Bioelectromics of a photosynthetic microalgae assisted microbial fuel cell for wastewater treatment and value added production

Abstract

Power generation and recovery of value-added products using microalgae, Haematococcus lacustris is tested in a dual chamber photosynthetic microalgae-assisted microbial fuel cell (PMA-MFCt₁). The microalgal cells in conical flask act as control. The performance was compared to another, test PMA-MFCt₂. The control MFC in second test had electrode wires not connected (PMA-MFC_nw). The PMA-MFCt₁ set had microalgal catholytic media replenished unlike in PMA-MFCt₂. A comparative PMA₀-MFC, was used without microalgae and only water as catholyte. The results demonstrated maximum power density (PDmax) of 33.76 mW m^-2 in PMA-MFCt₁, 15.36 mW m^-2 in PMA-MFCt₂ and 8.05 mW m^-2 in PMA₀-MFC. The non replenishment of catholytic media in PMA-MFCt₂ set resulted in nutrient limitations, poor photosynthesis, and disrupted redox reactions. Further lowest PDmax in PMA₀-MFC proves that microalgae are excellent source of free nascent oxygen required for redox reaction. Taxonomic identity of microbes at the anode via 16 S rRNA showed the dominance of catalytic microbes mainly Proteobacteria. The different kinds of carotenoids from microalgae were estimated by UV-Vis and liquid chromatography-mass spectrometry (LC-MS) analysis. The microalgal growth, evaluated in terms of biomass dry weight (DW), was 118 mg L^-1, after 40 days of PMA-MFCt₁ operation, which was lesser than in control (conical flask) 123 mg L^-1. The pigments including total chlorophyll (a + b), and total carotenoids were 699.7 µg g^-1 and 224.6 µg g^-1, respectively, on day 16. Microalgal performance in PMA-MFCt₂ and its control (PMA-MFC_nw) was 10% and 32.52% inferior than in PMA-MFCt₁ and its control. The continuous replenishment of media in PMA-MFCt₁ maintained microalgal cells in continuous state of multiplication and photosynthesis resulting into higher bioelectricity generation and bioproducts than PMA-MFCt₂, and PMA-MFC_nw.

Keywords: Haematococcus lacustris; Bioelectricity; Carotenoids; Lipids; Metagenomics, microalgae; Microbial fuel cell; Wastewater treatment.

Fig. 1

Average physiochemical parameters intensity changes of five fed batch cycles for 40 days in a PMA-MFCt₁, (A) Anode, (B) Cathode, (C) Control, (D) Anode of PMA-MFCt_2, PMA-MFC_nw, PMA₀-MFC and (E) Cathode of PMA-MFCt_2, PMA-MFC_nw, PMA₀-MFC.

Fig. 2

Polarization curve of reactors (A) PMA-MFCt₁, (B) PMA-MFCt₂, (C) PMA₀-MFC and (D) COD removal of PMA-MFCt₁, PMA-MFCt_2, PMA-MFC_nw, PMA₀-MFC.

Fig. 3

Summary of microbial community profiles in the multilevel Krona function annotation diagram depicting taxonomic hierarchies where each cycle represents taxonomic levels of the bacterial communities in wastewater of PMA-MFCt₁ (A) sample of 1st day and (B) sample of day 40 at anodic chamber; (C) Heatmap is representing the abundance of different taxonomic groups of bacteria and the number are representing the percentage of abundance of a particular group on day 1 and day 40.

Fig. 4

Haematococcus lacustris in cathode of PMA-MFCt₁ from day 1 to day 40 (A) Cell count (B); Specific growth and Logistic kinetics with respect to cell count in PMA-MFCt₁ and control flask (C) total weight of biomass of control flask and PMA-MFCt₁ reactor from day 1 to 40 days, (D) Semi log plot growth kinetics with respect to biomass in PMA-MFCt₁ and control flask (E) total lipid percentage and lipid concentration MFC reactor and (F) Chlorophyll ‘a’, ‘b’, ‘a + b’ and total carotenoids in PMA-MFCt₁ and Astaxanthin in PMA-MFCt₁ and control flask.

Fig. 5

Astaxanthin and other carotenoids identified by liquid chromatography-mass spectrometry (LC-MS) from Haematococcus lacustris cells at cathode in a PMA-MFCt₁ for the presence of, (A) Astaxanthin fragment at RT 3.68 at m/z 279.08 detected in day 1 sample and on day 40, the LCMS showed (B) Halocynthiaxanthin at RT 6.71 min at m/z 233.08; (C) Docosahexaenoic acid (DHA) at 4.36 min at m/z 326; (D) at 7.11 min at m/z 139.08 fragments of fatty acid hexacosanedioic acid, carotenoids neoxanthin and vioxanthin; (E) astaxanthin diester at RT 8.23 m/z 1119.58; (F) Astaxanthin mono esters at retention time (RT) 15.92 min at m/z 579.58 and 596.83 and palmitic acid at 15.92 min at m/z 255.33.