Effect of growth conditions on cell wall composition and cadmium adsorption in Chlorella vulgaris: A new approach to biosorption research.

Biosorption of toxic metals in microalgae is a process relying on the presence of cell wall reactive groups acting as binding sites. This work studied the effect of culture conditions on the outer cell wall composition of C. vulgaris and cadmium biosorption. The experiments were conducted in continuous culture under light and nitrogen limitation at two growth rates (0.4 and 0.2 d-1). Functional groups were profiled using ATR-FTIR spectrometry, and total cadmium biosorption was assayed. Significant differences in composition were attested the most salient being the absence of carboxyl groups in the light deprived states and a larger number of carbohydrates and amino groups in the nitrogen deprived cultures, particularly amino groups from deacetylated D-glucosamine polysaccharides. Higher biosorption was obtained with the nitrogen-restricted biomass, reaching a maximum of 11.9 mgCd/gbiomass, as compared to a minimum of 8.0 mgCd/gbiomass achieved in the light-restricted states. The increased biosorption exhibited by nitrogen-restricted strains was attributed to the deacetylated amino groups that have enhanced cation affinity. This work has shown that the characteristics of the outer cell wall can be engineered by culture conditions to improve biosorption, providing a new approach that opens up new research frontiers for the biosorption of hazardous metals.

Enzymatic Pretreatment of Microalgae: Cell Wall Disruption, Biomass Solubilisation and Methane Yield Increase.

Anaerobic digestion of microalgal biomass for biogas production may be limited due to the cell wall resulting in an inefficient bioconversion. Enzymatic pretreatments are applied for inducing cell damage/lysis and organic matter solubilisation and this way increasing biogas production. We evaluated enzymatic pretreatments in different conditions for comparing in relation to cell wall rupture, increase of soluble material and increase in biogas production through anaerobic digestion performance in BMP assay. Chlorella sorokiniana cultures were subjected to three different enzymatic pretreatments, each under four different conditions of enzyme/substrate ratio, pH and application time. The results showed increases over 21% in biogas productions for all enzymatic pretreatments. Enzymatic pretreatment was effective at damaging microalgae cell wall, releasing organic compounds and increasing the rate and final methane yield in BMP tests. We observed a synergistic activity between the mixtures enzymes, which would depend on operational conditions used for each pretreatment.

Assessing the Effect of Pretreatments on the Structure and Functionality of Microbial Communities for the Bioconversion of Microalgae to Biogas.

Microalgae biomethanization is driven by anaerobic sludge associated microorganisms and is generally limited by the incomplete hydrolysis of the microalgae cell wall, which results in a low availability of microalgal biomass for the methanogenic community. The application of enzymatic pretreatments, e.g., with hydrolytic enzymes, is among the strategies used to work around the incomplete hydrolysis of the microalgae cell wall. Despite the proven efficacy of these pretreatments in increasing biomethanization, the changes that a given pretreatment may cause to the anaerobic sludge associated microorganisms during biomethanization are still unknown. This study evaluated the changes in the expression of the metatranscriptome of anaerobic sludge associated microorganisms during Chlorella sorokiniana biomethanization without pretreatment (WP) (control) and pretreated with commercial cellulase in order to increase the solubilization of the microalgal organic matter. Pretreated microalgal biomass experienced significant increases in biogas the production. The metatranscriptomic analysis of control samples showed functionally active microalgae cells, a bacterial community dominated by γ- and δ-proteobacteria, and a methanogenic community dominated by Methanospirillum hungatei. In contrast, pretreated samples were characterized by the absence of active microalgae cells and a bacteria population dominated by species of the Clostridia class. These differences are also related to the differential activation of metabolic pathways e.g., those associated with the degradation of organic matter during its biomethanization.

Physical Pretreatment Methods for Improving Microalgae Anaerobic Biodegradability.

Microalgae may be a potential feedstock for biogas production through anaerobic digestion. However, this process is limited by the hydrolytic stage, due to the complex and resistant microalgae cell wall components. This fact hinders biomass conversion into biogas, demanding the application of pretreatment techniques for inducing cell damage and/or lysis and organic matter solubilisation. In this study, sonication, thermal, ultrasound, homogeneizer, hydrothermal and steam explosion pretreatments were evaluated in different conditions for comparing their effects on anaerobic digestion performance in batch reactors. The results showed that the highest biomass solubilisation values were reached for steam explosion (65-73%) and ultrasound (33-57%). In fact, only applied energies higher than 220 W or temperatures higher than 80 °C induced cell wall lysis in C. sorokiniana. Nonetheless, the highest methane yields were not correlated to biogas production. Thermal hydrolysis and steam explosion showed lower methane yields in respect to non-pretreated biomass, suggesting the presence of toxic compounds that inhibited the biological process. Accordingly, these pretreatment techniques led to a negative energy balance. The best pretreatment method among the ones evaluated was thermal pretreatment, with four times more energy produced that demanded.