Efficient saccharification of microalgal biomass by Trichoderma harzianum enzymes for the production of ethanol

Microalgal and/or cyanobacterial biomass could be more advantageous than current plant feedstocks for the production of ethanol and/or other fermentation products. Enzymatic saccharification of microalgal biomass currently requires harsh pretreatments and significant amounts of expensive hydrolytic enzymes. The purpose of this study was: i) to analyze the quality of biomass of a cell-wall deficient microalga as a saccharification substrate for enzymatic hydrolysis; and ii) to identify an alternative source of enzymes for the saccharification of algal biomass. We showed that a newly isolated strain of the fungus Trichoderma harzianum was able to grow at the expense of intact cells of the alga Chlamydomonas reinhardtii as a sole source of nutrients. The fungal spent medium exhibited pectinolytic, amylolytic, cellulolytic, and proteolytic activities and efficiently saccharified either dry and milled biomass or intact cells of the microalga. Biomass of a cell-wall deficient C. reinhardtii strain was saccharified at a similar efficiency than biomass of the parental strain with T. harzianum enzymes, indicating full capacity of these enzymes to circumvent the cell-wall recalcitrance to hydrolysis. Algal biomass saccharification yielded up to 22.4 g reducing sugars.L-1 and could be quantitatively converted into ethanol by fermentation with Saccharomyces cerevisiae. The efficiency of conversion of algal biomass into ethanol showed in this study is among the highest reported so far and encourages research for scaling up production of T. harzianum enzymes as well as the evaluation of biomass of other algal strains as feedstocks for the production of ethanol.