Elsevier

Journal of Biotechnology

Volume 257, 10 September 2017, Pages 47-57
Journal of Biotechnology

Biomass and oil production by Chlorella vulgaris and four other microalgae — Effects of salinity and other factors

https://doi.org/10.1016/j.jbiotec.2016.11.029Get rights and content

Highlights

  • The nominally freshwater microalga Chlorella vulgaris was grown in seawater.

  • Oil productivity in sweater was double the productivity in freshwater.

  • Biomass yield on phosphate was ∼27% higher in seawater relative to freshwater.

  • Biomass loss in the dark obeyed first-order kinetics.

  • The calorific value of the seawater-grown biomass exceeded 25 kJ g−1.

Abstract

Five nominally freshwater microalgae (Chlorella vulgaris, Choricystis minor, Neochloris sp., Pseudococcomyxa simplex, Scenedesmus sp.) with a known ability to produce high-levels of lipids for possible use as fuel oils were evaluated for their ability to thrive and produce lipids in seawater and brackish water. Only C. vulgaris was found to thrive and produce lipids in full strength seawater. Seawater tolerant strains of C. vulgaris are unusual. Lipid productivity in nutrient sufficient seawater exceeded 37 mg L−1 d−1 and was nearly 2-fold greater than in freshwater. Although other microalgae such as C. minor had higher lipid productivities (e.g. 45 mg L−1 d−1), they did not thrive in seawater. The lipid content of the C. vulgaris biomass was nearly 16% by dry weight. The calorific value of the seawater-grown C. vulgaris biomass exceeded 25 kJ g−1. Compared to continuously illuminated cultures, a 12/12 h light-dark cycle reduced lipid productivity of C. vulgaris by ∼30%, but did not affect the lipid content of the biomass. Biomass yield on phosphate was nearly 27% higher in seawater compared to in freshwater. While C. vulgaris has been extensively studied in freshwater, it has not been examined to any detail in full strength seawater. Studies in seawater are essential for any future large scale production of algal oils for biofuels: seawater is available cheaply and in large amounts whereas there is a global shortage of freshwater.

Introduction

Microalgae are attracting much attention as potential sources of fuel oil and essential medicinal oils. Microalgae can be grown photosynthetically using water, carbon dioxide, mineral salts and freely available sunlight. Microalgae are generally more efficient oil producers than commercial oil crops such as oil palm and soybean (Chisti, 2007). Growing microalgae does not require arable land. Concentrated carbon dioxide, the main carbon source for photoautotrophic culture of microalgae, is available from coal-fired electricity generating facilities, for example. Water is a necessary resource for algal culture. Depending on species, microalgae can be grown in freshwater, including domestic wastewater, brackish water and seawater. This work assesses the ability of five nominally freshwater microalgae to produce biomass and lipids in saline media, particularly in full-strength seawater. The microalgae used, had all been previously identified as potentially good producers of lipids but only in freshwater media. Biomass and lipid productivities in freshwater, brackish water and seawater are reported. One species (Chlorella vulgaris) that was found to be highly productive in seawater-based media was studied in more detail for biomass and oil production in full-strength seawater.

Although wastewater can be used to grow algae such as Chlorella (Chiu et al., 2015), its supply is insufficient for producing any significant amount of algal oil for fuel (Chisti, 2013). Furthermore, the average lipid productivity of microalgae in effluent of municipal wastewater treatment plants has been reported to be less than 10 mg L−1 d−1 (Chiu et al., 2015). When higher productivities are observed (Chiu et al., 2015), they are invariably a consequence of heterotrophic growth on dissolved organic carbon present in municipal wastewaters. Therefore, wastewater is not a substitute for seawater for large-scale production of algal fuel oils. Existing studies of microalgae using freshwater media are of little relevance to any future large-scale production of algal oils for fuels (Chisti, 2013). This is because freshwater is in short supply globally and its use for producing microalgae will undoubtedly compete with its existing uses in production of food and fodder.

