Temporal acclimation of Microchloropsis gaditana CCMP526 in response to hypersalinity
Introduction
With the ever-increasing human population, demand for fossil fuel has increased drastically over the past decade. The restricted availability of arable land poses a serious challenge to grow plants for second-generation biofuel. Another alternative is to use microalgae as a third-generation source for producing biofuel. Microalgae can grow under variety of conditions and can be cultivated even in recycled municipal wastewater. Algae require less water than their terrestrial counterparts and can synthesize valuable co-products which can be used for applications ranging from pharmaceutical to cosmological purposes (Specht and Mayfield, 2014, Wang et al., 2015). The biomass can be used as a feedstock for animals and fish (Duong et al., 2015, Pulz and Gross, 2004). Another interesting trait of microalgae is that they accumulate lipids during unfavourable conditions. This property has been studied under various abiotic stress conditions such as pH, light, temperature, nutrient limitation, etc., (Markou & Nerantzis, 2013). One of the limitations in this approach is that the growth rate becomes compromised during stress conditions, which makes the process uneconomical. However, bioprospecting strategies along with multi-stage extraction steps can make the whole process economically feasible (Stranska-Zachariasova et al., 2016).
A two-stage cultivation can also be employed to decouple growth and lipid accumulation (Ra et al., 2015), but including an additional unit operation increases the cost of bio-fuel production. This might be overcome by modelling the reaction, optimizing the extraction process and reactor design (Likozar and Levec, 2014, Likozar et al., 2016, Šoštarič et al., 2012). Rather than inducing nutrient limitation, a naturally occurring stress in open raceway ponds was employed to understand the acclimation strategy in M. gaditana. These shallow ponds are prone to evaporation during the day-time, particularly in tropical regions, which leads to increase in salinity of the culture medium. Varying salinity levels have shown to induce lipid accumulation in several Nannochlorpsis strains (Gu et al., 2012b, Martínez-Roldán et al., 2014).
Microchloropsis gaditana CCMP526, previously known as Nannochloropsis gaditana CCMP526, is a marine microalga known for its potential to accumulate high levels of lipid (∼50% dry cell weight) and a high eicosapentaenoic acid content (Fawley et al., 2015, Sukenik et al., 1989). Being a marine strain, CCMP526 is susceptible to changes in salinity in its natural environment. Effects of light, CO2, temperature and nitrate limitation have been very well documented in CCMP526 (Corteggiani Carpinelli et al., 2014, Figueroa et al., 1997, Huertas et al., 2000, Simionato et al., 2011), which shows a range of biochemical changes during adverse conditions. Several studies have focussed on the effect of salinity on the physiology of other marine microalgae (Johnson et al., 1968, Takagi and Yoshida, 2006) as well as freshwater strains (Husic and Tolbert, 1986, Yoshida et al., 2004). For example, the marine green alga Dunaliella tertiolecta has been reported to accumulate osmolytes in the form of glycerol (Avron, 1992) and also combat high salt conditions by excluding Na+ ions from its cells (Katz & Pick, 2001). Some species closely related to Microchloropsis gaditana such as Nannochloropsis oculata and Nannochloropsis salina have been shown to accumulate lipid during adverse conditions (Bartley et al., 2013, Gu et al., 2012b, Pal et al., 2011). However, relatively few studies have focussed on understanding the acclimation strategy of microalgae towards high salt conditions.
In this study, we aimed at understanding the time course of acclimation of CCMP526 to hypersaline conditions by focussing on changes in primary functions such as photosynthesis, pigment synthesis, carbohydrate and lipid accumulation during short-term (hours) and medium-term (several days) hypersaline stress.
Section snippets
Growth conditions
Microchloropsis gaditana CCMP526 was cultivated in 0.2 μm-filtered sea water (collected from the Gippsland Lakes, Gippsland, Victoria, Australia), supplemented with Guillard’s f/2 nutrients (Guillard, 1975) and 17 mM sodium nitrate, at 25 °C using 500 ml glass bottles (Schott Duran, Germany). Cultures (300 ml) were mixed by bubbling with sterile air (0.2 μm filtered) supplied at a flow rate of 2.5 L min−1. Illumination was provided at 150 μmol·photons·m−2·s−1 (Philips, TLD36W, Amsterdam, The
Growth under different salinity conditions
The growth of M. gaditana CCMP526 was strongly influenced by the salinity conditions (two-way ANOVA: F (3, 72) = 875.1, P < .0001) and cultivation time (two-way ANOVA: F (8, 72) = 1207, P < .0001). Dunnett’s multiple comparisons test revealed that the growth rate (cell number) was not significantly affected until after the first 6 h of inoculation (P > .05), except for the 55 PSU treatment at 30 min (P = .0011) and for the 100 PSU treatment at 30 min and 3 h (P = .0001 and .0428 respectively).
Conclusions
Our study reveals the acclimation strategy of M. gaditana CCMP526 to hypersaline conditions. Temporal analysis of growth, photosynthetic parameters and various biomolecules suggests that salinity can be employed to induce lipid accumulation in M. gaditana, which is of industrial importance. Additionally, high saline conditions induced production of carbohydrates and pigments which could be of high interest in pharmacological and cosmetics areas. The ability of CCMP526 to grow at high salinity
Acknowledgements
The authors acknowledge the technical support from the members of Prof. John Beardall’s lab. This work was supported by the IITB-Monash Research Academy, IITB, Mumbai, India; Reliance Industries Limited, India (IMURA0304).
Conflicts of interest
The authors declare that there is no conflict of interest.
References (49)
- et al.
Effects of salinity on growth and lipid accumulation of biofuel microalga Nannochloropsis salina and invading organisms
Biomass Bioenergy
(2013) - et al.
Chromosome scale genome assembly and transcriptome profiling of Nannochloropsis gaditana in nitrogen depletion
Mol. Plant
(2014) - et al.
Effects of high irradiance and temperature on photosynthesis and photoinhibition in Nannochloropsis gaditana Lubián (Eustigmatophyceae)
J. Plant Physiol.
(1997) Osmoregulation in Dunaliella, Part II: photosynthesis and starch contribute carbon for glycerol synthesis during a salt stress in Dunaliella tertiolecta
Plant Physiol. Biochem.
(2007)- et al.
Effects of dissolved inorganic carbon availability on growth, nutrient uptake and chlorophyll fluorescence of two species of marine microalgae
Aquacult. Eng.
(2000) - et al.
Plasma membrane electron transport coupled to Na+ extrusion in the halotolerant alga Dunaliella
Biochim. Biophys. Acta
(2001) - et al.
On the relationship between non-photochemical quenching and photoprotection of Photosystem II
Biochim. Biophys. Acta
(2012) - et al.
Effect of process conditions on equilibrium, reaction kinetics and mass transfer for triglyceride transesterification to biodiesel: experimental and modeling based on fatty acid composition
Fuel Process. Technol.
(2014) - et al.
Transesterification of oil to biodiesel in a continuous tubular reactor with static mixers: modelling reaction kinetics, mass transfer, scale-up and optimization considering fatty acid composition
Fuel Process. Technol.
(2016) - et al.
Protein measurement with the Folin phenol reagent
J. Biol. Chem.
(1951)