Jeong Jun Yoon

Anaerobic digestibility of algal bioethanol residue.

The aim of this work was to investigate anaerobic digestibility of algal bioethanol residue from saccharification and fermentation processes. A series of batch anaerobic digestion tests using saccharification and fermentation residue showed that the maximum methane yields of saccharification residue and fermentation residue were 239 L/kg VS (Volatile Solids) and 283 L/kg VS (Volatile Solids), respectively. Energy recovered by anaerobic digestion of the residue was 2.24 times higher than that from the ethanol produced in the main process. 5-HMF (5-hydroxymethylfurfural), a saccharification byproduct, could retard methanogenesis at over 3g/L however, the inhibition was prevented by increasing cell biomass concentration. Anaerobic digestion of residue has the potential to enhance bioenergy recovery and environmental sustainability of algal bioethanol production.

Predominance of cluster I Clostridium in hydrogen fermentation of galactose seeded with various heat-treated anaerobic sludges.

To identify the key bacterial populations in hydrogen fermentation of galactose, a fermentor seeded with a heat-treated sludge was operated. After 27h of fermentation, the proportion of butyric acid increased to 69.4wt.% and the gas production yield reached 1.0molH2/molgalactose. In the pyrosequencing of 16S rDNA, an increase of the proportion of the phylum Firmicutes from 4.2% to 92% (mostly cluster I Clostridium) was observed. To verify the predominance and the ubiquity of the cluster, five fermentors seeded with different heat-treated anaerobic sludges having different feedstock compositions and digestion temperatures were investigated using qPCR analyses. The abundance of the cluster increased >100-fold during the fermentation, regardless of the inocula. Moreover, the abundance was negatively correlated with the lag time of hydrogen production and positively correlated with the hydrogen production rate, demonstrating the relevance of the cluster to hydrogen production. Taken together, the results clearly revealed the importance of cluster I Clostridium in the hydrogen fermentation of galactose.

Evidence of syntrophic acetate oxidation by Spirochaetes during anaerobic methane production.

To search for evidence of syntrophic acetate oxidation by cluster II Spirochaetes with hydrogenotrophic methanogens, batch reactors seeded with five different anaerobic sludge samples supplemented with acetate as the sole carbon source were operated anaerobically. The changes in abundance of the cluster II Spirochaetes, two groups of acetoclastic methanogens (Methanosaetaceae and Methanosarcinaceae), and two groups of hydrogenotrophic methanogens (Methanomicrobiales and Methanobacteriales) in the reactors were assessed using qPCR targeting the 16S rRNA genes of each group. Increase in the cluster II Spirochaetes (9.0±0.4-fold) was positively correlated with increase in hydrogenotrophic methanogens, especially Methanomicrobiales (5.6±1.0-fold), but not with acetoclastic methanogens. In addition, the activity of the cluster II Spirochaetes decreased (4.6±0.1-fold) in response to high hydrogen partial pressure, but their activity was restored after consumption of hydrogen by the hydrogenotrophic methanogens. These results strongly suggest that the cluster II Spirochaetes are involved in syntrophic acetate oxidation in anaerobic digesters.

HRT dependent performance and bacterial community population of granular hydrogen-producing mixed cultures fed with galactose.

The effects of hydraulic retention times (HRTs-6, 3 and 2 h) on H2 production, operational stability and bacterial population response in a continuously stirred tank reactor (CSTR) were evaluated using galactose. A peak hydrogen production rate (HPR) of 25.9 L H2/L-d was obtained at a 3 h HRT with an organic loading rate (OLR) of 120 g/L-d, while the maximum hydrogen yield (HY) of 2.21 mol H2/mol galactose was obtained at a 6 h HRT (60 g galactose/L-d). Butyrate was dominant and the lactate concentration increased as HRT decreased, which significantly affected the HY. Biomass concentration (VSS) decreased from 16 to 3g/L at a 2 h HRT, leading to failure. A 3 h HRT supported the favorable growth of Clostridium species, as indicated by an increase in their populations from 25.4% to 27%, while significantly reducing Bacilli populations from 61.6% to 54.2%, indicating that this was the optimal condition.

Feasibility of anaerobic digestion from bioethanol fermentation residue.

The focus of this study was the reuse of red algal ethanol fermentation residue as feedstock for anaerobic digestion. Levulinic acid and formic acid, the dilute-acid hydrolysis byproducts, inhibited methanogenesis at concentrations over 3.0 and 0.5 g/L, respectively. However, the inhibition was overcome by increasing inoculum concentration. A series of batch experiments with the fermentation residue showed increased methane yield and productivity at higher inoculum concentration. The maximum methane conversion rate of 84.8% was found at 5 g COD/L of fermentation residue at 0.25 g COD/g VSS of food-to-microorganism (F/M) ratio. The red algal ethanol fermentation residue can possibly be used as a feedstock in anaerobic digestion at appropriate concentration and F/M ratio.

Evolutionary engineering of Saccharomyces cerevisiae for efficient conversion of red algal biosugars to bioethanol.

The aim of this work was to apply the evolutionary engineering to construct a mutant Saccharomyces cerevisiae HJ7-14 resistant on 2-deoxy-D-glucose and with an enhanced ability of bioethanol production from galactose, a mono-sugar in red algae. In batch and repeated-batch fermentations, HJ7-14 metabolized 5-fold more galactose and produced ethanol 2.1-fold faster than the parental D452-2 strain. Transcriptional analysis of genes involved in the galactose metabolism revealed that moderate relief from the glucose-mediated repression of the transcription of the GAL genes might enable HJ7-14 to metabolize galactose rapidly. HJ7-14 produced 7.4 g/L ethanol from hydrolysates of the red alga Gelidium amansii within 12 h, which was 1.5-times faster than that observed with D452-2. We demonstrate conclusively that evolutionary engineering is a promising tool to manipulate the complex galactose metabolism in S. cerevisiae to produce bioethanol from red alga.