Influence of photobioreactor set-up on the survival of microalgae inoculum

Photobioreactor configurations in cultivating microalgae biomass for biorefinery

Microalgae, highly prized for their protein, lipid, carbohydrate, phycocyanin, and carotenoid-rich biomass, have garnered significant industrial attention in the context of third-generation (3G) biorefineries, seeking sustainable alternatives to non-renewable resources. Two primarily cultivation methods, open ponds and closed photobioreactors systems, have emerged. Open ponds, favored for their cost-effectiveness in large-scale industrial production, although lacking precise environmental control, contrast with closed photobioreactors, offering controlled conditions and enhanced biomass production at the laboratory scale. However, their high operational costs challenge large-scale deployment. This review comprehensively examines the strength, weakness, and typical designs of both outdoor and indoor microalgae cultivation systems, with an emphasis on their application in terms of biorefinery concept. Additionally, it incorporates techno-economic analyses, providing insights into the financial aspects of microalgae biomass production. These multifaceted insights, encompassing both technological and economic dimensions, are important as the global interest in harnessing microalgae's valuable resources continue to grow.

Numerical study of bacteria removal from microalgae solution using an asymmetric contraction-expansion microfluidic device: A parametric analysis approach

Microalgae have been remarkably taken into account due to their wide applications in the biopharmaceutical, nutraceutical and bio-energy fields. However, contamination of microalgae with bacteria still appears to be a concern, adversely impacting products' quality and process efficiency. Microalgae decontamination with conventional techniques is usually expensive and time-consuming. Moreover, damage to microalgae cells is highly possible. Asymmetric contraction-expansion microchannels (Asym-CEMCs) are promising passive microfluidic devices that can overcome conventional techniques' drawbacks with their standing-out features. However, the flexibility of Asym-CEMCs performance arising from their various tunable geometrical parameters results in the fact that their performance for separating a target particle cannot be predicted without an investigation. In this work, for the first time, Asym-CEMCs were numerically studied for the removal of a very conventional bacteria, B. subtilis (1 μm), from one of the most popular microalgae, C. vulgaris (5.7 μm). The influences of the microchannel aspect ratio, length and width ratios of the expansion-to-contraction zones, and the total flow rate on the separation resolution and focusing width of the particles were investigated by a 3D numerical model. The aspect ratio had the strongest influence on the Asym-CEMC performance, however, the length ratio had no considerable effect on the results. A decrease in the aspect ratio augmented the shear-induced lift force and Dean drag force, leading to a significant separation resolution improvement. Microalgae decontamination was also enhanced by an increase in the total flow rate and expansion-to-contraction width ratio. Finally, a locally optimized Asym-CEMC with an aspect ratio of one and expansion-to-contraction width and length ratios of 4.7 and 2.07, respectively, was proposed, leading to complete microalgae decontamination with a high normalized separation resolution of 0.6. In a word, Asym-CEMCs with tailored dimensions are promising for successfully decontaminating microalgae from bacteria.

Overview and Challenges of Large-Scale Cultivation of Photosynthetic Microalgae and Cyanobacteria

Microalgae and cyanobacteria are diverse groups of organisms with great potential to benefit societies across the world. These organisms are currently used in food, feed, pharmaceutical and cosmetic industries. In addition, a variety of novel compounds are being isolated. Commercial production of photosynthetic microalgae and cyanobacteria requires cultivation on a large scale with high throughput. However, scaling up production from lab-based systems to large-scale systems is a complex and potentially costly endeavor. In this review, we summarise all aspects of large-scale cultivation, including aims of cultivation, species selection, types of cultivation (ponds, photobioreactors, and biofilms), water and nutrient sources, temperature, light and mixing, monitoring, contamination, harvesting strategies, and potential environmental risks. Importantly, we also present practical recommendations and discuss challenges of profitable large-scale systems associated with economical design, effective operation and maintenance, automation, and shortage of experienced phycologists.

