Elsevier

Bioresource Technology

Volume 272, January 2019, Pages 606-610
Bioresource Technology

Short Communication
Enhanced biomass and lipid accumulation of mixotrophic microalgae by using low-strength ultrasonic stimulation

https://doi.org/10.1016/j.biortech.2018.10.058Get rights and content

Highlights

Abstract

Ultrasonic treatment was applied to enhance the biomass and lipid accumulation of mixotrophic microalgae. The optimal microalgal ultrasonic power, ultrasonic frequency, ultrasonic interval and growth phase were 20 W, 20 Hz, 2 s and logarithmic phase, respectively. The maximum biomass concentration and lipid content reached 2.78 g L−1 and 28.5%, which were 26.9% and 37% higher than those of the control group. Microscope analysis shows that ultrasonic exposure caused tiny cracks or holes on the surface of cell walls, but did not damage the integrity of algal cell structure. After ultrasonic stimulation, the permeability of membrane and the transport of nutrients were improved, and the utilization rate of substrate and pigment concentration increased 22.7% and 18.4%, respectively. However, excessive ultrasonic irradiation significantly inhibited the cell growth and lipid accumulation of microalgae. This study indicates the feasibility and efficiency of using low-strength ultrasound in promoting biomass and lipid production of microalgae.

Introduction

Microalgae based biodiesel is one of the potential alternatives to traditional fossil fuels to satisfy the increasing energy needs and fuel crisis, which is due to its advantages of easy to cultivate, high level of lipids and less land needed (Shin et al., 2018). Several strategies have been used to improve the microalgal cell growth and lipid production, which include the optimization of culture medium (e.g., type of carbon source, salts and vitamins), physical parameters (e.g., pH, light intensity and temperature) and type of culture mode (e.g., phototrophic, heterotrophic and mixotrophic growth) (Ren et al., 2015, Zhou et al., 2018, Zhu, 2015).

Algal cells can be cultivated in open ponds or closed photobioreactors by using light and CO2 as carbon and energy sources, which is known as photosynthetic cultivation (Yan et al., 2016b). Cultivation of microalgae in heterotrophic mode does not need light, and organic compounds serve as the sole carbon source (Chen et al., 2018). Mixotrophic cultivation can use simultaneously inorganic and organic compounds as carbon source, and thus microalgae under mixotrophic mode can provide more energy and intermediates for microalgal metabolism (Nagarajan et al., 2017). Moreover, owing to the lower requirements for light intensities, energy cost of mixotrophic cultivation is lower than that of photoautotrophic culture (Ren et al., 2014). Therefore, mixotrophic culture of microalgae has attracted an increasing number of interests.

In addition, sonication is a clean mechanical way and has been applied to improve the aerobic or anaerobic digestion of waste activated sludge (Xiao and Ju, 2018). Suitable irradiation can induce the release of intact, individual microorganisms and small particles, without demand of high energy to rupture the cell walls (Xiao and Ju, 2018). Interest in the application of sonication in microalgal industry has increased rapidly (Joyce et al., 2014, Zhu, 2015). Ultrasonic wave has been used in controlling bloom and assisting lipid extraction from algae (Han et al., 2016b). The ultrasonic treatment could lead to the improvement of enzyme activity, cell membrane permeability and substrate transportation (Guo et al., 2011, Wang et al., 2012). However, only a few previous studies tested the effect of ultrasonic treatment on the growth of Anabaena variabilis in phototrophic condition (Han et al., 2016a). Little information is available on the beneficial aspects of ultrasonic treatment in the biomass and lipid accumulation of mixotrophic microalgae. To date, no detailed literature has provided the optimal ultrasonic treatment parameters for mixotrophic cultivation of microalgae.

In this study, ultrasonic treatment was adopted to enhance the lipid production performance of mixotrophic microalgae. The ultrasonic treatment powers, frequencies, intervals and algal growth phases were optimized. Variations of microalgal growth, substrate and pigment concentration were investigated. Moreover, the mechanism of ultrasonic stimulation on microalgae was discussed. This study aims to develop new strategies for improving the microalgal biomass and lipid productivities.

Section snippets

Microalgal strain and culture condition

The microalgae mutant Scenedesmus sp. Z-4 had high lipid production ability and was employed in the present study (Liu et al., 2015). Glucose-supplemented BG-11 medium was used as culture medium for algal growth. The compositions of the medium are as follows (g L−1): Glucose, 9; NaNO3, 0.8; Na2CO3, 0.02; K2HPO4, 0.04; ferric ammonium citrate, 0.006; MgSO4·7H2O, 0.075; CaCl2, 0.027; EDTA, 0.001; citric acid, 0.006; and 1 mL L−1 trace mental solution. The compositions of trace mental solution are

Effect of ultrasonic power

Various ultrasonic powers (0, 10, 20, 30, 40 and 50 W) were designed to investigate the effect of ultrasonic power on cell growth and lipid production (Fig. 1A). When ultrasonic power increased from 0 to 20 W, biomass concentration increased slightly from 2.21 to 2.74 g L−1, while lipid yield increased from 21.3% to 28.3%. Further increase of ultrasonic power (30–50 W) inhibited the biomass and lipid production. This indicates that lower ultrasonic power can stimulate the algal growth and lipid

Conclusions

The low-strength ultrasonic stimulation can markedly improve the algal biomass concentration and lipid content, which reached the maximum values of 2.78 g L−1 and 28.5% at an ultrasonic power of 20 W, frequency of 20 Hz and interval of 2 s under the logarithmic growth phase. The ultrasonic treatment had little effect on the survival ratio of algal cells, but can promote the permeability and substrate utilization of mixotrophic microalgae, resulting in significant increase in the biomass and

Acknowledgments

This study was supported by the National Natural Science Foundation of China (Nos. 51708156, 51678186 and 51478139), the China Postdoctoral Science Foundation (Nos. 2018 T110311, 2017 M620117 and 2017 M611375), the Heilongjiang Postdoctoral Science Foundation (No. LBH-Z17063), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2017DX13), the Fundamental Research Funds for the Central Universities (No. HIT.NSRIF.2019046), and the China

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These authors contributed equally to this paper.

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