Improving algal biofuel production through nutrient recycling and characterization of photosynthetic anomalies in mutant algae species
Continued use of fossil fuels is now widely recognized as unsustainable because of diminishing supplies and the contribution of these fuels to the increased carbon dioxide concentration in the environment. Algae represent a promising new source of feedstock for the production of renewable, carbon neutral, transportation fuels. However, significant economic and technical challenges remain to be solved for scaling-up of algal biofuel production. This dissertation has examined two innovative approaches to improve algal biofuel production: (1) an integrated waste to algae biofuel production process that recycles wastewater nutrients into multiple cycles of algal growth, and (2) characterization of potentially advantageous photosynthetic anomalies observed in a mutant strain of green alga species Chlamydomonas reinhardtii. A novel system for algal biofuel production was proposed, which integrates algal biomass production, wastewater treatment and conversion of biomass to bio-crude oil. In this system, low-lipid but fast-growing algae were cultivated in wastewater, and the biomass was harvested and fed into a hydrothermal liquefaction (HTL) process for biofuel production. The post-HTL wastewater (PHWW) accumulates most of the nutrients from the incoming biomass and this can subsequently be fed back to the algae culturing system to recycle nutrients for multiple cycles of algae growth. A series of algae cultivation and hydrothermal conversion experiments were conducted, which showed that a consortium of algae and bacterial can be cultured in PHWW and capture both nutrients and organics. In our tests, 86% of organics (represented as chemical oxygen demand COD), 50% of nitrogen, and 25% of phosphorus were removed from the PHWW, and other previous research has shown that mixed algal-bacterial bioreactors can remove more than 90% of these contaminants when the process is optimized. Our results also showed that low-lipid alga-bacterial biomass can be successfully converted into a self-separating bio-crude oil, with refined oil yield between 30% and 50%. Approximately 70% of the nitrogen content in the incoming HTL feedstock ended up in the aqueous PHWW product, which provides a significant opportunity of nutrient recycling. A series of investigations were carried out to characterize the biophysical and biochemical difference between a spontaneous mutant of the green alga Chlamydomonas reinhardtii and the wild type cells (the mutant is called IM and the wild type is called WT). Growth curve experiments were carried out to quantify the biomass production of IM and WT as function of different light intensities. Results of this research showed that under low light intensity (10 micro mol photons m-2 s-1), IM had 35% higher cell number and 25% higher cell mass per unit volume of algae suspension than the WT. At 640 micro mol photons m-2 s-1 light intensity, both IM and WT cultures had similar cell mass, but the IM exhibited 35% lower cell number per unit volume of algae suspension than the WT. In addition, Photosystem II activity was characterized by fluorescence transients. The IM mutant had a 9% higher variable to minimal fluorescence (Fv/Fo), 10% higher ‘performance index’ (PI (abs)), a 9% higher φPo /(1-φPo ), and a 7% lower dissipation of energy per reaction center (DIo/RC) in comparison to the WT. These results suggest that IM has higher efficiency of primary photochemistry, lower rate of heat dissipation and therefore, a stronger overall photosynthetic driving force. Thus, elucidating these distinctive characteristics of the IM mutant could help accelerate development of practical biofuel production processes to meet global fuel demands.
algal biofuel, wastewater, hydrothermal liquefaction, nutrient recycling, Chlamydomonas reinhardtii, photosynthesis, fluorescence transient, environmental stress
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign