harvesting techniques which diff er in cost and energy. While

Chaetomorpha linum is present in unattached form in both estuarine systems and coastal lagoons subject to eutrophication. It can live all the year and can reach high biomass values, estimated around 3.5–5 kgfwt m−2.

Ulva laetevirens (as U. rigida C. Agardh) is present in attached and unattached form in estuaries and shallow eutrophic lagoons. During the growing season it may reach 15–20 kgfwt m−2 and large free-fl oating thalli.

Gracilaria bursa-pastoris is one of the few Rhodophyceae able to live in eutrophic coastal lagoons where in some periods it becomes the dominant species of the drifting bed. It is present in attached and unattached form in coastal lagoons and both hemispheres.

Pterocladiella capillacea, commonly lives on rocky hard substrata, often on vertical rock-faces, from the inter-tidal level to about 20 m depth, in wave-exposed areas. This species is widely distributed in the Mediterranean Sea, but lives in both hemispheres.

Codium vermilaria lives on hard horizontal substrata either in sheltered or lightly wave-exposed areas, between 0 and 50 m depth, in shady places. This species is distributed in the boreal hemisphere from North Atlantic Ocean to Mediterranean Sea.

Table 3. Macroalgae.



macroalgae and plants require simple operations, microalgae, due to their size and, sometimes, fragility, demand sophisticated equipment and handling operations.

Th e choice of harvesting methods depends on a few factors: (i) type of algae that has to be harvested (fi lamentous, unicellular, etc.); (ii) whether harvesting occurs continuously or discontinuously; (iii) what the energy demand is per cubic meter of algal suspension; and (iv) what the investment costs are.36,39,54

Th e technologies mainly used with microalgae are centrifugation, sedimentation, fi ltration, screening and straining, and fl occulation. Various fl occulants have been used, covering a large variety of chemical structures such as metal compounds,55 cationic polymers,56 and natural polymers such as chitin.57 Th ey have been employed not only at laboratory scale, but also at industrial scale. Such ‘induced fl occulation’ may be accompanied by a ‘spontaneous or auto-fl occulation’ that can be caused by pH varia-tion of the culture medium upon CO2 consumption. For example, an increase of pH may cause the precipi-tation of phosphates (essentially Ca-phosphate) which causes fl occulation of algae. Aggregation of algae, produced by organic secreted substances58 or aggrega-tion with bacteria,59 may also occur that facilitates their sedimentation.

Centrifugation is a very popular technique today, but still it presents some drawbacks such as the rate of separation. Most advanced technologies are based on the use of membranes (tubular, capillary, or hollow-fi ber membranes) that are becoming more and more popular.60 Th e size of the pore decreases in the order from tubular (5–15 mm) to capillary (1 mm) to hollow-fi ber (0.1 μm) and the risk of plugging in-creases with the decrease of the pore diameter.

Th e harvesting of macroalgae and plants requires more immediate and less sophisticated technologies. Th e technique depends on the fact that the biomass is grown fl oating-unattached or fl oating-attached to a hard substrate. In the former case, the biomass can be easily collected using a net (as in fi shing); in the latter case it must be cut from the substrate. Automated or manual devices can be used for the collection.61

Aquatic biomass compositionAquatic biomass contains several pools of chemicals at diff erent concentrations depending on the physical stresses or genetic manipulation induced on the

organism. Table 4 compares the composition of microalgae, macroalgae, and grass. Very oft en, despite being members of the same marine algae species, the chemical compositions were found to be diff erent according to the harvesting site.62

In general, microalgae and macroalgae can be used in diff erent sectors:

1. Energy (hydrocarbons, hydrogen, methane, methanol, etc.).

2. Foods and chemicals (proteins, oils and fats, sterols, carbohydrates, sugars, alcohols, etc.).

3. Other chemicals (dyes, perfumes, vitamins/supplements, etc.).

Aquatic biomass can be used as a raw, unprocessed food as they are rich in carotenoids, chlorophyll, phycocyanin, amino acids, minerals, and bioactive compounds.

Besides their nutritional value, these compounds have applications in the pharmaceutical fi eld as immune-stimulating, metabolism increasing, choles-terol reducing, anti-infl ammatory, and antioxidant agents;63 they are also rich in omega-3 fatty acids, which have a signifi cant therapeutic importance inherent in the ability to act as an anti-infl ammatory to treat heart disease.

