Pilot-scale continuous recycling of growth medium for the mass culture of a halotolerant Tetraselmis sp. in raceway ponds under increasing salinity: A novel protocol for commercial microalgal biomass production
Introduction
Currently there is substantial interest in the utilisation of microalgae towards the mitigation of the present and future world crises in food, fuel and environment. Microalgae are very diverse and represent a rich source of phytochemicals which can be used in human food, animal feed, aquaculture, for pharmaceutical, cosmetic and health food products, and conceivably for biofuel and associated by-products (Brennan and Owende, 2010, Pulz and Gross, 2004, Spolaore et al., 2006). As an alternative form of agriculture, microalgae cultivation has much commercial appeal because it can potentially yield better biomass productivity rates than land crops over similar land areas (Posten and Schaub, 2009, Schenk et al., 2008). Furthermore, it offers the possibility to use resources that are otherwise under-utilised (e.g. non-arable land, saline water, wastewater, etc.) or that are accumulating to polluting levels (e.g. excess nutrients leading to eutrophication of waterbodies, build-up of CO2 in the atmosphere which causes the greenhouse effect, etc.).
To date, much of the successful mass cultivation of microalgae is limited to the production of high value products, high revenues of which offset the high capital and operational expenditures incurred in generating and processing the biomass. Some of the most cost-prohibitive components of microalgae biomass production are directly associated with the large volume of water, which needs to be processed for cultivation and harvesting (Borowitzka and Moheimani, 2013, Fon Sing et al., 2013; Molina Grima et al., 2003). According to the life-cycle assessments of microalgae cultivation reported by Clarens et al., 2010, Flesch et al., 2013 and Yang et al. (2011), the cost-effectiveness of production could be substantially improved by minimising the water and nutrient footprint through the continuous recycling of the culture medium and by using non-potable sources of water. However, while culture medium recycling seems to confer certain advantages, ultimately the prospect of culture reuse depends largely on the suitability of the harvested water for further continuous cultivation. This is because as opposed to freshly made medium, the recycled medium, if untreated, potentially carries over and accumulates all of the dissolved chemical compounds and suspended particles remaining after the harvesting process. For instance, cell wall debris, contaminating organisms (e.g. other algal species, bacteria, etc.), dissolved organic compounds and other potentially-growth inhibiting chemicals released from the cells commonly foul the return water. If the water is left untreated prior to returning to the ponds, these chemical compounds and particles can quickly lead to increased bacterial activity and culture deterioration (Ben-Amotz, 1995, Chini-Zittelli et al., 1999, Rodolfi et al., 2003). In addition to this, the gradual increase in inorganic salts (salinity) due to evaporation must also be considered, especially in open cultivation systems where brackish or saline water is used. This change in salinity can potentially impact on the culture in two ways, namely: by a gradual dominance of more halotolerant species of microalgae, and/or in steady decline in the density of the desired microalga due to its inability to cope with osmotic changes and changing salt ratios. Another potential challenge of recycling medium with increasing salinity is the precipitation of calcium salts, especially in calcium-laden water, thereby causing loss of alkalinity and other minerals such as iron and phosphorus (Shimamatsu, 2004).
Clearly more exhaustive research should be carried out to enhance and optimise current processes used to grow monocultures of microalgae in recycled culture medium. Considering the difficulties of recycling culture media and the need to use non-potable water for improving the economics and sustainability of microalgal biomass production, the real challenge lies in the ability to sustain monocultures of microalgae for as long as possible in a recycled culture medium of potentially increasing salinity without any loss in biomass productivity. This study investigates the effect of culture medium recycling on culture health and biomass productivity of a halotolerant strain of Tetraselmis sp. grown at increasing salinity and continuously for long periods in open raceway ponds under outdoor field conditions. Thus, the proposed protocol is a direct contribution to the development of cost-efficient and sustainable production of biomass from saline microalgae in the field by a rational recycling of the culture medium.
Section snippets
Location and microalgal species
The experiment was carried out from August to December 2012 in outdoor open raceway mixed ponds in a semi-arid remote area in Karratha, Western Australia, Australia (20S 45′47.72″, 116E 44′9.88″). Tetraselmis sp. MUR 233, originally from the Murdoch University Algae Culture Collection (Perth, Western Australia) was used as test organism. This alga was maintained in semi-continuous cultivation mode in outdoor raceway ponds at the same location for at least two years prior to this experiment.
Ponds
Two 2 m
Long-term cultivation under increasing salinity
The semi-continuous culturing of Tetraselmis MUR 233 in both non-recycled and recycled media lasted for almost 5 months (127 days) without any major interruptions or culture loss. Throughout this period, the cultures benefited from abundant sunlight, night and day temperatures above 10 and 28 °C, respectively, and no rain (Fig. S1 in Supporting information). For the purpose of the experiment and due to technical constraints that prevented the assessment of the impact of variation in solar
Conclusions
This proof-of-concept study demonstrates that (1) the expected baseline productivity from a halotolerant microalgae culture grown under increasing salinity in a semi-arid climate is consistently greater than 15 g AFDW m−2 d−1, (2) Tetraselmis MUR 233 can be grown continuously with minimal freshwater input, and in recycled culture medium without any decline in biomass productivity and, (3) the electro-flocculation harvesting technique circumvents the wastage of water and nutrients and the need for
Acknowledgements
Financial support for this study was provided by the Australian Research Council’s Linkage Project Funding Scheme (Project LP 100200616) in partnership with SQC Pty Ltd. The authors also acknowledge the support and technical assistance provided by the Muradel Pty Ltd. staff.
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