Lipid productivity in limnetic Chlorella is doubled by seawater added with anaerobically digested effluent from kitchen waste

Abstract

Background: An economical strategy for producing microalgae as biofuel feedstock is driven by the freshwater and nutrients input. In this study, seawater was applied to limnetic algal cultivation and the behavior of algae in seawater media was observed including growth, lipid synthesis, and ultrastructure. To make seawater cater algae, a kind of wastewater, anaerobically digested effluent from kitchen waste (ADE-KW), was used as nutrient sources.

Results: Pure seawater cannot support the growth demand of freshwater microalga, due to high salinity and lack of nutrients. However, it is the conditions triggered the algae to synthesize lipids of 60%, double of lipid content in standard medium BG11. Introducing 3 or 5% ADE-KW (volume percentage) into seawater made algal growth reach the level attained in BG11, while lipid content compared favourably with the level (60%) in pure seawater. This method achieved the goal of fast growth and lipid accumulation simultaneously with the highest lipid productivity (19 mg/L day) at the exponential stage, while BG11 obtained 10.55 mg/L day at the stationary stage as the highest lipid productivity, almost half of that in seawater media. Moreover, the condition for highest lipid productivity enlarged algal cells compared to BG11. Under the condition for highest lipid productivity, Chlorella sorokiniana SDEC-18 had enlarged cells and increased settling efficiency compared to BG11, which facilitated harvest in an energy saving way.

Conclusions: The results suggested that combining seawater with ADE-KW to cultivate microalgae had a double function: nutrients and water for algal growth, and high salinity for stimulating lipid accumulation. If this technology was operated in practice, freshwater and non-waste nutrient consumption would be completely obviated.

Keywords: Anaerobically digested effluent from kitchen waste; Limnetic microalgae; Lipid; Seawater.

Fig. 1

Biomass concentration (a) and maximum growth rate (b) of Chlorella sorokiniana SDEC-18 grown in BG11 and seawater supplemented with different volume percentages (0, 1, 3, 5, 8, and 15%) of anaerobically digested effluent from kitchen waste. *Data followed by different letters are significantly different by Duncan’s test at p < 0.05

Fig. 2

Lipid content (a) and lipid productivity (b) for the whole cultivation period of Chlorella sorokiniana SDEC-18 cells grown in BG11 and seawater supplemented with different volume percentages (0, 1, 3, 5, 8, and 15%) of anaerobically digested effluent from kitchen waste. *Data for the same growth stage followed by different letters are significantly different by Duncan’s test at p < 0.05

Fig. 3

Images under (a) a fluorescence microscope and (b) a confocal microscope, and fluorescence intensity (c) for Nile Red-stained neutral lipid in Chlorella sorokiniana SDEC-18 cultivated in BG11 and seawater supplemented with different volume percentages (0 and 3%) of anaerobically digested effluent from kitchen waste. Shown in Fig. 3b are hydrocarbon oils stained using the neutral lipid-binding stain Nile Red (yellow) and chlorophyll autofluorescence (red). An individual cell was defined by chlorophyll autofluorescence from the chloroplast in each cell. Each image is an overlay of Nile Red signal, chlorophyll autofluorescence signal, and a bright-field image. Scale bar, 20 μm

Fig. 4

Cell ultrastructure of Chlorella sorokiniana SDEC-18 cells grown in BG11 and seawater supplemented with different volume percentages (0, 3, and 15%) of anaerobically digested effluent from kitchen waste. L, lipid droplets; S, starch granules; C, chloroplast; N, nucleolus; W, cell wall

Fig. 5

Settling efficiency of Chlorella sorokiniana SDEC-18 in BG11, and in seawater with 0 and 3% anaerobically digested effluent from kitchen waste after 10-day cultivation. a Settling efficiency. b Smearing phenotype in centrifuge bottle. c Appearance of flocs in algal cultures. Scale bar, 20 μm

Fig. 6

Morphological observation of Chlorella sorokiniana SDEC-18 under an optical microscope (left column) and a scanning electron microscope (right column) for the tested algae cultivated in BG11 and in seawater supplemented with 0 and 3% by volume percentages of anaerobically digested effluent from kitchen waste (ADE-KW)

Fig. 7

Cell diameters of Chlorella sorokiniana SDEC-18 cells grown in BG11 and seawater supplemented with different volume percentages (0, 1, 3, 5, 8, and 15%) of anaerobically digested effluent from kitchen waste. *Data followed by different letters are significantly different by Duncan’s test at p < 0.05

Fig. 8

Relative electrical conductivity of Chlorella sorokiniana SDEC-18 grown in BG11 and seawater supplemented with different volume percentages (0, 1, 3, 5, 8, and 15%) of anaerobically digested effluent from kitchen waste. The final relative electrical conductivity is summarized in the inset. *Data followed by different letters are significantly different by Duncan’s test at p < 0.05