Abstract
The constraints and advantages in cellulolytic enzymes production by fungal biofilms for a consolidated bioconversion process were investigated during this study. The biofilm cultivations were carried out in reactors designed for consolidated bioprocessing Multispecies Biofilm Membrane reactors, (MBM) where an aerobic fungal biofilm produces the lignocellulolytic enzymes while a fermenting microorganism forms the fermentation product at anaerobic conditions. It was shown that although mycelial growth was limited in the MBM reactors compared to submerged cultivations, the secretion of cellulolytic enzymes per cell dry weight was higher. When Trichoderma reesei was used as the sole enzyme producer, cellobiose accumulated in the liquid medium as the result of the deficiency of β-glucosidase in the fungal secretome. To enhance β-glucosidase activity, T. reesei was co-cultivated with A. phoenicis which is a β-glucosidase overproducer. The two fungi formed a multispecies biofilm which produced a balanced cellulolytic cocktail for the saccharification of plant biomass. The mixed biofilm reached a 2.5 fold increase in β-glucosidase production, compared to the single T. reesei biofilm. The enzymatic systems of single and mixed biofilms were evaluated regarding their efficiency on cellulosic substrates degradation. Washed solids from steam pretreated beechwood, as well as microcrystalline cellulose were used as the substrates. The enzymatic system of the multispecies biofilm released four times more glucose than the enzymatic system of T. reesei alone from both substrates and hydrolyzed 78 and 60% of the cellulose content of washed solids from beechwood and microcrystalline cellulose, respectively.
Highlights
Biological decay, bioremediation and biodegradation of plant biomass are performed by the combined action of diverse microorganisms or by complex microbial communities
This study showed the efficiency of fungal biofilms as cellulolytic enzyme producers using two filamentous fungi T. reesei and A. phoenicis
A. phoenicis could not grow on Avicel as the sole microorganism, due to the lack of CBH activity in its secretome, it was possible to form a multi-species biofilm with T. reesei in the MBM reactors, proving the versatility of the reactor design to host multi-species fungal biofilms
Summary
Biological decay, bioremediation and biodegradation of plant biomass are performed by the combined action of diverse microorganisms or by complex microbial communities. A paradigm of cellulolytic multispecies biofilms from nature is the cattle rumen where cellulose degradation is mainly performed by such multispecies biofilms which are formed on the lignocellulosic particles. The successful degradation of cellulose by these microbes is based on the dynamic equilibrium among the different species: Rumen microorganisms benefit from each other’s metabolism, in order to effectively access and consume their preferred substrates (Prescott et al, 2002; Nagaraja, 2016). These interactions create distinguished ecological niches, making the rumen suitable for many different microorganisms. The aim was to make the production of lignocellulosic ethanol economically attractive, by decreasing the cost of enzymes production and transport as well as the number of unit operations (Jouzani and Taherzadeh, 2015)
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