Effect of Nutrients on the Growth and Photosynthetic Ability of a Hypersaline Diatom

Effect of Nutrients on the Growth and Photosynthetic Ability of a Hypersaline Diatom
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Murdoch University
Caitlin May: 30981943
Effect of Nutrients on the Growth and Photosynthetic Ability of a Hypersaline Diatom
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Murdoch University
Caitlin May: 30981943

Introduction

Recently there has been an increase of interest in the industrial applications of microalgae, in particular diatoms, for producing high value products. Diatoms are a group of eukaryotic, unicellular microorganisms generally characterized by an exterior cell wall comprised of amorphous silica (Falciatore & Bowler, 2002). Diatoms constitute a large proportion of phytoplanktonic algae and are of high ecological relevance, as they are believed to be responsible for approximately 40% of marine primary productivity (Falciatore & Bowler, 2002; Young & Beardell, 2005).
Over the past few decades numerous species of diatoms have been screened for their ability to produce natural substances such as poly-unsaturated fatty acids (fish oils), antibiotics, vitamins, hydrocarbons, pharmaceutically active compounds and foodstuffs for both human consumption and aquaculture industries (Bozarth et al., 2008; Cenciani et al., 2011; Converti et al., 2009). Of particular interest however are their fuel production capabilities, most notably their ability to produce large amounts of natural oils with some diatom species reported to produce a lipid content of between 20%- 40% dry weight (Jiang et al., 2012; Kanda et al., 2011; Liu et al., 2008).
In addition to lipid production for biodiesels, diatom biomass also contains carbohydrates and proteins that can be used as alternate fuels via carbohydrate fermentation for ethanol, anaerobic digestion of proteins to produce methane and photobiological hydrogen production, making them a valuable resource for industrial applications (Bozarth et al., 2008; Cencani et al., 2011). In this regard it is believed that microalgae will play a major role in the mass production of biofuels and producing high value products over the coming years (Converti et al., 2009; Ramachandra et al., 2009).
Microalgae are known as some of the most efficient photosynthetic organisms on earth, with both their growth and lipid production capabilities directly linked to reductions in greenhouse gas emissions (Cenciani et al., 2011; Feng et al., 2011). This presents further benefits for their use as a renewable energy source as they can potentially sequester carbon dioxide released from industrial plants during their growth, effectively acting as a carbon sink (Cenciani et al., 2011; Converti et al., 2009;).
Due to these factors diatoms and other microalgae have emerged as one of the most promising alternatives for the production of renewable biofuels. This is largely due to their faster growth rates, photosynthetic efficiencies and higher oil yields in comparison to current terrestrial plant sources (Converti et al., 2009; Liu et al., 2008). Furthermore, microalgae are able to grow in areas not suited for conventional agriculture such as marginal lands, high salinity reservoirs and wastewater deposits, minimizing the displacement of farmland required for food crops (Liu et al., 2008).
Although mass cultivation of microalgae is steadily increasing worldwide the use of microalgae biofuels is yet to be commercialized due to its relatively high cost in comparison to current fuel sources. Key parameters affecting algal oil production include biomass productivity, overall lipid productivity and downstream processing costs (Feng et al., 2011; Liu et al., 2008; Zhang et al., 2010). Thus, developments in process optimization will be a determining factor in the future of this technology.
The overall cellular productivity and biochemical quality of lipids is largely dependent on microalgae growth conditions (Pahl et al., 2009). The quality and quantity of lipids produced within microalgae cells are known to vary immensely as a result of changes in growth parameters (Jiang et al., 2012; Mutlu et al., 2010). It is widely accepted that lipid accumulation occurs in cells under high stress conditions. This is due to intracellular changes that shift the usage of carbon from proliferation to energy storage in the form of lipids (Jiang et al., 2012). The biochemical changes in microalgae associated with unfavourable environmental conditions such as agitation, nutrients, temperature, pH and light intensity are known to be species- specific, and also strongly influenced by the interaction of these factors (Cenciani et al., 2011; Jiang et al., 2012; Khatoon et al., 2010; Saros & Fritz, 2000).
