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

Abstract

Microalgal and/or cyanobacterial biomass could be more advantageous than current plant feedstocks for theproduction of ethanol and/or other fermentation products. Enzymatic saccharification of microalgal biomasscurrently requires harsh pretreatments and significant amounts of expensive hydrolytic enzymes. The purpose ofthis 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 ofalgal biomass. We showed that a newly isolated strain of the fungus Trichoderma harzianum was able to grow atthe expense of intact cells of the alga Chlamydomonas reinhardtii as a sole source of nutrients. The fungal spentmedium exhibited pectinolytic, amylolytic, cellulolytic, and proteolytic activities and efficiently saccharifiedeither dry and milled biomass or intact cells of the microalga. Biomass of a cell-wall deficient C. reinhardtii strainwas saccharified at a similar efficiency than biomass of the parental strain with T. harzianum enzymes, indicatingfull 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 inthis study is among the highest reported so far and encourages research for scaling up production of T. harzianumenzymes as well as the evaluation of biomass of other algal strains as feedstocks for the production of ethanol.