• Energy Research

AbstractOxygen‐balanced mixotrophy (OBM) is a novel type of microalgal cultivation that improves autotrophic productivity while reducing aeration costs and achieving high biomass yields on substrate. The scale‐up of this process is not straightforward, as nonideal mixing in large photobioreactors might have unwanted effects in cell physiology. We simulated at lab scale dissolved oxygen and glucose fluctuations in a tubular photobioreactor operated under OBM where glucose is injected at the beginning of the tubular section. We ran repeated batch experiments with the strain Galdieria sulphuraria ACUF 064 under glucose pulse feeding of different lengths, representing different retention times: 112, 71, and 21 min. During the long and medium tube retention time simulations, dissolved oxygen was depleted 15–25 min after every glucose pulse. These periods of oxygen limitation resulted in the accumulation of coproporphyrin III in the supernatant, an indication of disruption in the chlorophyll synthesis pathway. Accordingly, the absorption cross‐section of the cultures decreased steeply, going from values of 150–180 m2 kg−1 at the end of the first batch down to 50–70 m2 kg−1 in the last batches of both conditions. In the short tube retention time simulation, dissolved oxygen always stayed above 10% air saturation and no pigment reduction nor coproporphyrin III accumulation were observed. Concerning glucose utilization efficiency, glucose pulse feeding caused a reduction of biomass yield on substrate in the range of 4%–22% compared to the maximum levels previously obtained with continuous glucose feeding (0.9 C‐g C‐g−1). The missing carbon was excreted to the supernatant as extracellular polymeric substances constituted by carbohydrates and proteins. Overall, the results point out the importance of studying large‐scale conditions in a controlled environment and the need for a highly controlled glucose feeding strategy in the scale‐up of mixotrophic cultivation.

Mathematical models were developed to predict biomass and hydrocarbon productivities and colony size (ouputs) of Botryococcus braunii showa cultures based on light intensity, temperature and dilution rate (inputs). These models predicted the following maximum values: biomass productivity, 1.3 g L-1 d-1; hydrocarbon productivity, 1.5 mg L-1 d-1; colony size, 320 µm under different culture conditions respectively. These values were confirmed experimentally. Additionally, the combination of inputs that simultaneously maximize all the possible outputs combinations were determined. The prediction for biomass-hydrocarbon-colony size were 1 g L-1 d-1, 12.05 mg L-1 d-1 and 156.8 µm respectively; biomass productivity-hydrocarbon productivities: 1 g L-1 d-1 and 13.94 mg L-1 d-1 respectively; biomass productivity-colony size: 1 g L-1 d-1 and 172.8 µm respectively; hydrocarbon productivity-colony size: 9 mg L-1 d-1 and 240 µm respectively. All these predictions were validated experimentally. These models might be very useful to implement a Botryococcus braunii showa large scale production.

Cellular agriculture could represent a more sustainable alternative to current food and nutraceutical production processes. Tisochrysis lutea microalgae represents a rich source of antioxidants and omega-3 fatty acids essential for human health. However, current downstream technologies are limiting its use. The present work investigates mild targeted acoustic treatment of Tisochrysis lutea biomass at different growth stages and acoustic frequencies, intensities and treatment times. Significant differences have been observed in terms of the impact of these variables on the cell disruption and energy requirements. Lower frequencies of 20 kHz required a minimum of 4500 J to disrupt 90% of the cells, while only 1000 J at 1146 kHz. Comparing these results with current industry standards such as bead milling, up to six times less energy use has been identified. These mild biomass processing approaches offer a certain tunability which could suit a wide range of microorganisms with only minor adjustments.

AbstractThe slow development of microalgal biotechnology stems from the failure in the design of large‐scale photobioreactors where light energy is efficiently utilized. Due to the light gradient inside the reactor and depending on the mixing properties, algae are subjected to certain light/dark cycles where the light period is characterized by a light gradient. These light/dark cycles will determine productivity and biomass yield on light energy. Air‐lift reactors can be used for microalgae cultivation and medium‐frequency light/dark cycles will be found in these systems. Light/dark cycles are associated with two basic parameters: first, the light fraction, i.e., the ratio between the light period and the cycle time and second, the frequency of the light/dark cycle. In the present work, light/dark cycles found in air‐lift reactors were simulated taking into account the light gradient during the light period. The effect of medium‐frequency cycle time (10–100 s) and light fraction (0.1–1) on growth rate and biomass yield on light energy of the microalgae Dunaliella tertiolecta was studied. The biomass yield and growth rates were mainly affected by the light fraction, while cycle time had little influence. Response surface methodology was used and a statistical model describing the effect of light fraction and cycle time on growth rate and biomass yield on light energy was developed. The use of the model as a reactor design criterion is discussed. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 170–179, 2003.

The Arabian Peninsula's advantageous climate, availability of non-arable land, access to seawater and CO2-rich flue gas, make it an attractive location for microalgae biomass production. Despite these promising aspects, the region has seen very few studies into the commercial feasibility of algae-based value chains. This work aims to address this gap through a techno-economic feasibility study of algae biomass production costs, comparing different photobioreactor types, locations, and production scales. Flat panel and raceway pond cultivation systems were found to be the most economically attractive cultivation systems, with biomass production costs as low as 2.9 €·kg-1. Potential cost reductions of up to 42.5% and 25% could be accomplished with improvements in photosynthetic efficiencies and increased culture temperatures, respectively. As of such, efforts to source local thermo- and photo- tolerant strains could be the key to unlock the potential of the region for algae commercialization, linking into food, feed and nutraceutical industries.

This work aimed to select a Tisochrysis lutea phenotype with higher biomass and fucoxanthin productivities using fluorescence-activated cell sorting (FACS). A novel phenotype was obtained after 2 rounds of selection, based on high-fucoxanthin fluorescence. The resulting phenotype forms cell aggregates, has no flagella, and was stable after 15 months. Optimal temperature (30 °C) and light (300 µmol m-2 s-1) were obtained at laboratory scale, identical to the original strain. The biomass productivity was higher than the original strain: 1.9× at laboratory scale (0.4 L), and 4.5× under outdoor conditions (190 L). Moreover, compared to the original strain, the productivity of fucoxanthin increased 1.6-3.1× and docosahexaenoic acid 1.5-1.9×. These are the highest ever reported outdoor productivities, obtained with a robust new phenotype from a T. lutea monoculture isolated with FACS without genetic manipulation. The resulting phenotype shows high potential for industrial production.