Due to the high product-distribution entropy, the extraction of a single product may have an economic benefi t if the product represents several tens % of the global dry-mass. If it is present at the level of few units %, then it should have a high market value for meet-ing economic criteria. As mentioned above, the ability of algal organisms to concentrate a type of resource (proteins, starch, lipids) upon stress may help to reduce the entropy and to increase the concentration of a given product in the biomass. Th is issue is particularly relevant when the use of aquatic biomass for energy purposes is considered. Due to the cost of cultivation, producing biomass with a high content of energy products should be a must.

Table 4. Dry biomass composition (%) (organic fraction).

Microalgae Macroalgae Grass

Saccharides 5–25 50–80 35

Lipids 20–40 8–24 3

Proteins 20–50 7–27 25

Fibres (lignin) – 33–50 37



Technologies for chemicals productionChemicals can be extracted from the biomass by using a variety of technologies of diff erent intensity (destructive, semi-destructive, and non-destructive). Th ere is a relation between the soft ness-hardness of the technology used and the complexity of the struc-ture of the chemicals extracted. Soft er technologies will aff ect less complex molecular structures that will be recovered unchanged. Hard technologies will destroy complex networks and complex molecules.

Biomass is suitable for the production of diff erent products such as: bio-oil, biodiesel, bioalcohol, biohydrogen, and biogas, all related to the production of energy.

Extraction techniqueTh e extraction of chemicals from microalgae and macroalgae may require diff erent technologies due to the diff erent size and quality of the cell membrane of the algae. Depending on the species strain, the cell membrane can result to be very hard, so that crushing of the membrane is recommended prior to the extrac-tion. Such crushing is quite eff ective if performed at low temperature, typically the liquid nitrogen tem-perature (183 K). Th is will obviously increase the cost of the extracted oil and lower its net energetic value.

Among the technologies used to produce chemicals from biomass, pressure and solvent extraction can be used for the extraction of bio-oil. Th e former tech-nique is not well suited to microalgae.

Extraction by using organic solvents (that may be toxic to animals and humans) is very oft en used. Th is may have a drawback due to the retention of solvent by the algal mass which may represent a risk if the extract is used as food, but has no consequences when the extract is used as fuel.

Supercritical carbon dioxide (scCO2) may substitute the organic solvent as it has some unique advantages and is considered a good candidate for algae treat-ment because it is a non-toxic and fully ‘green’ solvent.64 Despite the advantages, using scCO2 to extract valuable compounds from microalgae is not the prevailing technology in use today even though production costs are of the same order of magnitude as those related to classical processes. In fact, for such techniques quite anhydrous materials are recom-mended (water content below 5%), so energy should be consumed to dry the biomass.

Bench scale scCO2 experiments on microalgae have been performed on Botryococcus, Chlorella, Dunaliella, and Arthrospira from which diff erent types of valuable products have been extracted as hydrocarbons (up to 85% mass of cell from Botryococ-cus), paraffi nic and natural waxes from Botryococcus and Chlorella, strong antioxidants (astaxanthin, ß-carotene) from Chlorella and Dunaliella, and linolenic acid from Arthrospira.

Th e moderate temperatures and inert nature of CO2 have been shown to virtually eliminate the degrada-tion of the product extracted. In addition to the extract quality, the ability to signifi cantly vary the CO2 solvation power by changes in pressure and/or temperature adds operating fl exibility to the scCO2 extraction process that no other extraction method, including solvent extraction, can claim.65

For the scCO2 extraction, the biomass should be dried, then the cellular wall has to be broken in order to increase the extraction yield (it is possible to use liquid nitrogen, or a diff erent method).66 Sometimes methanol can be added as co-solvent in order to increase the extraction yield.

Bio-oil content of aquatic biomass Th ere is much interest lately in the use of microalgae for the production of biodiesel, although this is not the only producible fuel: biogas can also be produced, as well as bioethanol or biohydrogen. Th e quality and composition of the biomass will suggest the best option for the biofuel to be produced. A biomass rich in lipids will be suitable for the production of bio-oil and biodiesel, while a biomass rich in sugars will be better suited to the production of bioethanol. Th e anaerobic fermentation of sugars, proteins, and organic acids will produce biogas.