Present research in this area has been mainly focused on modified growth conditions, such as, silicon and trace metal deficiency, high salinity and nutrient limitation, in particular, nitrogen deprivation to enhance intracellular lipid production (Jiang et al., 2012; Li et al, 2008). Studies conducted in this area showed that a depletion in nitrogen levels leads to decreased protein production and relative increases in both lipid and carbohydrate storage (Converti et al., 2009; Isleten-Hosoglu et al., 2012; Jiang et al, 2012). However, the decrease in protein production results in decreased growth rates and hence, lowers overall productivity. Converti et al (2009) suggests that for the effective large scale production of biofuel from microalgae a compromise needs to be reached between an increase in lipid percentage and a slowdown in growth.
Besides nutrient depletion and medium composition other environmental factors such as temperature, pH and agitation significantly effect cellular growth and lipid production of microalgae (Jiang et al., 2012). Thus to optimize overall productivity the interactions between environmental factors and growth conditions need to be assessed. In general pH is regarded as one of the more complex and influential environmental factors affecting algal populations (Dubinsky & Rotem, 1974). Left unregulated algal populations have the ability to control the pH of their medium due to photosynthetic uptake and respiratory release of carbon dioxide. These changes in pH affect the carbonate-bicarbonate equilibria of the medium (Dubinsky & Rotem, 1974). It is widely accepted that oscillations in pH drastically affect cellular processes in microalgae, including their redox potential and ability to absorb nutrients (Dubinsky & Rotem, 1974).
Photosynthetic efficacy of microalgae is also known to have a marked effect on the overall productivity of the system. Photosynthetic performance affects the primary production of proteins, carbohydrates and other important cellular functions (Jiang et al., 2012). Microalgae have shown the ability to adapt and acclimatise their photosynthetic activity in response to changes in environmental conditions, such as nutrient availability, light and temperature changes (Juneau & Harrison, 2005). The effects of these factors on photosynthetic performance have been well documented (Jiang et al., 2012; Lippemeier et al., 1999; Lombardi & Maldonado, 2011; Young & Beardall, 2005).
As stated previously much is already known on the effects of nutrient limitation on intracellular lipid production. However, it is also necessary to investigate the appropriate sources of these nutrients and their interactions with environmental factors. This study aims to examine the effect and interactions between pH levels and nitrogen source (NaNO3, NH4Cl, urea) on the biochemical composition, photosynthetic response (chlorophyll fluorescence) and growth characteristics of a hypersaline diatom Cymbella sp. It is hypothesised that the combination of controlled pH levels and nitrogen source will have a significant effect on both cellular composition and growth.
Diatoms will be grown in semi-continuous cultures under constant temperature, mixing and light irradiance. Photosynthetic rates will be determined from dark-adapted samples using the pulse- amplitude- modulation (PAM) fluorescence method. This method measures the photosynthetic capacity of photosystem II (PSII) and has been widely used to assess the effects of changing environmental factors on the photosynthetic efficacy of algal populations (Juneau & Harrison, 2005; Lombardi & Maldonado, 2011; Lippemeier et al., 1999; Parkhill et al., 2001). Cellular composition will be evaluated through lipid, protein, dry weight and carbohydrate analysis. Lipids will then be further analysed by both thin layer and gas chromatography. It is hoped that the results found in this study will contribute to future investigations on microalga growth optimisation and increased lipid production.
Materials and Methods

References
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References: Lombardi, A.T., & Maldonado, M.T., (2011) The Effects of Copper on the Photosynthetic Response of Phaeocystis cordata, Photosynthesis Research, 108 (1), pp. 77-87 Mutlu, Y.B., Isik, O., Uslu, L., Koc, K., & Durmaz, Y