Botryococcus braunii growth and photosynthetic activity in biofilm

Botryococcus braunii is a green alga with high long chain hydrocarbon content suitable as a source of biofuel. Due to its external hydrocarbon production, mass cultivation of B. braunii can potentially reduce biofuel production energy and water requirements. However, if the aim is to mass produce biofuel, growing any microalgae with the conventional liquid based cultivation systems are found to be currently not economically viable. The alternative cultivation method is solid growth (biofilm) that can reduce energy and costs associated with harvesting and dewatering process. In the present work, I assessed the potential of successful biofilm growth and the overall lipid productivity of different B. braunii races, and conducted the preliminary economic assessment of B. braunii grown in biofilm compared to the liquid cultivation. In this project, I studied B. braunii (races A, B, L and S) growth on a biofilm for 10 weeks, under laboratory conditions (75 μmol photons m-2 s-1 and 25oC). Growth, photosynthetic activity and biofilm structure were observed. Three out of four tested races (B, L and S) were successfully grown to stationary phase, with no contamination. However, B. braunii UTEX2441 (race A) cultures were heavily contaminated by fungi from Week 5, thus I stopped recording growth measurements for this strain. Amongst the another three races, B. braunii BOT22 (race B) culture showed higher biomass productivity (3.80 mg dry weight cm-2 d-1) and lipid yield (1.11 mg dry weight cm-2). Irrespective of the race, almost all photosynthetic parameters (FV/F0, PIABS and the OJIP curve) showed that the biofilm culture was more stressed when grown at lag and stationary phases compared to the logarithmic phase. The most reliable photosynthetic parameter for estimating B. braunii growth condition was FV/F0. Confocal microscopy observation of B. braunii biofilm showed that biofilm thickness for all races B, L and S increased between Weeks 0 and 10, ranging from 62–92 μm from the initial thickness. The confocal microscopy observation also indicated that most of the lipids were concentrated on top, of the biofilm (between the depths of 20 and 60 μm covering 34-70% of the biofilm). In general the growth characteristics (biomass and lipid) and, photosynthetic performance of all races indicated that B. braunii BOT22 is the most promising strain for biofilm cultivation. The preliminary economic assessment was also conducted to compare economics of solid and liquid based B. braunii cultivation. The biofilm cultivation system (using algal turf system, ATS) required an area five times smaller than that of the liquid cultivation system (using paddle wheel driven raceway pond, ORP) to produce the same amount of biomass. Biofilm cultivation system of B. braunii was calculated to be more economical than conventional liquid based cultivation. The production costs of B. braunii biomass in ORPs was 16 times higher than the production costs when ATS was used. This difference is mainly due to less operational cost, especially for water and power cost, and higher B. braunii biomass productivity of solid based cultivation system (ATS) when compared to liquid based cultivation.