Efficient Harvesting of Nannochloropsis Microalgae via Optimized Chitosan-Mediated Flocculation

Food-grade rather than synthetic or chemical flocculants are needed for microalgae harvesting by settling, if used for food products. Chitosan is effective in harvesting freshwater microalgae, but it is expensive and typically not suitable for marine microalgae like Nannochloropsis. To minimize costs for food-grade flocculation, a number of potentially important parameters are considered, including chitosan solubility and optimized chitosan-mediated flocculation of Nannochloropsis sp. BR2 by a five-factor central composite design experiment. Results show that an optical density (440 nm) of 2 (0.23 g dry weight L-1), initial pH of 6, final pH of 10, and 22 ppm chitosan with a viscosity of 1808 cP provide optimum flocculation efficiency, which is predicted to be in the range of 97.01% to 99.93%. These predictions are verified on 4.5 and 8 L Nannochloropsis sp. BR2 cultures.

Development of a harvesting technique for large-scale microalgal harvesting for biodiesel production

Harvesting methods executed in lab and large-scale for efficient and sustainable harvesting of microalgae and supernatant reuse for recultivation.

Mechanism of algal aggregation by Bacillus sp. strain RP1137

Alga-derived biofuels are one of the best alternatives for economically replacing liquid fossil fuels with a fungible renewable energy source. Production of fuel from algae is technically feasible but not yet economically viable. Harvest of dilute algal biomass from the surrounding water remains one of the largest barriers to economic production of algal biofuel. We identified Bacillus sp. strain RP1137 in a previous study and showed that this strain can rapidly aggregate several biofuel-producing algae in a pH- and divalent-cation-dependent manner. In this study, we further characterized the mechanism of algal aggregation by RP1137. We show that aggregation of both algae and bacteria is optimal in the exponential phase of growth and that the density of ionizable residues on the RP1137 cell surface changes with growth stage. Aggregation likely occurs via charge neutralization with calcium ions at the cell surface of both algae and bacteria. We show that charge neutralization occurs at least in part through binding of calcium to negatively charged teichoic acid residues. The addition of calcium also renders both algae and bacteria more able to bind to hydrophobic beads, suggesting that aggregation may occur through hydrophobic interactions. Knowledge of the aggregation mechanism may enable engineering of RP1137 to obtain more efficient algal harvesting.

Influence of culture density, pH, organic acids and divalent cations on the removal of nutrients and metals by immobilized Anabaena doliolum and Chlorella vulgaris

The potential of alginate-immobilized Anabaena doliolum and Chlorella vulgaris was assessed for removal of nutrients (NO inf3 (sup-) and NH inf4 (sup+) ) and metals (Cr2O inf7 (sup2-) and Ni(2+)) at different biomass concentrations (0.05, 0.1, 0.25, 0.49 and 1.22 g dry wt l(-1)) and pH values (4 to 10). Though uptake of all these substances was higher in concentrated algal beads (0.25, 0.49 and 1.22 g dry wt l(-1)), their rate of uptake was significantly (P<0.001) lower than that of low (0.05 g dry wt l(-1)) cell density beads. For A. doliolum, there was no significant difference in uptake rates for beads having densities of 0.05 and 0.1 g dry wt l(-1). Chlorella vulgaris, however, showed maximum efficiency at 0.1 g dry wt l(-1). Uptake of both the nutrients and the metals was maximal at pH 7 followed by pH 8, 6, 9, 10, 5 and 4. Of the different substances (organic acids and divalent cations) used, humic acid was most efficient in decreasing metal uptake. Mg(2+) was, however, more efficient than Ca(2+) in decreasing Ni(2+) uptake. Immobilized algae with a cell density of 0.1 g dry wt l(-1) were the most efficient for nutrient and metal removal at pH 6 to 8.

Rapid aggregation of biofuel-producing algae by the bacterium Bacillus sp. strain RP1137

Algal biofuels represent one of the most promising means of sustainably replacing liquid fuels. However, significant challenges remain before alga-based fuels become competitive with fossil fuels. One of the largest challenges is the ability to harvest the algae in an economical and low-energy manner. In this article, we describe the isolation of a bacterial strain, Bacillus sp. strain RP1137, which can rapidly aggregate several algae that are candidates for biofuel production, including a Nannochloropsis sp. This bacterium aggregates algae in a pH-dependent and reversible manner and retains its aggregation ability after paraformaldehyde fixation, opening the possibility for reuse of the cells. The optimal ratio of bacteria to algae is described, as is the robustness of aggregation at different salinities and temperatures. Aggregation is dependent on the presence of calcium or magnesium ions. The efficiency of aggregation of Nannochloropsis oceanica IMET1 is between 70 and 95% and is comparable to that obtained by other means of harvest; however, the rate of harvest is fast, with aggregates forming in 30 s.

