Abstract
The process of milking microalgae is a promising approach to reduce the downstream costs for the production of valuable substances from microalgae by avoiding the steps of harvest, dewatering, and cell disruption of the common process chain (cultivation, harvesting, dewatering, cell disruption, extraction, purification). The green microalga Botryococcus braunii is particularly suited for this process due to its ability to produce large amounts of long-chain hydrocarbons accumulating in an extracellular matrix. The extracellular location of hydrocarbons is an enormous advantage in comparison with other microalgae that accumulate lipids in intracellular lipid bodies. At present, only a few B. braunii strains (UTEX 572, CCAP 807/2, SAG 807/1, FACHB 357, Bot22, and SCCAP 1761) have been examined for the process of long-term repetitive milking. In order to identify promising candidates for the milking process, twelve different B. braunii strains (SAG 30.81, SAG 807/1, UTEX 572, UTEX 2441, CCAP 807/2, ACOI 58, ACOI 1257, SCCAP K-1489, var. Showa, Bot22, SCCAP K-1761, and CCALA 779) were investigated in terms of growth, lipid accumulation, nutrient uptake, solvent compatibility, and extracellular hydrocarbon extractability. Based on these results, a ranking was defined in view of eligibility for non-destructive hydrocarbon extraction. Results indicate a particular potential for hydrocarbon milking for two of those twelve B. braunii strains. The strain Showa (71 out of 75 ranking points), which has not yet been examined for long-term repetitive milking, and the strain Bot22 (64 out of 75 ranking points) seem to be the most suitable strains for the milking process. They both possess good extractant compatibility including hydrocarbon extractability as well as high biomass and lipid productivity.
Highlights
Microalgae and their numerous advantages have been mentioned in several publications to date (Rosello Sastre and Posten 2010; Griehl and Bieler 2011; Santhosh et al 2016; Deviram et al 2020)
A suitable alga for the process of milking is the colony forming green microalga Botryococcus braunii which produces a high content of extracellular long-chain hydrocarbons which constitute up to 86% of the total dry weight (Borowitzka 2018)
Compared with other plants using nearly 85% of the carbon to generate biomass and only 10% for fatty acid production, B. braunii utilizes only 45% to build biomass and nearly the same amount to produce hydrocarbons (Melis 2013). This specific feature of carbon partitioning during photosynthesis is responsible for the high hydrocarbon content during growth phase on the one hand, and for a relatively low growth rate on the other hand (Jackson et al 2017), which makes the classical treatment of this alga not feasible (Griehl et al 2015)
Summary
Microalgae and their numerous advantages have been mentioned in several publications to date (Rosello Sastre and Posten 2010; Griehl and Bieler 2011; Santhosh et al 2016; Deviram et al 2020). A suitable alga for the process of milking is the colony forming green microalga Botryococcus braunii which produces a high content of extracellular long-chain hydrocarbons which constitute up to 86% of the total dry weight (Borowitzka 2018). Compared with other plants using nearly 85% of the carbon to generate biomass and only 10% for fatty acid production, B. braunii utilizes only 45% to build biomass and nearly the same amount to produce hydrocarbons (Melis 2013) This specific feature of carbon partitioning during photosynthesis is responsible for the high hydrocarbon content during growth phase on the one hand, and for a relatively low growth rate on the other hand (Jackson et al 2017), which makes the classical treatment of this alga not feasible (Griehl et al 2015). SAG 807/1, CCAP 807/2, SCCAP 1761, and Bot have been used for the process of milking, while strains SAG 30.81, ACOI 1257, ACOI 58, UTEX 2441, UTEX 572, CCALA 779, SCCAP 1489, and Showa only been investigated for growth so far
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