Chlorella biomass is a human food supplement and has recognized benefits as a nutritional additive to animal feeds (Chiu et al., 2015, Griffiths et al., 2014, Kotrbáček et al., 2015). Chlorella grown in freshwater media is used as an aquaculture feed, but is too expensive for use as bulk animal feed (Kotrbáček et al., 2015). Extensive work on freshwater production of C. vulgaris biomass and oils has been reported (Griffiths et al., 2014, Liang et al., 2009, Liu and Chen, 2016, Liu and Hu, 2013, Lohman et al., 2015, Ördög et al., 2016, Potvin et al., 2011, Rajanren et al., 2015, Ras et al., 2011, Scragg et al., 2002, Singhasuwan et al., 2015, Sirisansaneeyakul et al., 2011, Wong et al., 2016), but studies in seawater-based media are rare. In nitrogen-limited freshwater batch cultures, the lipid productivity of C. vulgaris has ranged from 3 to 173 mg L−1 d−1 with an average being ∼47 mg L−1 d−1 (Griffiths et al., 2014). In nutrient sufficient media, lipid productivity of many microalgae is generally lower than in nutrient-limited culture (Ördög et al., 2016, Scragg et al., 2002).

Apparently, many strains of C. vulgaris do not withstand full strength seawater (Alyabyev et al., 2007, Matos et al., 2015) and this explains a lack of studies in seawater for this microalga. Electrical conductivity of freshwater-based culture media is generally less than 1000 μS cm−1. In contrast, a seawater based medium will have a minimum conductivity at 25 °C of around 54,000 μS cm−1. In one study, increasing conductivity of the culture medium from around 1000 μS cm−1 (i.e. a freshwater-based medium) to only around 2800 μS cm−1 reduced biomass productivity of C. vulgaris from around 0.1 g L−1 d−1 to less than 0.02 g L−1 d−1 (Matos et al., 2015).

A desalination concentrate with a maximum total dissolved salts concentration of 2.3 g L−1 mixed with a freshwater-based medium (Bold’s basal medium) to the level of 25% by volume, has been used to culture C. vulgaris (Matos et al., 2015), but the dissolved salts concentration in the final medium was only about 0.6 g L−1 higher than in a typical freshwater-based medium. In contrast, seawater contains around 40 g L−1 of dissolved salts, or nearly 67-fold greater than the saline medium used by Matos et al. (2015).

An unidentified Chlorella sp. was reported to grow in full strength seawater (Choi and Lee, 2016). Apparently, the maximum lipid content of the biomass was about 20% by dry weight in a nutrient sufficient medium (Choi and Lee, 2016), but no growth or lipid productivity data were reported. According to Shen et al. (2015), growth of a C. vulgaris was not affected by a sea salt concentration of up to 50 g L−1 and the biomass produced under nutrient deficient conditions contained nearly 40% w/w lipids. In short, there is a general lack of data on seawater culture of C. vulgaris. This work aims to address this knowledge gap. In addition, comparative data are provided on four other nominally freshwater microalgae grown in freshwater, brackish water, and seawater media.

Section snippets

Microalgae

The following five freshwater microalgae (green algae, Chlorophyta) were used: Chlorella vulgaris; Choricystis minor; Neochloris sp.; Pseudococcomyxa simplex; and Scenedesmus sp. Scenedesmus sp. had been isolated by Dr. T. Mazzuca Sobczuk at Massey University, New Zealand. The other algae had been purchased from Landcare Research, Lincoln, New Zealand. The identity of C. vulgaris had been confirmed by rbcL gene sequencing (Luangpipat, 2013). For the other microalgae, the identity was as

Salinity effects on algal growth

Because the interest was in eventually using the abundant seawater for culturing the algae for oils, all algae were assessed for growth capability in BG11 formulated in seawater. For comparison, growth was also measured in the same medium formulated with brackish water (i.e. a 1:1 by volume mixture of artificial seawater and freshwater) and freshwater. The typical growth profiles are shown in Fig. 2. The total lipids contents of the biomass at the end of the culture (Fig. 2) and its calorific

Concluding remarks

Of the five nominally freshwater microalgae tested, only C. vulgaris thrived in a full strength seawater medium. Biomass productivity in seawater was almost the same as in freshwater, but seawater enhanced lipid productivity by nearly 2-fold compared to freshwater. The biomass grown in a nutrient sufficient seawater medium contained nearly 16% w/w lipids, or more than double the lipid content of the biomass grown in freshwater. The calorific value of the biomass was nearly 25 kJ g−1 and was not

Acknowledgements

We are grateful to Neste Oil Corporation, Porvoo, Finland, for funding this research and allowing it to be published. Nakhon Sawan Rajabhat University, Nakhonsawan, Thailand, funded Dr. T. Luangpipat through a scholarship.

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