Wastewater from textile digital printing as a substrate for microalgal growth and valorization

This study aims at evaluating an innovative biotechnological process for the concomitant bioremediation and valorization of wastewater from textile digital printing technology based on a microalgae/bacteria consortium. Nutrient and colour removal were assessed in lab-scale batch and continuous experiments and the produced algae/bacteria biomass was characterized for pigment content and biomethane potential. Microbial community analysis provided insight of the complex community structure responsible for the bioremediation action. Specifically, a community dominated by Scenedesmus spp. and xenobiotic and dye degrading bacteria was naturally selected in continuous photobioreactors. Data confirm the ability of the microalgae/bacteria consortium to grow in textile wastewater while reducing the nutrient content and colour. Improvement strategies were eventually identified to foster biomass growth and process performances. The experimental findings pose the basis of the integration of a microalgal-based process into the textile sector in a circular economy perspective.

Impact of photobioreactor design on microalgae-bacteria communities grown on wastewater: Differences between thin-layer cascade and thin-layer raceway ponds

Thin-layer (TL) photobioreactors (PBRs) are characterised by high productivity. However, their use is limited to lab/pilot-scale, and a deeper level of characterisation is needed to reach industrial scale and test the resistance of multiple microalgae. Here, the performance and composition of eight microalgal communities cultivated in the two main TLs design (thin-layer cascade (TLC) and thin-layer raceway pond (RW)) were investigated through Illumina sequencing. Chlorella vulgaris showed robustness in both designs and often acted as an "invasive" species. Inoculum and reactor type brought variability. Eukaryotic microalgae inocula led to a more robust and stable community (higher similarity), however, RWs were characterised by a higher variability and did not favour the eukaryotic microalgae. The only cyanobacterial inoculum, Nostoc piscinale, was maintained, however the community was variable between designs. The reactor design had an effect on the N cycle with the TLC and RW configurations, enhancing nitrification and denitrification respectively.

Centrate as a sustainable growth medium: Impact on microalgal inocula and bacterial communities in tubular photobioreactor cultivation systems

Centrate is a low-cost alternative to synthetic fertilizers for microalgal cultivation, reducing environmental burdens and remediation costs. Adapted microalgae need to be selected and characterised to maximise biomass production and depuration efficiency. Here, the performance and composition of six microalgal communities cultivated both on synthetic media and centrate within semi-open tubular photobioreactors were investigated through Illumina sequencing. Biomass grown on centrate, exposed to a high concentration of ammonium, showed a higher quantity of nitrogen (5.6% dry weight) than the biomass grown on the synthetic media nitrate (3.9% dry weight). Eukaryotic inocula were replaced by other microalgae while cyanobacterial inocula were maintained. Communities were generally similar for the same inoculum between media, however, inoculation with cyanobacteria led to variability within the eukaryotic community. Where communities differed, centrate resulted in a higher richness and diversity. The higher nitrogen of centrate possibly led to higher abundance of genes coding for N metabolism enzymes.

Profiling microalgal cultures growing on municipal wastewater and fertilizer media in raceway photobioreactors

Microalgae cultivation is proposed as an effective system for pathogens reduction and wastewater depuration, however, a full characterisation of the risks is still needed. Two raceways were inoculated with Scenedesmus, one using wastewater and the other using a fertilizer medium. Microbial community and pathogen presence were explored by next generation sequencing (NGS), commercial qPCR array and plate counts. These methods proved to be complementary for a full characterization of community structure and potential risks. Media and sampling locations contributed to shape communities and pathogenic loads. The main pathogenic genera detected were Arcobacter and Elizabethkingia (mainly in wastewater) with an important presence of Aeromonas (all samples). A lower presence of pathogens was detected in fertilizer samples, while wastewater showed a reduction from inlet to outlet. Raceways showed potential as an effective biotreatment, with most of the retained pathogens released in the outlet and only a minor part settled in the biomass.

