Outdoor cultivation of Chlorella vulgaris FSP-E in vertical tubular-type photobioreactors for microalgal protein production

Abstract

Aquafeeds rely heavily on fishmeal as the major protein source. However, the recent increase in the price of fishmeal and sharp decrease in fish resources have resulted in a major threat to the aquaculture industry. Therefore, cost-effective and sustainable fishmeal alternatives should be found. While microalgae are considered a promising alternative to fishmeal, the cultivation scale should be enlarged and its protein production cost should be much lower. In this study, Chlorella vulgaris FSP-E was grown in 50-liter outdoor photobioreactors (PBRs) to produce microalgal proteins. Using the optimum urea concentration (18.6 mM), inoculum size (0.2 g/L) and aeration rate (0.05 vvm), the maximum biomass productivity (268.1 mg/L/d) and protein productivity (155.4 mg/L/d) were obtained. Semi-batch operation with a 50% medium replacement ratio attained a high protein content and productivity of 52.2% and 125.2 mg/L/d, respectively. The resulting microalgal protein contained 60.3% indispensable amino acids, thus possessing a high commercial value.

Introduction

According to a report by The Food and Agriculture Organization of the United Nations, human consumption of fish increased from 137.3 million tons in 2006 to 153.6 million tons in 2011. However, the amount of fish captured from freshwater and seas has been maintained at around 90 million tons every year, while fish production from the aquaculture sector increased from 47.3 to 63.6 million tons over the period 2006 to 2011 [1]. Given the depletion of wild fish resources and the need to allow those resources to recover naturally, the aquaculture sector has seen a rapid growth to supplement the global demand for fish [1]. High-value fish species in aquaculture are often carnivorous, consuming more than 400 g/kg of protein. This high protein diet is commonly provided by fishmeal — the main protein source in aquafeed [2], [3], [4]. The increasing demand for fishmeal has caused a significant rise in the price of fishmeal [5], thus sparking the search for alternatives, with microalgal protein being a promising source. In addition, algal biomass also contains other valuable components, such as pigments, omega-3 fatty acids, minerals and vitamins, which further increase the nutritional value of algae-based feed [6], [7]. Previous studies showed that serving algal as animal feed additive resulted in enhancements growth, physiological activity, feed utilization efficiency, stress response, starvation tolerance, disease resistance and carcass quality of the cultured fish [5]. However, one of the major challenges of using algal as aquafeed is how to effectively cultivate the protein-rich microalgae in an outdoor large scale to produce proteins with high quality and low cost. It is well known that temperature variations, inconsistent sunlight supply, contamination, inefficient mixing, and light penetrating efficiency are vital factors for the outdoor phototrophic microalgae growth. Considering the lower operation cost and relatively large scale production, commercial production of microalgal biomass is still commonly achieved via outdoor cultivation. Therefore, it is required to develop high efficiency and cost-effective photobioreactors for outdoor algae cultivation. Chen et al. [6] reported that using 50 L outdoor vertical tubular-type photobioreactors (PBRs) was favorable for photoautotrophic growth of microalgae due to better mixing efficiency and higher CO₂ retention time. Operation strategy, especially inoculum size, aeration rate and nitrogen sources, is the critical factor affecting the growth of microalgae, as the nitrogen source limitation significantly influences the production of microalgal biomass and cellular components, such as total proteins, carbohydrates and lipids [8], [9]. Previous research shows that using an appropriate inoculum size could effectively promote cell growth [10]. Besides, microalgae growth in a photobioreactor was influenced by nutrient uptake, gas–liquid mass transfer and strategies [11], [12], [13]. In addition, carbon dioxide concentration and feeding rate could also be used to improve microalgae biomass and protein production [14], [15]. Meanwhile, using preferable bioreactor mode, such as continuous and semi-batch operations, could also enhance the productivity of biomass as well as target products of microalgae [16].

This study was undertaken to first compare the performance of biomass production and protein productivity of Chlorella vulgaris FEP-E in indoor and outdoor microalgal cultivation using 50 L PBRs. Next, specific engineering strategy was applied to promote cell growth and protein production of C. vulgaris FEP-E in the 50 L PBRs. The operation factors that were optimized to achieve the best cell growth and protein production included nitrogen concentration, inoculum size and aeration rate. Finally, to assess the applicability of the microalgae-based protein production system, the outdoor vertical tubular-type photobioreactor was operated on a semi-batch mode for a prolonged period under the optimal operation conditions. The goals of this study are to develop feasible cultivation strategies to enhance microalgal protein productivity in outdoor cultivation, and to evaluate the potential of producing proteins from microalgae as an alternative to fishmeal in aquafeed applications.