Novel photobioreactor design for the culture of Dunaliella tertiolecta - Impact of color in the growth of microalgae

Advancements of microalgal upstream technologies: Bioengineering and application aspects in the paradigm of circular bioeconomy

Sustainable energy transition has brought the attention towards microalgae utilization as potential feedstock due to its tremendous capabilities over its predecessors for generating more energy with reduced carbon footprint. However, the commercialization of microalgae feedstock remains debatable due to the various factors and considerations taken into scaling-up the conventional microalgal upstream processes. This review provides a state-of-the-art assessment over the recent developments of available and existing microalgal upstream cultivation systems catered for maximum biomass production. The key growth parameters and main cultivation modes necessary for optimized microalgal growth conditions along with the fundamental aspects were also reviewed and evaluated comprehensively. In addition, the advancements and strategies towards potential scale-up of the microalgal cultivation technologies were highlighted to provide insights for further development into the upstream processes aimed at sustainable circular bioeconomy.

Sunlight filtered via translucent-colored polyvinyl chloride sheets enhanced the light absorption capacity and growth of Arthrospira platensis cultivated in a pilot-scale raceway pond

In this research, the effects of filtered sunlight traveling through translucent-colored polyvinyl chloride (PVC) sheets on the photoconversion efficiency of Arthrospira platensis are investigated. Filtered sunlight improves the phycobilisome's capacity to completely absorb and transport it to intracellular photosystems. Findings indicated that filtered sunlight via orange-colored PVC sheet increased biomass dry weight by 21% (2.80 g/L), while under blue-colored PVC sheet decreased by 32% (1.49 g/L), when compared with translucent-colored (control) PVC sheet (2.19 g/L) after 120 h of culture. The meteorological conditions during the 1st week of cultivation reported higher light flux than the subsequent weeks. Furthermore, sunlight filtered through orange PVC sheet enhanced protein, allophycocyanin, phycocyanin, chlorophyll-a and carotenoids synthesis by 13%, 15%, 13%, 22%, and 27%, respectively. This practical and inexpensive solar radiation filtration system supports large-scale production of tailored bioactive compounds from microalgae with high growth rate.

Recent advances in CO2 fixation by microalgae and its potential contribution to carbon neutrality

Many countries and regions have set their schedules to achieve the carbon neutrality between 2030 and 2070. Microalgae are capable of efficiently fixing CO₂ and simultaneously producing biomass for multiple applications, which is considered one of the most promising pathways for carbon capture and utilization. This work reviews the current research on microalgae CO₂ fixation technologies and the challenges faced by the related industries and government agencies. The technoeconomic analysis indicates that cultivation is the major cost factor. Use of waste resources such as wastewater and flue gas can significantly reduce the costs and carbon footprints. The life cycle assessment has identified fossil-based electricity use as the major contributor to the global warming potential of microalgae-based CO₂ fixation approach. Substantial efforts and investments are needed to identify and bridge the gaps among the microalgae strain development, cultivation conditions and systems, and use of renewable resources and energy.

Bioenergy, Biofuels, Lipids and Pigments—Research Trends in the Use of Microalgae Grown in Photobioreactors

This scientometric review and bibliometric analysis aimed to characterize trends in scientific research related to algae, photobioreactors and astaxanthin. Scientific articles published between 1995 and 2020 in the Web of Science and Scopus bibliographic databases were analyzed. The article presents the number of scientific articles in particular years and according to the publication type (e.g., articles, reviews and books). The most productive authors were selected in terms of the number of publications, the number of citations, the impact factor, affiliated research units and individual countries. Based on the number of keyword occurrences and a content analysis of 367 publications, seven leading areas of scientific interest (clusters) were identified: (1) techno-economic profitability of biofuels, bioenergy and pigment production in microalgae biorefineries, (2) the impact of the construction of photobioreactors and process parameters on the efficiency of microalgae cultivation, (3) strategies for increasing the amount of obtained lipids and obtaining biodiesel in Chlorella microalgae cultivation, (4) the production of astaxanthin on an industrial scale using Haematococcus microalgae, (5) the productivity of biomass and the use of alternative carbon sources in microalgae culture, (6) the effect of light and carbon dioxide conversion on biomass yield and (7) heterotrophy. Analysis revealed that topics closely related to bioenergy production and biofuels played a dominant role in scientific research. This publication indicates the directions and topics for future scientific research that should be carried out to successfully implement economically viable technology based on microalgae on an industrial scale.