Employing newly developed plastic bubble wrap technique for biofuel production from diatoms cultivated in discarded plastic waste

Algal culturing in photobioreactors for biofuel and other value-added products is a challenge globally specifically due to expensive closed or open photobioreactors associated with the high cost, problems of water loss and contamination. Among the wide varieties of microalgae, diatoms have come out as potential source for crude oil in the form of Diafuel™ (biofuel from diatoms). However, culturing diatoms at large scale hypothesized as diatom solar panels for biofuel production is still facing a need for facile and economical production of value-added products. The aim of this work was to culture diatom (microalgae) in a closed system by sealing the reactor rim tightly with very cheap priced and used plastic bubble wrap material which is generally discarded in a lodging and transportation of goods. To optimize it, different plastic wraps discarded from a plastic industry were tested first for their permeability to gases and impermeability to water loss. It was found that among different varieties of plastic bubble wraps, low density polyethylene (LDPE) bubble wrap material which was used to seal glass containers as photobioreactors allowed harvest of maximum Diafuel™ (37%), lipid (35 μgmL^-1), highest cell count (1152 × 10² cells mL^-1), maximum CO₂ absorbance (0.084) with almost no water loss and nutrient uptake for 40 days of experiments. This was due to its permeability to gases and impermeability to water. To check usability of such LDPE bubble wrap on other microalgae it was therefore tested on the red-green microalgae Haematococcus pluvialis, which showed scope to be scaled up for astaxanthin production using discarded bubble wrap packing material. This study thus would open up a new way for decreasing plastic disposal and with reuse for sustainable development and application of diatom in biofuel production which could find applications in environmental and industrial sectors.

Applications of synthetic light-driven microbial consortia for biochemicals production

Synthetic microbial consortia provide a versatile and efficient platform for biochemicals production through the labor division. Especially, microbial communities composed of phototrophs and heterotrophs offer a promising alternative, as they can directly convert carbon dioxide (CO₂) into chemicals. Within this system, photoautotrophic microbes can convert CO₂ into organic carbon for microbial growth and metabolites synthesis by the heterotrophic partners. In return, heterotrophs can provide additional CO₂ to support the growth of photoautotrophic microbes. However, the unmatched growing conditions, low stability and production efficiency of synthetic microbial consortia hinder their further applications. Thus, design and construction of mutualistic and stable synthetic light-driven microbial consortia are urgently needed. In this review, the progress of synthetic light-driven microbial consortia for chemicals production was comprehensively summarized. In addition, space-efficient synthetic light-driven microbial consortia in hydrogel system were reviewed. Perspectives on orderly distribution of light-driven microbial consortia associated with 3D printing technology in biomanufacturing were also addressed.

Mass Cultivation of Microalgae: I. Experiences with Vertical Column Airlift Photobioreactors, Diatoms and CO2 Sequestration

From 2015 to 2021, we optimized mass cultivation of diatoms in our own developed vertical column airlift photobioreactors using natural and artificial light (LEDs). The project took place at the ferrosilicon producer Finnfjord AS in North Norway as a joint venture with UiT—The Arctic University of Norway. Small (0.1–6–14 m3) reactors were used for initial experiments and to produce inoculum cultures while upscaling experiments took place in a 300 m3 reactor. We here argue that species cultivated in reactors should be large since biovolume specific self-shadowing of light can be lower for large vs. small cells. The highest production, 1.28 cm3 L−1 biovolume (0.09–0.31 g DW day−1), was obtained with continuous culture at ca. 19% light utilization efficiency and 34% CO2 uptake. We cultivated 4–6 months without microbial contamination or biofouling, and this we argue was due to a natural antifouling (anti-biofilm) agent in the algae. In terms of protein quality all essential amino acids were present, and the composition and digestibility of the fatty acids were as required for feed ingredients. Lipid content was ca. 20% of ash-free DW with high EPA levels, and omega-3 and amino acid content increased when factory fume was added. The content of heavy metals in algae cultivated with fume was well within the accepted safety limits. Organic pollutants (e.g., dioxins and PCBs) were below the limits required by the European Union food safety regulations, and bioprospecting revealed several promising findings.

The structure and diversity of microalgae-microbial consortia isolated from various local organic wastes

Pure microalgae cultivation in organic wastes may be hampered by their low adaptation to extreme growth conditions and by the risk of microbial contamination. This work aimed to isolate self-adapted microalgae-microbial consortia able to survive in organic wastes characterized by extreme conditions, to be then proposed for technological application in removing carbon and nutrients from wastes' streams. To do so, sixteen organic wastes with different origins and consistency were sampled. Twelve microbial consortia were isolated from wastes and their eukaryotic and prokaryotic compositions were analyzed by next generation sequencing. Eight eukaryotic communities were dominated by Chlorophyta, led by Chlorella, able to survive in different wastes regardless of chemical-biological properties. Tetradesmus, the second most represented genus, grew preferentially in substrates with less stressing chemical-physical parameters. Chlorella and Tetradesmus were mostly isolated from cow slurry and derived wastes which proved to be the best local residual organic source.

