**3. Algal polysaccharides**

Ethanol production from algae is based on fermentation of algal polysaccharides which are starch, sugar and cellulose. For microalgae, their carbohydrate content (mostly starch) can be reached to 70% under specific conditions [29]. Microalgal cell walls are divided into inner cell wall layer and outer cell wall layer. Outer cell layer can be trilaminar outer layer and thin outer monolayer. Also there can be no outer layers as well [30]. Outer cell walls of microalgae contain certain polysaccharides such as pectin, agar and alginate. However their composition can be vary from species to species [30]. On the contrary, inner cell walls of microalgae constitute mostly cellulose, hemicellulose and other materials [30]. Due to have cellulose in their cell walls and starch, microalgae are considered as a feedstock for production of bioethanol [31]. Most of their cell wall polysaccharides and starch can be fermented for bioethanol production [32].

Similarly, carbohydrate content of macroalgae is found 25-50% in the green algae, 30-60% in the red algae and 30-50% in the brown algae. Macroalgae species which have the highest polysaccharide content are *Ascophyllum* (42-70%)*, Porphyra* (40-76%) and *Palmaria* (38-74%). High carbohydrate content of algal species are presented in Table 3 [32]. Polysaccharides in the cell wall of macroalgae are composed of cellulose and hemicelluloses. Cellulose and hemicellu‐ loses content of macroalgae compose 2-10% of dry weight. Lignin is only exists in *Ulva* species andit constitutes 3% ofdry weight[27].Differently from microalgae, alginate, mannitol, glucan and laminarin are the most abundant polymers in macroalgal structure [42]. Alginates are


**Table 3.** Carbohydrate content of algal species

mannitol, alginate which are used in various sectors [25]. They are separated from microalgae with having low lipid content and different from lignocellulosic material with having less or

Microalgae stand out as biodiesel feedstock with the ability of lipid production and high photosynthetic efficiency. As for macroalgae, they are utilized for biogas or bioethanol production with their carbohydrates [26]. First studies as algal biofuels are focused on biodiesel production. However, there is a potential for carbohydrates in the structure of algae which can be utilized for ethanol production after various hydrolysis processes. Algal cells in the water don't need structural biopolymers such as hemicellulose and lignin which are necessary for terrestrial plants [4]. This simplifies the process of bioethanol production. Marine algae can produce high amount of carbohydrate every year. Also it is expected that algae will meet the demand of biofuel feedstock due to harvest in a short time than other biofuel raw materials [27]. Microalgae which have high amount of starch such as *Chlorella, Dunaliella, Chlamydomonas, Scenedesmus* are very useful for bioethanol production. In addition to that, microalgae don't need energy consumption for distribution and transportation of molecules like starch. Like microalgae, macroalgae are also raw materials that can be used in ethanol production. Absence of lignin or having less lignin in the structure, simplifies the hydrolysis stages [4,28]. Although it changes with the algal species, they have various amounts of heteropolysaccharides in their structures. Whereas red algae contain carrageenan and agar, brown algae have laminaran and

Ethanol production from algae is based on fermentation of algal polysaccharides which are starch, sugar and cellulose. For microalgae, their carbohydrate content (mostly starch) can be reached to 70% under specific conditions [29]. Microalgal cell walls are divided into inner cell wall layer and outer cell wall layer. Outer cell layer can be trilaminar outer layer and thin outer monolayer. Also there can be no outer layers as well [30]. Outer cell walls of microalgae contain certain polysaccharides such as pectin, agar and alginate. However their composition can be vary from species to species [30]. On the contrary, inner cell walls of microalgae constitute mostly cellulose, hemicellulose and other materials [30]. Due to have cellulose in their cell walls and starch, microalgae are considered as a feedstock for production of bioethanol [31]. Most of their cell wall polysaccharides and starch can be fermented for bioethanol production [32].

Similarly, carbohydrate content of macroalgae is found 25-50% in the green algae, 30-60% in the red algae and 30-50% in the brown algae. Macroalgae species which have the highest polysaccharide content are *Ascophyllum* (42-70%)*, Porphyra* (40-76%) and *Palmaria* (38-74%). High carbohydrate content of algal species are presented in Table 3 [32]. Polysaccharides in the cell wall of macroalgae are composed of cellulose and hemicelluloses. Cellulose and hemicellu‐ loses content of macroalgae compose 2-10% of dry weight. Lignin is only exists in *Ulva* species andit constitutes 3% ofdry weight[27].Differently from microalgae, alginate, mannitol, glucan and laminarin are the most abundant polymers in macroalgal structure [42]. Alginates are

no lignin in their structure [6].

144 Biofuels - Status and Perspective

mannitol in their structure [6].

**3. Algal polysaccharides**

polymers which are obtained from cell walls of various brown algae. They consist of mannur‐ onic acidand L-gluronic acid monomers and they are extracted from cell walls by using sodium carbonate. Although they are usually used as stabilizer in pharmaceutical industry, they also used in paper and adhesive manufacture, oil, photography and textile industries [43-45]. Caragenan is another polysaccharide which is obtained from red algae. It is used as stabilizer in food, textile and pharmaceutical industry. Agar is also acquired from red algae. Like caragenans and alginates, it is extracted with hot water and used as stabilizer and gelling agent. 90% ofproducedagarisutilizedin the foodindustry,the remaining 10% isusedin microbiolog‐ ical and biotechnological field [44,46]. Mannitol which is a structure in brown algae is a sugar alcohol, especially found in *Laminaria* and *Ecklonia.* Mannitol content of macroalgae can change with season and environmental conditions. Mannitol can be used in pharmaceutical, paint, leather and paper manufacture. In addition to that, mannitol can be utilized in food industry as a coating material [27]. Laminarin is a polysaccharide which helps the immune system by increasing the B cells, provides protection against infection by bacterial pathogens and severe irradiation. Another polysaccharide from macroalgae is ulvan. It is mainly presented in *Ulva sp.* and it is source of sugars for production of fine chemicals [27].
