Abstract
BackgroundAlgal harvesting is a major cost which increases biofuel production cost. Algal biofuels are widely studied as third-generation biofuel. However, they are yet not viable because of its high production cost which is majorly contributed by energy-intensive biomass harvesting techniques. Biological harvesting method like fungal-assisted harvesting of microalgae is highly efficient but poses a challenge due to its slow kinetics and poorly understood mechanism.ResultsIn this study, we investigate Aspergillus fumigatus–Chlorella pyrenoidosa attachment resulting in a harvesting efficiency of 90% within 4 h. To pinpoint the role of extracellular metabolite, several experiments were performed by eliminating the C. pyrenoidosa or A. fumigatus spent medium from the C. pyrenoidosa–A. fumigatus mixture. In the absence of A. fumigatus spent medium, the harvesting efficiency dropped to 20% compared to > 90% in the control, which was regained after addition of A. fumigatus spent medium. Different treatments of A. fumigatus spent medium showed drop in harvesting efficiency after periodate treatment (≤ 20%) and methanol–chloroform extraction (≤ 20%), indicating the role of sugar-like moiety. HR-LC–MS (high-resolution liquid chromatography–mass spectrometry) results confirmed the presence of N-acetyl-d-glucosamine (GlcNAc) and glucose in the spent medium. When GlcNAc was used as a replacement of A. fumigatus spent medium for harvesting studies, the harvesting process was significantly faster (p < 0.05) till 4 h compared to that with glucose. Further experiments indicated that metabolically active A. fumigatus produced GlcNAc from glucose. Concanavalin A staining and FTIR (Fourier transform infrared spectroscopy) analysis of A. fumigatus spent medium- as well as GlcNAc-incubated C. pyrenoidosa cells suggested the presence of GlcNAc on its cell surface indicated by dark red dots and GlcNAc-specific peaks, while no such characteristic dots or peaks were observed in normal C. pyrenoidosa cells. HR-TEM (High-resolution Transmission electron microscopy) showed the formation of serrated edges on the C. pyrenoidosa cell surface after treatment with A. fumigatus spent medium or GlcNAc, while Atomic force microscopy (AFM) showed an increase in roughness of the C. pyrenoidosa cells surface upon incubation with A. fumigatus spent medium.ConclusionsResults strongly suggest that GlcNAc present in A. fumigatus spent medium induces surface changes in C. pyrenoidosa cells that mediate the attachment to A. fumigatus hyphae. Thus, this study provides a better understanding of the A. fumigatus-assisted C. pyrenoidosa harvesting process.
Highlights
Algal harvesting is a major cost which increases biofuel production cost
Role of extracellular metabolites in C. pyrenoidosa–A. fumigatus harvesting When A. fumigatus pellets and C. pyrenoidosa cells, suspended in respective spent medium (Fig. 1a), were mixed at 1:5 ratio, C. pyrenoidosa harvesting efficiency of 90% was observed after 4 h (Fig. 1b)
This A. fumigatus–C. pyrenoidosa ratio and its harvesting efficiency were optimized in our earlier study [12] and has been referred to as control for all the experiments performed in the present study
Summary
Algal biofuels are widely studied as third-generation biofuel They are yet not viable because of its high production cost which is majorly contributed by energy-intensive biomass harvesting techniques. The commercialization of algal biofuels still lags behind due to high cost of investment towards energy-intensive biomass harvesting techniques like centrifugation, membrane filtration and chemical based flocculation from photobioreactors [4,5,6]. Chemical processes are highly efficient, but the requirement of a lot of flocculant dosages renders the harvested biomass contaminated with undesirable chemicals [7, 8]. Induced harvesting of algal cells is being explored as a replacement for the conventional algal dewatering processes which contribute towards 3–15% of the algal biomass production cost [9, 10]. In spite of having the potential for becoming a cost-effective process for algal harvesting, the lack of knowledge regarding the causative factors for the algal attachment to fungal pellets limits its application
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