Hydrolysis

Microalgal biomass can be pre-treated via hydrolysis using the cellulosic enzymes obtained from fungi, protozoa or bacteria. Widely used cellulosic enzymes are cellu — lase from Thrichoderma reseei and cellulase from Aspergillus niger. The cellulase enzyme consists of three main components [63]; (1) 1,4-b-D-glucan glucanohydro — lases (endoglucanases); break down the cellulose crystallinity, (2) 1,4-b-D-glucan cellobiohydrolases and 1,4-b-D-glucan glucanohydrolases (exoglucanases); the exog- lucanases hydrolyse the individual cellulose fibres into simple sugars and cellobiohy — drolases attack the chain ends producing cellobiose, (3) b-D-glucoside glucohydrolases (b-glucosidases); release glucose monomers by hydrolysing the disaccharides and tetrasaccarides of cellulose and form glucose that is ready to be used in the fermenta­tion process. Even though it is a common practice for biomass to be pre-treated prior to enzymatic hydrolysis, microalgal biomass can undergo the hydrolysis process directly without any pre-treatment due to its non-lignin composition. This makes production of bioethanol from microalgal biomass more economical.

Table 6 Microorganisms commonly used for industrial ethanol production Natural sugar utilization pathways Major products

Organism

Glu

Man

Gal

Xyl

Ara

EtOH

Other

O2 needed

pH

Anaerobic bacteria

+

+

+

+

+

+

+

Neutral

Escherichia coli

+

+

+

+

+

+

Neutral

Zymomonas mobilis

+

+

Neutral

Saccharomyces

+

+

+

+

Acidic

cerevisiae

Pichia stipitis

+

+

+

+

+

+

+

Acidic

Filamentous fungi

+

+

+

+

+

+

Acidic

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