Potential pre-concentration methods for Nannochloropsis gaditana and a comparative study of pre-concentrated sample properties

We compared potential pre-concentration techniques for Nannochloropsis gaditana (Nng) by testing natural sedimentation; flocculation with aluminium sulphate, polyaluminium chloride and chitosan; and induced pH. Promising flocculation efficiencies and concentration factors were obtained in a short time with alkalinity-induced flocculation at an adjusted pH of 9.7 and with chitosan at an adjusted pH of 9.9 using a concentration of 30mgL(-1). The sedimentation rates of alkalinity-induced flocculation were also evaluated. Additionally, viscosity, particle size distribution and Ca/Mg ions were analysed for pre-concentrated samples of N. gaditana (Nng) and the previously studied Phaeodactylum tricornutum (Pht) which were obtained by various different harvesting methods under optimal conditions. The rheological properties of the concentrated algae suspensions of two microalgal species showed Newtonian behaviour. The mean diameters of the flocs were between 39 and 48μm. The Ca/Mg analysis showed that Mg(+2) is the triggering ion for alkalinity-induced flocculation in the conditions studied.

Microalgae and wastewater treatment

Organic and inorganic substances which were released into the environment as a result of domestic, agricultural and industrial water activities lead to organic and inorganic pollution. The normal primary and secondary treatment processes of these wastewaters have been introduced in a growing number of places, in order to eliminate the easily settled materials and to oxidize the organic material present in wastewater. The final result is a clear, apparently clean effluent which is discharged into natural water bodies. This secondary effluent is, however, loaded with inorganic nitrogen and phosphorus and causes eutrophication and more long-term problems because of refractory organics and heavy metals that are discharged. Microalgae culture offers an interesting step for wastewater treatments, because they provide a tertiary biotreatment coupled with the production of potentially valuable biomass, which can be used for several purposes. Microalgae cultures offer an elegant solution to tertiary and quandary treatments due to the ability of microalgae to use inorganic nitrogen and phosphorus for their growth. And also, for their capacity to remove heavy metals, as well as some toxic organic compounds, therefore, it does not lead to secondary pollution. In the current review we will highlight on the role of micro-algae in the treatment of wastewater.

Recovery of microalgal biomass and metabolites: process options and economics

Commercial production of intracellular microalgal metabolites requires the following: (1) large-scale monoseptic production of the appropriate microalgal biomass; (2) recovery of the biomass from a relatively dilute broth; (3) extraction of the metabolite from the biomass; and (4) purification of the crude extract. This review examines the options available for recovery of the biomass and the intracellular metabolites from the biomass. Economics of monoseptic production of microalgae in photobioreactors and the downstream recovery of metabolites are discussed using eicosapentaenoic acid (EPA) recovery as a representative case study.

Chitosan as flocculant for concentrating Euglena gracilis cultures

The practical criteria for the usefulness of an algal separation process for laboratory routine being effectiveness and time consumption, we tested the feasibility of a flocculation procedure to harvest large volumes of Euglena gracilis in culture. This procedure turned out to be a technically viable system which avoided tedious centrifugation and preserved E. gracilis flagellar apparatus integrity. E. gracilis cultures were treated with chitosan, a by-product derived from chitin from the exoskeleton of crustaceans. Since this polymer carries a positive charge, it functions as a polycationic coagulating agent by adsorbing onto particles in suspension and by bridging together into agglomerates, or flocs. A 96-98% reduction of suspended cells in cultures with 200 mg/l of chitosan, at pH 7.5, was obtained.

The potential of microalgal biotechnology: A review of production and uses of microalgae

An overview of the various aspects, promises and limitations of microalgal biotechnology is presented. The factors of importance in microalgal cultivation as well as the culture systems are briefly described. Microalgal biomasses can fulfil the nutritional requirements of aquatic larvae and organisms. The biochemical composition of algae can be improved by the manipulation of culture conditions. The nutritive value of the microalgal biomasses for human and animal consumption is also commented upon as well as some socio-economical aspects. Among the sources of required nutrients (N, P), wastewaters and manures can upgraded as culture media for microalgae the safety of which has to be evaluated. Harvesting of the biomass is one of the bottlenecks. The various techniques, physical, physico-chemical and biological are outlined and their feasibility and economic interest examined. Microalgal biomasses can be submitted to various technological transformations. Various processes are reviewed in the light of their effects on safety and nutritional value. The possible extraction of fine chemicals and the preparation of protein concentrates is also reported on. The various uses of microalgae lead to a possible competition, to be evaluated, between systems for the production of food, energy and chemicals. The review finally covers the application of genetic manipulation to microalgae.