The challenges which the large scale microalgal industry is facing are associated with the high cost of key operations such as harvesting, nutrient supply and oil extraction. The high-energy input for harvesting makes current commercial microalgal biodiesel production economically unfeasible and can account for up to 50% of the total cost of biofuel production. Co-cultivation of fungal and microalgal cells is getting increasing attention because of high efficiency of bio-flocculation of microalgal cells with no requirement for added chemicals and low energy inputs. Moreover, some fungal and microalgal strains are well known for their exceptional ability to purify wastewater, generating biomass that represents a renewable and sustainable feedstock for biofuel production. We have screened the flocculation efficiency of the filamentous fungus A. fumigatus against 11 microalgae representing freshwater, marine, small (5 µm), large (over 300 µm), heterotrophic, photoautotrophic, motile and non-motile strains. Some of the strains are commercially used for biofuel production. Lipid production and composition were analysed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources contained in wheat straw and swine wastewater, respectively. Co-cultivation of algae and A. fumigatus cells showed additive and synergistic effects on biomass production, lipid yield and wastewater bioremediation efficiency. Analysis of fungal-algal pellet's fatty acids composition suggested that it can be tailored and optimised through co-cultivating different algae and fungi without the need for genetic modification.

BackgroundThe microalgal-based industries are facing a number of important challenges that in turn affect their economic viability. Arguably the most important of these are associated with the high costs of harvesting and dewatering of the microalgal cells, the costs and sustainability of nutrient supplies and costly methods for large scale oil extraction. Existing harvesting technologies, which can account for up to 50% of the total cost, are not economically feasible because of either requiring too much energy or the addition of chemicals. Fungal-assisted flocculation is currently receiving increased attention because of its high harvesting efficiency. Moreover, some of fungal and microalgal strains are well known for their ability to treat wastewater, generating biomass which represents a renewable and sustainable feedstock for bioenergy production.ResultsWe screened 33 fungal strains, isolated from compost, straws and soil for their lipid content and flocculation efficiencies against representatives of microalgae commercially used for biodiesel production, namely the heterotrophic freshwater microalgae Chlorella protothecoides and the marine microalgae Tetraselmis suecica. Lipid levels and composition were analyzed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources from wheat straw and swine wastewater, respectively. The biomass of fungal-algal pellets grown on swine wastewater was used as feedstock for the production of value-added chemicals, biogas, bio-solids and liquid petrochemicals through pyrolysis. Co-cultivation of microalgae and filamentous fungus increased total biomass production, lipid yield and wastewater bioremediation efficiency.ConclusionFungal-assisted microalgal flocculation shows significant potential for solving the major challenges facing the commercialization of microalgal biotechnology, namely (i) the efficient and cost-effective harvesting of freshwater and seawater algal strains; (ii) enhancement of total oil production and optimization of its composition; (iii) nutrient supply through recovering of the primary nutrients, nitrogen and phosphates and microelements from wastewater. The biomass generated was thermochemically converted into biogas, bio-solids and a range of liquid petrochemicals including straight-chain C12 to C21 alkanes which can be directly used as a glycerine-free component of biodiesel. Pyrolysis represents an efficient alternative strategy for biofuel production from species with tough cell walls such as fungi and fungal-algal pellets.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0210-6) contains supplementary material, which is available to authorized users.

BackgroundNumerous strategies have evolved recently for the generation of genetically modified or synthetic microalgae and cyanobacteria designed for production of ethanol, biodiesel and other fuels. In spite of their obvious attractiveness there are still a number of challenges that can affect their economic viability: the high costs associated with (1) harvesting, which can account for up to 50 % of the total biofuel’s cost, (2) nutrients supply and (3) oil extraction. Fungal-assisted bio-flocculation of microalgae is gaining increasing attention due to its high efficiency, no need for added chemicals and low energy inputs. The implementation of renewable alternative carbon, nitrogen and phosphorus sources from agricultural wastes and wastewaters for growing algae and fungi makes this strategy economically attractive.ResultsThis work demonstrates that the filamentous fungi, Aspergillus fumigatus can efficiently flocculate the unicellular cyanobacteria Synechocystis PCC 6803 and its genetically modified derivatives that have been altered to enable secretion of free fatty acids into growth media. Secreted free fatty acids are potentially used by fungal cells as a carbon source for growth and ex-novo production of lipids. For most of genetically modified strains the total lipid yields extracted from the fungal-cyanobacterial pellets were found to be higher than additive yields of lipids and total free fatty acids produced by fungal and Synechocystis components when grown in mono-cultures. The synergistic effect observed in fungal-Synechocystis associations was also found in bioremediation rates when animal husbandry wastewater was used an alternative source of nitrogen and phosphorus.ConclusionFungal assisted flocculation can complement and assist in large scale biofuel production from wild-type and genetically modified Synechocystis PCC 6803 strains by (1) efficient harvesting of cyanobacterial cells and (2) producing of high yields of lipids accumulated in fungal-cyanobacterial pellets.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0364-2) contains supplementary material, which is available to authorized users.

There is growing interest in the ability of high rate algal ponds (HRAP) to treat wastewater. This method reduces the costs of algal production while treating the wastewater quicker and more efficiently than standard lagoon practices. Two parallel HRAPs were used in this study to treat secondary effluent. Nitrogen levels were significantly reduced with a mean reduction of 71% for ammonia and 64% for total nitrogen. The use of the HRAPs significantly increased the algal biomass levels compared to the algal growth in the storage lagoons, with a mean increase of 274%. Beneficial use of algae can be used to reduce treatment costs; so being able to predict and optimise the amount of algal biomass produced in HRAPs is vital. However, most models are complicated and require specific, detailed information. In this study, a predictive microalgal growth model was developed for HRAP by adapting two previously established models: the Steele and Monod models. The model could predict algal growth based on temperatures and solar radiation and account for limiting ammonia concentrations in an elevated pH environment with natural variations in the algal community. This model used experimental data that would be readily available to any established HRAP study.