The microalgal research field is currently lacking a unified theoretical computing system to explain various experimental results related to microalgal growth. Thus a novel universal theoretical model was created to predict microalgal growth with carbon dioxide (CO2) fixation in any cultivation system. First, a new "light-effect colorimetric method" was proposed to estimate the actual f(2) value of suspended microalgal cells during regular experimentation using a formula explaining the photosynthetic effective electron transfer rate (ETR = PFD center dot phi II center dot f(1)center dot f(2)), which only requires the use of a simple spectrophotometer. A mathematical relationship between photosynthetic electron transfer and the microalgal growth rate was then identified based on this modified ETR by simplifying factors influencing the cultivation conditions (e.g., nutrients and CO2) into the slope and intercept of this formula. Subsequently, software was written to allow the above relationship to stimulate any 3-D microalgal cultivation system. Many example cases were conducted to clarify the significance and application of this theoretical model. It was found that the average ETR of a cultivation system describes microalgal tolerance to high CO2 concentrations. A photobioreactor at any given location under a certain light condition has a theoretical maximum yield of microalgal biomass, irrespective of how the other cultivation conditions change. A new concept of "biological similarity" is proposed as a basic principle for scaling up microalgal experiments with photosynthetic CO2 fixation to perform a repeated growth curve with < 5 % error. Finally, a "multi-batch dilution method" was demonstrated to increase the microalgal biomass yield by 64.4 % over a short cultivation period. General application of this calculation model would change the empirical status of microalgal engineering designs.

Microalgae are significantly affected by the spectra composition with various wavelengths. The development of light harvesting pigments can be controlled with specific wavelength of filtered light received by microalgae. Coverage of open raceway pond using transparent colored polyvinyl chloride sheets (PVCS) to filter light spectra, was assessed for the capacity to enhance biomass growth rate. Results showed that orange PVCS filtered light spectra at wavelengths from 480 to 665 nm, increased biomass dry weight (3.3 g/L) by 61% compared with control condition (white PVCS = 350-750 nm). Light spectra filtered through orange PVCS were more easily absorbed by the light harvesting pigment protein complex (phycobilisome) of Arthrospira platensis cells and subsequently transferred to intracellular photosynthesis reaction centers. Therefore, A. platensis cells cultivated with light spectra filtered through orange PVCS contained 62.7 mg/L chlorophyll-a and 23.5 mg/L carotenoid, which were 40% and 29% higher than control condition (with white PVCS).

Microalgae with a high growth rate and a carbon fixation rate present a promising potential to produce diverse renewable energy products and reduce greenhouse gas emissions. However, some microalgal species may have limited light-use efficiency and specific growth rate. To alleviate these issues, Nannochloropsis oceanica was subjected to 137Cs–γ radiation to obtain the desired mutant with enhanced light-use efficiency and increased biomass productivity. The N. oceanica mutant ZJU700 showed a 26.7% increase in biomass productivity after nuclear irradiation at a 700 GY dosage. It was found that the mutant had a 30.2% higher oxygen evolution rate than the wild type cells. High-throughput transcriptome sequencing showed that expression of photosynthetic related genes in the mutant were much higher than in wild type cells. Expression of the psbO gene increased by 455% in mutant ZJU700, contributing to an increased oxygen evolution rate by splitting water. Three up-regulated genes, namely petC, petF, and petH, resulted in enhanced electron transport during the photoreaction process. Up-regulation of many genes involved in the Calvin cycle indicated that CO2 fixation rate was likely to be increased to produce more carbohydrates in mutant, thereby contributing to the increased biomass productivity in mutant ZJU700.

To simultaneously promote biomass yield and astaxanthin content of Haematococcus pluvialis, ammonium ferric citrate (AFC) was employed to stimulate light harvest in photosynthesis during the green stage and oxidation induction in astaxanthin accumulation during the red stage. AFC not only improved chlorophyll synthesis by 22.5% to provide more electrochemical potential energy in the green stage, but also alleviated photosystem II damage to maintain a high level of effective quantum yield by enhancing carotenoid production. The citrate derived from AFC stimulated acetyl-CoA and NADPH production through citric acid cycle and transaminase cycle during the red stage, resulting in an increased lipid content by 1.77-fold. The astaxanthin content in H. pluvialis cells cultivated with 5 μM AFC was 12.5% higher than that without AFC, which was attributed to severe oxidative stress caused by AFC through Haber-Weiss reaction. These results provided a new approach to reduce emission of greenhouse gasses with producing high-value products.

In this study, the Tesla-valve (TV) baffles were used to optimize the flow field in a column photobioreactor (PBR) in order to promote mass transfer of CO2 gas in the solution. The TV baffles were composed of many tilted plates with central holes and curved arcs facing downwards, installed along inner rising section of the column PBR. Many clockwise and anti-clockwise vortices were generated during the rising flow while passing through proposed TV baffles. An optimum TV baffle structure (30° plate angle, 8 cm arc width) decreased mixing time by 36.4% and increased the mass transfer coefficient by 50%. The TV baffles supported the movement of the A.platensis cells between light and dark regions to enhance their photochemical efficiency ϕPSII by 24.6% and Fv/Fm by 12.7%. Therefore, the biomass yield increased by 28.1% and exhibited an increased helix pitch and trichome length in comparison with traditional column PBR without baffles.

For the comprehensive understanding of regulated carbon metabolism associated with lipid accumulation in microalgae Chlorella sp. cultivated with nitrogen starvation and phosphorus repletion (N-P+), the discrepancy among gene expression levels in carbon flow from CO2 to fatty acid synthesis were characterized through de novo sequencing, transcriptome assembly, annotation, and differential expression analysis. The biomass production of 1900 mg L−1 and lipid content of 677.55 mg (1010 cells)−1 in Chlorella sp. under N-P+ condition were 1.46-fold and 1.49-fold of those under nitrogen starvation and phosphorus limitation (N-Plim) condition, respectively. The expression levels of many enzymes related to carbon metabolism under N-P+ condition were upregulated, including carbon fixation, pyruvate metabolism, glycolysis, TCA cycle and fatty acid synthesis pathways. CO2 fixation accelerated through the upregulated Calvin cycle and glycolysis pathways. The rapid synthesis of pyruvate and upregulation of key rate-limiting enzymes involved in fatty acid synthesis favored lipid accumulation.

A novel microporous fibrous-diaphragm aerator (FDA) was proposed to generate pressurized gas with check valve to penetrate through millions of micropores (6-126 mu m) of expanded fibrous diaphragm on internal support, thus decreasing bubble generation diameter and increasing bubble residence time for improving microalgal growth with CO2 fixation in a raceway pond. When installation angle of FDA internal support increased from 0 degrees to 45 degrees, bubble generation time and diameter first decreased (to valley bottoms of 4 ms and 0.45mm at 22 degrees) and then increased. Compared to traditional strip aerator, bubble generation time and diameter decreased by 50% and 60% through FDA with support installation angle of 22 degrees and average pore diameter of 28 mu m, while gas-solution mixing time decreased by 22% and mass transfer coefficient increased by 40%, leading to increased actual photochemical efficiency (by 80%) and increased biomass yield (by 38.5%) of Arthrospira cells with pure CO2 aeration through FDA.