Valorization of untreated rice bran towards bioflocculant using a lignocellulose-degrading strain and its use in microalgal biomass harvest

Abstract

BackgroundMicroalgae are currently considered as a promising feedstock for the production of biofuels and high-value products. However, the efficient harvest of microalgal biomasses from their culture broth is a major challenge. The harvesting of algal biomass by flocculation combined with gravity sedimentation is more convenient and cost-effective than traditional methods such as centrifugation and filtration. Compared to inorganic and chemically synthetic flocculants, bioflocculants are a suitable choice for microalgal harvest due to their biodegradable and nontoxic properties. Nonetheless, the high production costs associated with expensive substrates hinder the commercial applications of bioflocculants. Previous studies have shown that the hydrolysates of lignocellulosic biomasses from dilute acid hydrolysis can be utilized as an inexpensive carbon source for the production of bioflocculants. However, the toxic by-products generated in the dilute acid hydrolysis step limit the efficiency of subsequent fermentation. The strains that produce bioflocculants by using untreated lignocellulosic materials can circumvent the pretreatment process, as well as promote the application of bioflocculants in microalgal harvest.ResultsUnder alkaline fermentation conditions, the alkaliphilic strain Bacillus agaradhaerens C9 secreted 1.69 IU/mL of alkali-tolerant xylanase and 0.06 IU/mL of cellulase, indicating that this particular strain can efficiently convert untreated rice bran into bioflocculant (RBBF-C9), thereby circumventing rice bran pretreatment for downstream fermentation. The optimal fermentation conditions that result in the highest bioflocculant yield (12.94 g/L) were as follows: 20 g/L of untreated rice bran, 3 g/L of yeast extract, and 20 g/L of Na2CO3 at 37 °C for 24 h. RBBF-C9 contained 74.12% polysaccharides and 4.51% proteins, and was estimated to be 137 kDa. Furthermore, the bioflocculant RBBF-C9 exhibited good flocculating efficiency (91.05%) of oil alga Chlorella minutissima UTEX2341 when 60 mg/L of RBBF-C9 was added into the algal culture broth.ConclusionsThis study demonstrated that untreated rice bran is a suitable inexpensive substrate for the production of bioflocculants, and thus provides a novel approach in utilizing rice bran. The extracted bioflocculants may be potentially used in biomass harvesting of the oil algae C. minutissima UTEX2341 from the culture broth.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0780-6) contains supplementary material, which is available to authorized users.

Improving enzymatic digestibility of wheat straw pretreated by a cellulase-free xylanase-secreting Pseudomonas boreopolis G22 with simultaneous production of bioflocculants

“…1 a). The xylanase activity of G22 was the highest of four tested bioflocculant-producing bacteria, the latter of which, showed maximum xylanase activities of 0.46 U mL −1 ( C. cellulans L804) [ 26 ], 1.69 U mL −1 ( B. agaradhaerens C9) [ 27 ], and 1.03 U mL −1 ( Pseudomonas sp. GO2) [ 21 ].…”

“…The bioflocculant yields of G22 were comparable to that of most other bioflocculant-producing strains, which produced bioflocculant yields of 0.205–2.93 g L −1 [ 50 , 51 ]. However, these yields were lower than that of some high-producing strains, such as C. cellulans L804 (4.75 g L −1 ) [ 26 ], and B. agaradhaerens C9 (12.94 g L −1 ) [ 27 ]. The decrease in flocculating efficiency is most likely due to the secretion of bioflocculant-degrading enzymes via the bacteria and cell autolysis [ 24 , 52 ].…”

“…Recently, certain lignocellulolytic enzyme-producing bacteria were reported to produce polysaccharide bioflocculants via directly using untreated lignocellulosic biomass as a carbon source. Liu et al [ 26 , 27 ] reported that Cellulosimicrobium cellulans L804 and Bacillus agaradhaerens C9 could convert untreated biomass into bioflocculants by secreting various lignocellulolytic enzymes. Cellulases and xylanases secreted by the strain Pseudomonas sp.…”

Screening for Bio-flocculant Producing Bacterial Strains from Asa River in Ilorin Kwara State

“…Flocculants can be of chemical and biological origins where the latter is referred to as natural macromolecule polymers or bio-flocculants produced by microorganisms that are capable of flocculating a number of suspended solids, including cells and colloidal solids, while being harmless and biodegradable (Chen et al, 2017). These bio-flocculants have been widely reported and applied to wastewater treatment, heavy metal removal and sludge dewatering and in the harvest of various microalgae (Ndikubwimana et al, 2016;Liu et al, 2017). However, the high cost of production of bio-flocculants still hinders their application commercially.…”

“…However, the high cost of production of bio-flocculants still hinders their application commercially. It has been proposed that costs of production can be reduced by either screening for high production strains or utilization of low-cost substrates as culture medium (Chen et al, 2016;Liu et al, 2017). A number of novel bio-flocculantproducing bacteria have been reported to be isolated from various environments such as wastewaters, activated sludges and soil samples, and they have been used to produce high yields of bioflocculants by optimizing their conditions of fermentation (Peng et al, 2014;Liu et al, 2015b) but little effort has been vested in the use of indigenous open water as a potential source of bio-flocculating bacteria that could possess better flocculating activity.…”