Abstract
Background: While a major driver for technological innovation, microalgae-based biofuels remain economically non-viable. However, as microalgae cultivation continues to scale-up to industrial production levels, algal biorefinery strategies must emerge in order to effectively valorize the biomass (crop) and help stabilize the economic viability of a microalgae-based industry. Of the numerous potential markets for microalgae (co) products, the animal nutrition and aquaculture sectors may be highly promising areas to focus for generating revenues. The aim of this paper was to generate novel primary chemical and biochemical composition data on whole and lipid-extracted algal biomass produced from a proprietary freshwater strain of Scenedesmus sp. AMDD.
Methods: Scenedesmus sp. AMDD biomass was produced in a ‘Brite-Box’ photobioreactor and half was lipid-extracted by automated Soxhlet solvent extraction. Using established and new methods, both fractions were characterized for their chemical and biochemical composition including proximate, amino acid, fatty acid, carotenoid and elemental composition. The data are discussed with an emphasis towards their application to terrestrial livestock and aquaculture feeds.
Results: Under the particular cultivation conditions employed in this study, Scenedesmus sp. AMDD had a growth rate of 0.8±0.1 d-1, biomass production of 0.4±0.0 g DW L-1 and daily productivity of 40.0±2.9 mg DW L-1 d-1. On a DW basis, whole algal biomass (WAB) contained low ash (3%), moderate protein (32-35%), esterifiable lipid (11-13%) and starch (8-9%), high total carbohydrate (35-38%), fibre (26-29%) and energy (23 MJ kg-1). Lipid-extracted biomass (LEB) contained higher protein (40- 44%), total carbohydrate (40-48%), starch (10-12%) and fibre (30-35%) and lower residual esterifiable lipid (<1%) and energy (20 MJ kg-1). Scenedesmus sp. AMDD protein had favorable amino acid profiles with high essential amino acid indices (0.9-1.0); was rich in first-limiting essential amino acids lysine (1.8-2.3%), methionine (0.7-1.0%) and tryptophan (0.2-0.7%). Algal lipid (% of total fatty acids) was predominantly composed of polyunsaturated fatty acids (PUFA, 45-52%), high in monounsaturated fatty acids (MUFA, 31-38%) and low in saturated fatty acids (SFA, 16%). Scenedesmus sp. AMDD lipid was particularly rich in α-linolenic acid (18:3n-3; 18-23%), total n-3 PUFA (30-38%) and lower in total n-6 PUFA (13-15%); which resulted in attractive n-3:n-6 ratios (2-3:1). Scenedesmus sp. AMDD biomass was rich in potassium (0.6-0.7%), phosphorous (0.5-0.7%), iron (1081- 1777 mg kg-1) and lutein (0.3% [WAB only]) and was virtually absent of contaminating heavy metals. A species-specific nitrogen-to-protein conversion factor of N×5.55 was validated for use with Scenedesmus sp. AMDD harvested during exponential growth phase.
Conclusion: Due to its relatively high contents of protein (32-44%) and carbohydrate (35-48%), Scenedesmus sp. AMDD biomass could be considered a protein-rich algal meal with a market value similar to those produced from terrestrial oilseed crops such as soy, canola and corn. However, the carbohydrate fraction contained low levels of starch (23-26% of total carbohydrate) and a high proportion of fibre (74-77% of total carbohydrate). As a result, relatively unprocessed Scenedesmus sp. AMDD biomass may not be well digested by monogastric animals and may be better suited for ruminant animals that have a higher capacity for digestion of cellulosic materials. In order to enter the higher-value monogastric animal feed sector, the biomass would likely need to be upgraded to produce algal protein concentrates (APCs) with higher protein levels, concomitant with reduced total carbohydrate and fibre levels. In vitro investigations are currently underway to evaluate Scenedesmus sp. AMDD biomass for nutrient digestibility, metabolizable energy content, ruminal gas production, enteric methane abatement potential, ruminal fatty acid biohydrogenation and assessment for potential consumption risk by monogastric and ruminant animals.
Keywords: Amino acids, carotenoids, composition, fatty acids, microalgae, minerals, proximate.
Graphical Abstract
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