Several species of microalgae are very rich in lipids (up to 70–80% dry weight, with a good average standard of 30–40%) and this makes a given species-strain more or less suitable for bio-oil production. Th e highest values are relevant to particular growing conditions. In a commercial culture what is of interest is the productivity of a pond, i.e. production per unit time.

Table 5 shows, as a comparison, the amount (L) of oil per hectare per year of diff erent types of biomass including microalgae.67,68

Macroalgae, in general, present a lower content of lipids than microalgae, and a larger variability.69 Th e



lipid content largely depends on the cultivation technique and on the time of the year macroalgae are collected.70,71 Th ese are, thus, key issues to be taken into consideration in the development of a commer-cial exploitation of such biomass.

Comparing microalgae and macroalgae, it must be considered that macroalgae are produced at lower costs than microalgae. Th e energy value of an alga cannot be stated just on the basis of the specifi c amount of oil produced but also from other very important parameters, such quality, the possibility of producing another form of energy from the residue obtained aft er the lipid extraction, etc.

The quality of bio-oilAlthough algae biomass can be thermally processed to aff ord an oily product, the acidity and composition of the liquid are such that its direct use is not suited and complex processing is needed before its use. We consider here the extraction of lipids. Th ey are a mixture containing more than a single type of fatty-acid (FA), most frequently the lipid fraction of algae (both microalgae and macroalgae) contains a large

variety of FAs, with diff erent number of unsatura-tions, as shown in Table 6. Th is is an important issue for assessing the energetic value of a biomass. Th e number of unsaturations in an FA is important as it determines the usability of the compound as a fuel. In fact, the optimal conditions for having a biodiesel with good combustion properties is the presence of only one unsaturation in the C-chains.72 Th erefore, the higher the number of unsaturations, the lower the quality of the biodiesel. Th is brings us to the conclu-sion that the biodiesel extracted from aquatic biomass oft en may need a hydrogenation treatment in order to reduce its unsaturation number73 and produce a better quality fuel.

It has been experimentally demonstrated that the product-distribution and unsaturation-distribution can depend on the CO2 concentration. An increase in the CO2 concentration of up to 10% in the gas-phase has almost doubled the total concentration of FAs (from 29.1 to 55.5%) and in particular that of FAs 16:0, 18:1, 20:4, and 20:5 in C. linum. In general, it has been found that the number of unsaturations may increase with the concentration of CO2.74,75

Bio-oil, such as extracted, can be directly used in thermal processes or in combustion, but cannot be used in diesel engines as it presents a Low Heating Value-(LHV) (8–12 MJ/kg) and high viscosity. To the latter use it can be converted into biodiesel through a transesterifi cation reaction (Scheme 1) in order to increase the LHV to 36 MJ/kg.

Considering biodiesel from an environmental point of view, it includes several benefi ts: the reduction of carbon monoxide (50%) and carbon dioxide (78%) emissions,76 the elimination of SO2 emission as biodiesel does not include sulphur; and the reduction of particulate. As biodiesel is non-toxic and biode-gradable, its use and production is rapidly increasing, especially in Europe, the United States, and Asia. A growing number of fuel stations are making biodiesel

Table 5. Yields (L ha−1 y−1) of various types of biomass.

Biomass Yield (L ha−1 y−1)

Corn 170

Soybeans 455 to 475

Saffl ower 785

Sunfl ower 965

Rapeseed 1200

Jatropha 1890

Coconut 2840

Palm 6000

Microalgae 47 250 to 14 2000

Fatty acid Species and percentage of a given compound in the species

Compound N of Catoms/unsaturated bonds Fucus sp

Nereocystisluetkeana Ulva lactuca

Enteromorpha compressa

Padiva pavonica

Laurencia obtuse

Saturated C12→C20 15.6% 27.03% 15.0% 19.6% 23.4% 30.15

Monounsaturated C14→C20 28.55% 15.84% 18.7% 12.3% 25.8% 9%

Polyunsaturated C16/2→C16/4, C18/2→C18/4, C20/2

55.86% 57.11% 66.3% 68.1% 50.8% 60.9%

Table 6. Distribution of fatty acids in lipids present in some macroalgae.



available to consumers, and a growing number of large transport fl eets use some proportion of biodiesel in their fuel. Table 7 reports some fuel properties of diff erent types of biodiesel.