Quorum quenching strategy for biofouling control in membrane photobioreactor

This study aims to reduce membrane fouling in membrane photobioreactor (MPBR) through the quorum quenching (QQ) strategy. For this purpose, the QQ beads (immobilized Rhodococcus sp. BH4) were added to the MPBR, and antifouling ability was evaluated in consideration of the changes in transmembrane pressure (TMP), extracellular polymeric substance (EPS), microbial community, and cake layer morphology on the membrane surface. The results showed that the TMP of control MPBR (MPBR-C) reached 818 mbar and 912 mbar on the operation hours of 35 and 170, while the TMP of experimental MPBR (MPBR-QQ) was only 448 mbar and 676 mbar, respectively. The QQ strategy effectively reduced the EPS content in MPBR. The microscopic observations indicated that the QQ diminished the cake layer formation and pore-blocking on the membrane surface. Comparisons of 16S and 18S gene communities revealed minor differences between bacterial and eukaryotic species in MPBRs at phylum and class levels.

Sustainable valorization of algae biomass via thermochemical processing route: An overview

Biofuels have become an attractive energy source because of the growing energy demand and environmental issues faced by fossil fuel consumption. Algal biomass, particularly microalgae, has excellent potential as feedstock to be converted to bio-oil, biochar, and combustible syngas via thermochemical conversion processes. Third-generation biofuels from microalgal feedstock are the promising option, followed by the first-generation and second-generation biofuels. This paper provides a review of the applications of thermochemical conversion techniques for biofuel production from algal biomass, comprising pyrolysis, gasification, liquefaction, and combustion processes. The progress in the thermochemical conversion of algal biomass is summarized, emphasizing the application of pyrolysis for its benefits over other processes. The review also encompasses the challenges and perspectives associated with the valorization of microalgae to biofuels ascertaining the potential opportunities and possibilities of extending the research into this area.

Using green alga Haematococcus pluvialis for astaxanthin and lipid co-production: Advances and outlook

Astaxanthin is one of the secondary carotenoids involved in mediating abiotic stress of microalgae. As an important antioxidant and nutraceutical compound, astaxanthin is widely applied in dietary supplements and cosmetic ingredients. However, most astaxanthin in the market is chemically synthesized, which are structurally heterogeneous and inefficient for biological uptake. Astaxanthin refinery from Haematococcus pluvialis is now a growing industrial sector. H. pluvialis can accumulate astaxanthin to ∼5% of dry weight. As productivity is a key metric to evaluate the production feasibility, understanding the biological mechanisms of astaxanthin accumulation is beneficial for further production optimization. In this review, the biosynthesis mechanism of astaxanthin and production strategies are summarized. The current research on enhancing astaxanthin accumulation and the potential joint-production of astaxanthin with lipids was also discussed. It is conceivable that with further improvement on the productivity of astaxanthin and by-products, the algal-derived astaxanthin would be more accessible to low-profit applications.

A multicriteria decision analysis for the evaluation of microalgal growth and harvesting

Biomass obtained from microalgae research studies gained momentum in recent years because of their extensive application potential in multiple industries such as high-value nutraceuticals, bioproducts, cosmetics, animal feed industries, and biofuels while being a sustainable and environmentally friendly option. Although they have high biomass yields and rapid growth rates there are some limitations and challenges that remain for large-scale commercialized cultivation and harvesting methods of microalgae. Since there are multiple pathways related to efficient cultivation and harvesting methods to be viable, this study adopted, TOPSIS (Technique for Order Preference by Similarity to Ideal Solution), a multicriteria decision-making tool, to find the most acceptable alternative by using excel spreadsheets to evaluate the information that is derived from literature and pilot-scale studies. As a result, tubular (helical) and plate (flat panel) photobioreactors (PBRs) for cultivation and chemical harvesting (with chitosan) and bio-flocculation for harvesting were deemed suitable, while plastic bag PBR and suspended air flotation were deemed unsuitable.