Hongxiang Lu

Comparison in dark hydrogen fermentation followed by photo hydrogen fermentation and methanogenesis between protein and carbohydrate compositions in Nannochloropsis oceanica biomass

The thermodynamic comparison in dark fermentation between amino acids and reducing sugars released from Nannochloropsis oceanica biomass are investigated for the first time. The total utilisation efficiencies of amino acids and reducing sugars are both about 95% in dark fermentation. But the consumption time of most amino acids is about 2 times as long as that of most reducing sugars in dark fermentation. A three-stage method comprising dark fermentation, photofermentation and methanogenesis is proposed to improve hydrogen and energy yields from N. oceanica biomass. Overall, the maximum hydrogen yield of 183.9 ml/g-total volatile solids (TVS) and the methane yield of 161.3 ml/g-TVS are achieved from N. oceanica biomass through the three-stage method. The total energy yield of hydrogen and methane from microalgae biomass through the three-stage method is 1.7 and 1.3 times higher than those through the two-stage (dark fermentation and methanogenesis) and single-stage (methanogenesis) methods, respectively.

Enhancing growth rate and lipid yield of Chlorella with nuclear irradiation under high salt and CO2 stress

In order to produce biodiesel from microalgae cultured with abundant seawater, Chlorella sp. was mutated with (137)Se-γ ray irradiation and domesticated with f/2 seawater culture medium (salinity=3 wt.%) under 15 vol.% CO2 stress. Biomass yield of the mutant increased by 25% compared with wild species and lipid content increased to 54.9%. When nitrogen and phosphorus concentrations in the initial substrate increased, the increased propagation speed of the mutant resulted in decreased cell diameter by 26.6% and decreased cell wall thickness by 69.7%. The dramatically increased biomass yield of the mutant with sufficient initial substrate and relative nitrogen starvation in the later growth period with continuous 15 vol.% CO2 led to an increased lipid yield of 1.0 g/L. The long-chain unsaturated fatty acids increased, whereas short-chain saturated fatty acids decreased.

Mutation of Spirulina sp . by nuclear irradiation to improve growth rate under 15% carbon dioxide in flue gas

Spirulina sp. was mutated by γ-rays from 60Co nuclear irradiation to improve growth and CO2 fixation rate under 15vol.% CO2 (in flue gas from a power plant). Mutants with enhanced growth phenotype were obtained, with the best strain exhibiting 310% increment in biomass yield on day 4. The mutant was then domesticated with elevated CO2 concentration, and the biomass yield increased by 500% after domestication under 15vol.% CO2, with stable inheritance. Ultrastructure of Spirulina sp. shows that the fractal dimension of Spirulina cells decreased by 23% after mutation. Pore size in the cell wall of Spirulina mutant increased by 33% after 15vol.% CO2 domestication. This characteristic facilitated the direct penetration of CO2 into cells, thus improving CO2 biofixation rate.

Simultaneous enhancement of microalgae biomass growth and lipid accumulation under continuous aeration with 15% CO2

To overcome the opposing trends in biomass yield and lipid accumulation, Chlorella PY-ZU1 cultures were continuously aerated with 15% CO2 to simultaneously enhance biomass yield (2.78 g L−1) and lipid content (47.04%). Microalgal cells consumed almost all the nitrate in the culture after 1 day to synthesize 24 mg L−1 of chlorophyll, which supported a peak growth rate of 0.675 g L−1 per day. Meanwhile, increased expression of key enzymes related to lipid synthesis (e.g., acetyl coenzyme A) enhanced lipid productivity to 192.10 mg L−1 per day. During the growth process, the carbon content of the dried biomass increased from 47.00% to 56.02% while the nitrogen content decreased from 6.36% to 1.99%. The unsaturated fatty acids decreased and saturated fatty acids increased, thus improving the anti-oxidation stability of microalgal biodiesel.

Transcriptome-based analysis on carbon metabolism of Haematococcus pluvialis mutant under 15% CO2

To elucidate the mechanism underlying the enhanced growth rate in the Haematococcus pluvialis mutated with 60Co-γ rays and domesticated with 15% CO2, transcriptome sequencing was conducted to clarify the carbon metabolic pathways of mutant cells. The CO2 fixation rate of mutant cells increased to 2.57gL-1d-1 under 15% CO2 due to the enhanced photosynthesis, carbon fixation, glycolysis pathways. The upregulation of PetH, ATPF0A and PetJ related to photosynthetic electron transport, ATP synthase and NADPH generation promoted the photosynthesis. The upregulation of genes related to Calvin cycle and ppdK promoted carbon fixation in both C3 and C4 photosynthetic pathways. The reallocation of carbon was also enhanced under 15% CO2. The 19-, 14- and 3.5-fold upregulation of FBA, TPI and PK genes, respectively, remarkably promoted the glycolysis pathways. This accelerated the conversion of photosynthetic carbon to pyruvate, which was an essential precursor for astaxanthin and lipids biosynthesis.

The oxidation product (NO3−) of NO pollutant in flue gas used as a nitrogen source to improve microalgal biomass production and CO2 fixation

In order to eliminate the inhibition effect of the toxic nitric oxide (NO) in flue gas on microalgal growth and CO2 fixation, NO was converted by a wet UV/H2O2 method to produce nitrate (NO3−), which then be used as a nitrogen source for microalgae to improve its growth. The growth ability and biomass compositions of the microalgae cultivated with the produced NO3− from NO gas were similar to those of the microalgae cultivated with equivalent moles of commercial NaNO3. The NO3− concentration produced from NO increased with UV lamp power, H2O2, and NO concentrations, resulting in an improved microalgal growth. The concentration of NO3− from 500 ppm NO wet-oxidized by 6% (v/v) H2O2 and 55 W UV light was up to 8.8 mM. When the produced nitrate was used as supplementary nitrogen source, the maximum growth productivity of Chlorella PY-ZU1 at 15% (v/v) CO2 reached 1.18 g L−1 per day (0.97 times higher than that cultivated with the standard medium). The peak fixation efficiency of 15% (v/v) CO2 was 69.6% (1.13 times higher than that cultivated with the standard medium).

Generating cycle flow between dark and light zones with double paddlewheels to improve microalgal growth in a flat plate photo-bioreactor

Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor, which strengthened the mass transfer and the mixing effect to improve microalgal growth with 15% CO2. Numerical fluid dynamics were used to simulate the cycle flow field with double paddlewheels. The local flow field measured with particle image velocimetry fitted well with the numerical simulation results. The horizontal fluid velocity in the photo-bioreactor was markedly increased from 5.8 × 10-5 m/s to 0.45 m/s with the rotation of double paddlewheels, resulting in a decreased dark/light cycle period. Therefore, bubble formation time and diameter reduced by 24.4% and 27.4%, respectively. Meanwhile, solution mixing time reduced by 31.3% and mass transfer coefficient increased by 41.2%. The biomass yield of microalgae Nannochloropsis Oceanic increased by 127.1% with double paddlewheels under 15% CO2 condition.