• Energy Research

Abstract Among biofuel production processes using microalgae biomass, biogas generation seems to be the least complex. Nevertheless, its efficiency is hampered due to the hard cell wall. In order to enhance its anaerobic biodegradability, the present investigation evaluated the effect of two pretreatments (low temperature autohydrolysis at 50 °C for 24 and 48 h incubation and alkaline (0.5, 2 and 5% w/w NaOH dosages)) on Chlorella vulgaris and Scenedesmus sp. The autohydrolysis resulted in 16 and 6% chemical oxygen demand (COD) solubilisation for Chlorella and Scenedesmus , respectively. During thermoalkaline pretreatment, COD in soluble phase (CODsol) was increased up to 19% for Chlorella and 17% for Scenedesmus sp. The highest carbohydrates solubilisation corresponded to 2 and 5% w/w NaOH dosage for 48 h at 50 °C for Chlorella (20%) and Scenedesmus (40–43%). When compared to Chlorella , Scenedesmus biomass exhibited higher carbohydrates solubilisation, although methane yield enhancement was low for both substrates. Best case scenario for Scenedesmus sp. (20% increase) was attained with 5% NaOH at 50 °C for 24 h. Despite the lower carbohydrates solubilisation observed for Chlorella , similar methane yields were similar to Scenedesmus sp. The low methane production enhancement was ascribed to the fact that the organic matter solubilised were exopolymers released during pretreatments rather than intracellular material.

Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.

The purpose of the study was comparison of two configurations of photobioreactors an open-type photobioreactor open to atmosphere and a tubular type photobioreactor closed to the atmosphere. Organic matter was fairly removed under both configurations at 50-60% and biomass carbon content on dry weight basis accounted for 45%. Both configurations were able to completely exhaust ammonium, however different mechanism removals were responsible for the different influent loads applied. In terms of nitrogen recovery by biomass assimilation, the open configuration ranged 38-47% whereas the closed type presented 31%. It is worth to mention that nitrification-denitrification was taking place under both photobioreactor configurations. Approximately 80% phosphate removal was achieved regardless the configuration and biomass P content was slightly higher in the closed-type reactor. For nutrient recycling, biomass harvesting is described as the key issue of this technology. Nevertheless, the closed configuration highlighted the great potential of the biofilm formation by retaining 96% of the total biomass produced.

Short-chain fatty acids (SCFAs) are considered building blocks for bioproducts in the so-called carboxylate platform. These compounds can be sustainably produced via anaerobic fermentation (AF) of organic substrates, such as microalgae. However, SCFAs bioconversion efficiency is hampered by the hard cell wall of some microalgae. In this study, one thermal and two enzymatic pretreatments (carbohydrases and proteases) were employed to enhance Chlorella vulgaris biomass solubilization prior to AF. Pretreated and non-pretreated microalgae were assessed in continuous stirred tank reactors (CSTRs) for SCFAs production. Aiming to understand microorganisms' roles in AF depending on the employed substrate, not only bioconversion yields into SCFAs were evaluated but microbial communities were thoroughly characterized. Proteins were responsible for the inherent limitation of raw biomass conversion into SCFAs. Indeed, the proteolytic pretreatment resulted in the highest bioconversion (33.4% SCFAs-COD/CODin), displaying a 4-fold enhancement compared with raw biomass. Population dynamics revealed a microbial biodiversity loss along the AF regardless of the applied pretreatment, evidencing that the imposed operational conditions specialized the microbial community. In fact, a reduced abundance in Euryarchaeota phylum explained the low methanogenic activity, implying SCFAs accumulation. The bacterial community developed in the reactors fed with pretreated microalgae exhibited high acidogenic activities, being dominated by Firmicutes and Bacteroidetes. Firmicutes was by far the dominant phylum when using protease (65% relative abundance) while Bacteroidetes was prevailing in the reactor fed with carbohydrase-pretreated microalgae biomass (40% relative abundance). This fact indicated that the applied pretreatment and macromolecule solubilization have a strong effect on microbial distribution and therefore in SCFAs bioconversion yields.

AbstractIncreasing yeast robustness against lignocellulosic-derived inhibitors and insoluble solids in bioethanol production is essential for the transition to a bio-based economy. This work evaluates the effect exerted by insoluble solids on yeast tolerance to inhibitory compounds, which is crucial in high gravity processes. Adaptive laboratory evolution (ALE) was applied on a xylose-fermentingSaccharomyces cerevisiaestrain to simultaneously increase the tolerance to lignocellulosic inhibitors and insoluble solids. The evolved strain gave rise to a fivefold increase in bioethanol yield in fermentation experiments with high concentration of inhibitors and 10% (w/v) of water insoluble solids. This strain also produced 5% (P > 0.01) more ethanol than the parental in simultaneous saccharification and fermentation of steam-exploded wheat straw, mainly due to an increased xylose consumption. In response to the stress conditions (solids and inhibitors) imposed in ALE, cells induced the expression of genes related to cell wall integrity (SRL1,CWP2,WSC2andWSC4) and general stress response (e.g.,CDC5,DUN1,CTT1,GRE1), simultaneously repressing genes related to protein synthesis and iron transport and homeostasis (e.g.,FTR1,ARN1,FRE1), ultimately leading to the improved phenotype. These results contribute towards understanding molecular mechanisms that cells might use to convert lignocellulosic substrates effectively.

Abstract Using residual material instead of sugars as substrate for oleaginous microorganisms is a promising approach that may reduce the production costs of microbial lipid. In this study, five oleaginous yeasts were screened for their ability to grow and produce lipid utilizing volatile fatty acids (VFAs), generated from anaerobic fermentation of microalgal biomass, as the only carbon and energy source. Yeasts growth and lipid accumulation capacity at three VFAs concentrations (i.e. 5, 10 and 15 g L−1) were evaluated. Regardless of VFAs concentration four of the five strains were able to grow in digestates reaching biomass yields from VFAs between 0.22 and 0.37 g g−1. The highest lipid content in dry biomass was observed in Cutaneotrichosporon curvatum and Cyberlindnera saturnus (36.9 and 33.9% on dry biomass, respectively) corresponding to lipid yields from VFAs of 0.11 and 0.13 g g−1, respectively. Oleic, palmitic and linoleic acids were the major fatty acids, accounting for more than 70% of the fatty acids contained in total yeast lipids, profile similar to that of common vegetable oils. The above findings suggest that microalgal biomass derived VFAs could be converted into yeast lipid suitable as feedstock in the chemical (including biofuel) industry.