Production of bioalcohol Aquatic biomass contain a variable quantity of simple sugars and starch with a low amount of cellulosic materials that are suited for the production of alcohols. Mainly ethanol is obtained by fermentation of such biomass, but other alcohols such as butanol can be produced in smaller amounts. Th e production of bioethanol from a biomass alternative to corn is a leading research theme these days.

Microalgae can be used to produce bioethanol29 as they are rich (up to >50% of the dry weight) in starch and glycogen.80,81

Similarly, macroalgae can be used for ethanol fermen-tation by converting their storage material to ferment-able sugars.82 Th e absence or low presence of lignin makes the enzymatic hydrolysis of algal cellulose more simple than in the case of terrestrial cellulosic biomass.

Very interesting is that bioethanol can be obtained using oleaginous algal residue aft er the extraction of oil which contains yet fermentable sugar.

Bioethanol from algae can be obtained from starch/cellulose which have to be extracted mechanically (ultrasound, disintegration, mechanical shear, etc.) from the cells or by using enzymes. Th e starch is then separated by extraction with water or an organic solvent and used for fermentation to yield bioethanol. Th e latter process can be carried out in a single step (using amylase) or a double step (saccharifi cation where the starch is hydrolyzed to simple sugars and fermentation using suitable yeast strain). At the end, ethanol needs purifi cation to obtain a concentration >95%.83

Besides starch, several algae, especially green algae, can accumulate cellulose as the cell wall carbohydrate, which can also be used for ethanol production. Like the cellulosic biomass from other plant sources, the cellulosic biomass from the algae can also be enzy-matically hydrolyzed using cellulase enzyme and converted into simple sugars which can then be easily fermented to ethanol.

Scheme 1. The conversion of a tryglyceride (lipid or bio-oil) into biodiesel (FAME).

Density (kg/L)

Ash content (%)

Flash point (°C)

Pour point (°C)

Cetane number

Calorifi c value (MJ/kg)

Ref

Algae 0.801 0.21 98 −14 52 40 77

Peanuts – – 271 −6.7 41.8 –

78Soya bean 0.885 – 178 −7 45 33.5

Sunfl ower 0.860 – 183 – 49 49

Diesel 0.855 – 76 −16 50 43.8

Biodiesel from marinefi sh oil

– – 103 – 50.9 41.4 79

Table 7. Fuel characteristics of different bio-oils.



Production of biogasTh e anaerobic fermentation of fresh organic materials is largely used for the production of biogas, a mixture of methane and CO2. Th e effi ciency of the process depends on several parameters and the quality of the biogas (ratio CH4/CO2) depends on the feed and the operative conditions.84,85 Reactors of diff erent types are used and both mesophilic micro-organisms and thermophilic bacteria are active.86 A large application of such technology is the production of biogas by fermentation of fresh municipal waste, thus turning municipal waste into energy (methane) for the community.

By their composition, algae, or aquatic biomass in general, are quite suited for conversion into biogas. It is clear that some kind of biomass may not be ad-equate for such applications. Literature data indicates that some species of microalgae are quite good for biogas production.87,88

ConclusionsAquatic biomass, i.e. microalgae, macroalgae, and plants, have a chemical composition that may vary according to growing conditions also within the same strain. To produce energy, the wild type are not always suitable as they can be sensitive to the culture conditions, so it is better to use a selected cultivated strain in order to have an optimal energetic yield.

Aquatic biomass are a source of several compounds which can be used as chemicals or to produce energy. In particular the coproduction of chemicals and fuels can be of great importance in order to make positive the economic balance. In fact, the cost of fuels derived from aquatic biomass is not competitive with that of fossil fuels. Th e correct application of the biorefi nery concept may result in the production of fuels at low cost if high-value chemicals are coproduced. In the near future aquatic biomass might contribute to the production of transport fuels in a signifi cant volume, supposing that the right conditions for its growth, collection, and processing are developed. In any case it seems that the coproduction of chemicals and fuels is necessary for profi table exploitation of aquatic biomass.

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Angela Dibenedetto

Angela Dibenedetto is Associate Professor at the Department of Chemistry, University of Bari (IT). Her scientifi c interests focus on CO2

utilization in synthetic, coordination and organometallic chemistry, catalysis, green chemistry, marine

biomass production by enhanced CO2 fi xation, marine biomass as source of fuels and chemicals applying the Biorefi nery concept. She is Director of the Interdepart-mental Centre on Environmental Methodologies and Technologies–METEA UniBa.

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Co2 Fixation