Elsevier

Bioresource Technology

Volume 209, June 2016, Pages 16-22
Bioresource Technology

Beneficial changes in biomass and lipid of microalgae Anabaena variabilis facing the ultrasonic stress environment

https://doi.org/10.1016/j.biortech.2016.02.103Get rights and content

Highlights

  • The lipid was efficiently accumulated within 1 day after the ultrasonic treatment.

  • The maximum lipid content of 46.9% was obtained in the algae Anabaena variabilis.

  • The lipid content was significantly improved at proper ultrasonic power and time.

  • The ultrasonic treatment did not negatively affect the protein content.

  • The suitable ultrasonic showed a high efficiency in enhancing lipid production.

Abstract

This study investigated the beneficial effects of ultrasonic treatment on the biomass, lipid and protein of the microalgae Anabaena variabilis. The microalgae after 11 days cultivation (initial algae) were treated at the powers of 200, 350 and 500 W for 10 min and then cultured continuously for 3 days (day 12–14). The power of 200 W induced the highest lipid content 37.8% on day 12. The subsequent experiments tested the ultrasonic treatment times of 5, 10, 20 and 40 min at 200 W in the initial algae. The significantly improved lipid content 46.9% and productivity 54.2 mg/L/d were obtained almost 1.46 and 1.86 times more than that of the control algae respectively after 1 day of continuous cultivation at 5 min. The proper ultrasonic treatment showed the feasibility and high efficiency in promoting lipid accumulation without negatively influencing the biomass, fatty acid profiles and protein content.

Introduction

Microalgae as a suitable and promising ingredient receive worldwide attention and research due to their applications in the production of edible nutrition, feed, dye and biofuel (Demirbas and Fatih Demirbas, 2011, Kiran et al., 2014, Menetrez, 2012). Compared with the second generation biomass, microalgae take the advantages of high biomass productivity and lipid yield, widespread existence, ease of management and less space occupation (Chisti, 2007, Maity et al., 2014). In the production of biofuels, microalgae can use sunlight and carbon dioxide to synthesize lipid under stress or stimulation conditions (Suali and Sarbatly, 2012, Venkata Mohan and Devi, 2014). The formed lipid as energy storage enables the microalgae cells endure the harsh environment and can be transesterified to produce biodiesel (Rawat et al., 2013). Improving the lipid content is a feasible way to maximize the biodiesel production by optimizing the cultivation conditions, designing efficient reactors and cultivation model and subjecting the algae to stress environments (Dogaris et al., 2015, Han et al., 2015, Rodolfi et al., 2009, Skorupskaite et al., 2015). The most common stress environments used are nutrients depletion, high temperature and light, pH values, salinity concentration (Venkata Mohan and Devi, 2014). Several studies have also mentioned the effects of ultrasonic exposure on the microalgae growth and intracellular compounds synthesis (Rajasekhar et al., 2012, Tang et al., 2004).

Ultrasonic is the sound waves of a frequency at or above 20 kHz. The caused compression and rarefaction cycles in water by the ultrasound radiation lead to the generation of cavitation bubbles or cavitation effect (Suslick, 1990). Millions of these produced bubbles can create implosive collapse accompanied by yielding localized extreme temperatures as high as 5000 °C, high pressures of 100 MPa and free radicals (Rajasekhar et al., 2012). The external conditions can induce the change of components synthesis including lipid, carbohydrate, protein and chlorophyll. Some researchers focus on the nitrogen starvation which can induce the lipid accumulation but limit the cell proliferation, nutrients utilization and carbon fixation (Khozin-Goldberg and Cohen, 2006, Rezanka et al., 2011, Zhou et al., 2014). The nitrogen starvation being time-consuming is usually applied throughout the growing period or the lipid-inducing cultivation phase. High salinity and temperature can induce lipid accumulation during the late cultivation time in a two-stage culture strategy (Venkata Mohan and Devi, 2014). Although few studies have tried to test the effects of ultrasonic on lipid accumulation of microalgae, ultrasonic wave has been used in controlling algae bloom, assisting lipid extraction and testing the effects on growth (Greenly and Tester, 2015, Lee et al., 2001, Ma et al., 2014, Rajasekhar et al., 2012, Tang et al., 2004). Ultrasonic wave can change the permeability of the membrane which affects the nutrients utilization leading to the variation of cell activity and material synthesis (Joyce et al., 2013, Tang et al., 2004). Hao et al. (2004) have observed the different effects of ultrasonic irradiation on the phycocyanin and chlorophyll a of Spirulina platensis at the frequencies of 20 kHz and 1.7 MHz. In a related study, Rajasekhar et al. (2012) investigated the growth of three species after sonication showing different membrane damage degree and ability in resistance to ultrasonic. The results showed that algae with gas-vacuolate and thin cell wall were more likely to be affected by the ultrasonic irradiation. Most of these works have focused on the harmful and inhibition effects of cells and growth under ultrasonic exposure; only little information is referred about the beneficial aspects of ultrasonic in microalgae growth. However, to our knowledge the researches have rarely focused on the beneficial effects of ultrasonic as a stress cultivation condition on the lipid accumulation of algae during the growth.

In this study, different ultrasonic treatment powers and times are tested to determine the biomass and lipid accumulation changes of the microalgae strain Anabaena variabilis. The selected strain is filamentous algae the morphological change of which can be easily observed under the microscope. Moreover, the present algae are rarely studied for the use of lipid production candidate. A cultivation model similar to the two-stage culture was adopted, which mainly focused on the second stage of lipid accumulation. The aim is to explore the feasibility of ultrasonic as stress condition in improving the lipid accumulation of microalgae during the late growth phase. Through the lab-scale trials an efficient method based on ultrasonic treatment can be drawn out to accelerate lipid production.

Section snippets

Microalgae strain

The microalgae used in this study were A. variabilis purchased from freshwater algae culture collection of the Institute of Hydrobiology in China (FACHB-Collection). The seed strains were cultured initially for 5–7 days in 200-mL Erlenmeyer flasks for activation following by the expanding culture in 1000-mL Erlenmeyer flasks at the temperature of 25 ± 1 °C for 5–7 days. The activated microalgae were centrifuged to form cell pellets which were re-suspended and transferred to the photobioreactors with

Microalgae growth

The microalgae were treated on day 11 by different ultrasonic powers to determine the effects on cells growth, the results of which were shown in Fig. 1. The microalgae under optimal conditions showed low growth activity after the cultivation for 11 days at the condition without ultrasonic (i.e. 0 W). After the ultrasonic treatment on day 11, the microalgae showed an interesting increase with the continued culture time. The maximum biomass concentration of 1.36 g/L was obtained at the ultrasonic

Conclusions

The effects of ultrasonic treatment on microalgae were studied to determine the beneficial aspects induced by ultrasound in the production of biomass and lipid. The results showed increment of biomass concentration on day 12 at the condition of 200 W and 5 min. Interestingly, the lipid content was significantly improved to 46.92% simultaneously under the same conditions leading to a maximum lipid productivity of 54.2 mg/L/d. The ultrasonic treatment indicated no negative effects on the properties

Acknowledgements

This work was gratefully financed by the National Science Fund for Excellent Young Scholars (51322811), Science and Technology Development Planning of Shandong Province (2012GGE27027), the Program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-12-0341).

References (34)

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