Engineering microbial metabolic energy homeostasis for improved bioproduction

Metabolic energy (ME) homeostasis is essential for the survival and proper functioning of microbial cell factories. However, it is often disrupted during bioproduction because of inefficient ME supply and excessive ME consumption. In this review, we propose strategies, including reinforcement of the capacity of ME-harvesting systems in autotrophic microorganisms; enhancement of the efficiency of ME-supplying pathways in heterotrophic microorganisms; and reduction of unessential ME consumption by microbial cells, to address these issues. This review highlights the potential of biotechnology in the engineering of microbial ME homeostasis and provides guidance for the higher efficient bioproduction of microbial cell factories.

Microalgal Co-Cultivation Prospecting to Modulate Vitamin and Bioactive Compounds Production

Microalgal biotechnology is gaining importance. However, key issues in the pipeline from species selection towards large biomass production still require improvements to maximize the yield and lower the microalgal production costs. This study explores a co-cultivation strategy to improve the bioactive compounds richness of the harvested microalgal biomass. Based on their biotechnological potential, two diatoms (Skeletonema marinoi, Cyclotella cryptica) and one eustigmatophyte (Nannochloropsis oceanica) were grown alone or in combination. Concentrations of ten vitamins (A, B₁, B₂, B₆, B₁₂, C, D₂, D₃, E and H), carotenoids and polyphenols, together with total flavonoids, sterols, lipids, proteins and carbohydrates, were compared. Moreover, antioxidant capacity and chemopreventive potential in terms inhibiting four human tumor-derived and normal cell lines proliferation were evaluated. Co-cultivation can engender biomass with emergent properties regarding bioactivity or bioactive chemical profile, depending on the combined species. The high vitamin content of C. cryptica or N. oceanica further enhanced (until 10% more) when co-cultivated, explaining the two-fold increase of the antioxidant capacity of the combined C. cryptica and N. oceanica biomass. Differently, the chemopreventive activity was valuably enhanced when coupling the two diatoms C. cryptica and S. marinoi. The results obtained in this pilot study promote microalgal co-cultivation as a valuable strategy aiming to boost their application in eco-sustainable biotechnology.

Using Phycocyanin as Spectral Converter on the Growth Parameters and Lipid Content of the Green Microalga Chlorella sp. in a Double Layer Flat Panel Photobioreactor

The focus of this research is spectral shifting of light using phycocyanin solution extracted from the blue-green microalga Spirulina platensis in order to increase biomass productivity of the green microalga Chlorella sp. Also, lipid and chlorophyll content of the green alga was investigated. With regard to the shift of the spectrum with the phycocyanin solution, a double layer flat panel photobioreactor and two different spectral shifting strategies were used. In each strategy, the effect of two different concentrations of the solution was investigated. In the first strategy, the light passes through the chamber containing the solution and then enters the microalga culture chamber. In the second strategy, the light first enters the culture chamber and then enters the chamber containing phycocyanin pigment. The results showed that the use of phycocyanin pigment by both strategies increased the biomass productivity (P) and the specific growth rate (μ_max) with a significant difference compared to the control system; the increase in P for first strategy was up to about 69%. Moreover, the use of phycocyanin solution with a lower concentration had a greater effect on the increase of total chlorophyll content; however, the solution with a higher concentration was more successful in the production of cell lipid content. Using the phycocyanin solution as spectral converter in a double layer flat panel photobioreactor increased the biomass productivity and chlorophyll content.

Orange light spectra filtered through transparent colored polyvinyl chloride sheet enhanced pigment content and growth of Arthrospira cells

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 Cultivation Technologies as an Opportunity for Bioenergetic System Development—Advantages and Limitations

Microalgal biomass is currently considered as a sustainable and renewable feedstock for biofuel production (biohydrogen, biomethane, biodiesel) characterized by lower emissions of hazardous air pollutants than fossil fuels. Photobioreactors for microalgae growth can be exploited using many industrial and domestic wastes. It allows locating the commercial microalgal systems in areas that cannot be employed for agricultural purposes, i.e., near heating or wastewater treatment plants and other industrial facilities producing carbon dioxide and organic and nutrient compounds. Despite their high potential, the large-scale algal biomass production technologies are not popular because the systems for biomass production, separation, drainage, and conversion into energy carriers are difficult to explicitly assess and balance, considering the ecological and economical concerns. Most of the studies presented in the literature have been carried out on a small, laboratory scale. This significantly limits the possibility of obtaining reliable data for a comprehensive assessment of the efficiency of such solutions. Therefore, there is a need to verify the results in pilot-scale and the full technical-scale studies. This study summarizes the strengths and weaknesses of microalgal biomass production technologies for bioenergetic applications.