Nikolaus I. Stellner

Indoor microalgae production under artificial illumination is constrained by facility footprint and energy demand, making biomass output per area the central design objective. Single-layer thin-layer cascade (TLC) systems are geometrically limited to a surface-to-footprint ratio (SF) of at most 1.0 m² m⁻². This study presents a stacked thin-layer cascade (STLC) photobioreactor validated at two scales sharing the same six-channel architecture: a laboratory-scale unit (2 L) and a pilot-scale unit (250 L). Both units achieved an SF well above the single-layer ceiling (3.65 and 4.2 m² m⁻²), demonstrating that vertical stacking is an inherent structural solution to the geometric limitation of open thin-layer cascade systems, independent of scale. Both units operated reliably for 11 to 13 days across a 125-fold difference in working volume, converging to comparable transitional Reynolds numbers and confirming that the film hydrodynamics central to the STLC concept remain consistent by design across scale. Footprint-normalized productivity reached 3.41 g DW m⁻² d⁻¹ at laboratory scale and 6.11 g DW m⁻² d⁻¹ at pilot scale, with pilot-scale protein reaching 42.1% DW prior to nitrogen depletion. Growth kinetics differed between scales, reflecting light/dark cycling regime, initial biomass density, and retention tank geometry as distinct, tunable operational variables. Closing the residual productivity gap requires jointly increasing photon flux density and standing biomass density, since low-density operation causes a large fraction of incident light to pass through the culture unabsorbed regardless of irradiance.

Commercially mature products obtained by fractionation or extraction of phototrophic microalgal biomass remain concentrated in four categories: whole-cell Spirulina/Chlorella, C-phycocyanin, astaxanthin, and DHA-rich oils. Little diversification of these fractionated, mid-tier products has followed the decline in upstream costs. Whole-cell feed and live-culture markets, agricultural biostimulants, and fermentation-derived ingredients are commercially active but lie outside this phototrophic downstream-processing scope. Reported open-pond biomass production costs have fallen from ~US$10 kg⁻¹ in the 1990s to sub-US$1 kg⁻¹ nth-plant projections, yet no substantial product diversification has occurred. This review brings together three complementary lines of evidence: a bibliometric analysis of 1995–2025 publications showing that downstream fractionation, biorefinery, and integrated process design account for only 9.3% of food-core microalgal research; institutional surveys documenting the same four dominant categories across Europe, China, and global markets; and a meta-analysis of 53 whole-biomass cost rows from 16 techno-economic assessments. These sources indicate consistently that downstream processing is a necessary, though not sole, constraint on commercial diversification. A four-tier unit-operation roadmap is proposed-cell disruption at commodity energy cost, fractionation with functional ingredient preservation, decolorization and desalting at food-ingredient unit cost, and standardized transferable workflows-each linked to a quantitative threshold and to the product categories it would unlock. Closing the microalgal processing technology gap now depends less on demonstrating feasibility than on meeting these thresholds.

The production of microbial lipids through single-cell oil (SCO) technologies has gained increasing attention as a sustainable alternative source of lipids for industrial applications. This development is driven by the limitations of plant-based oils, particularly their competition with food production and demand for arable land. Cutaneotrichosporon oleaginosus has been recognized as one of the most promising oleaginous microorganisms for efficient SCO production. To improve sustainability and economic viability, it is vital to understand the underlying metabolic mechanism of SCO production as well as needs and limitations in bioprocess engineering for the efficient utilization of carbon sources derived from diverse agricultural and industrial side streams. This review focuses on recent studies exploring the potential of SCO production through C. oleaginosus in a bioprocess context through the application of low-cost agro-industrial by-products as alternative carbon sources aiming to supply lipid raw materials for various industrial applications. C. oleaginosus can grow on different agro-industrial waste-derived substrates, including lignocellulosic biomass hydrolysates, biodiesel production process side streams, chitin-based by-products, cheese whey permeates, fungal biomass hydrolysates and algal biomass hydrolysates. These substrates contain various carbon sources, such as glucose, galactose, mannose, xylose, lactose, N-acetyl-glucosamine and glycerol, facilitating efficient SCO production. Additionally, the specific composition of SCO sourced from C. oleaginosus, including the presence of functional compounds like squalene and prevalent long-chain unsaturated fatty acids in its fatty acid profile, make it an ideal option to be used as a raw material in cosmetics, biofuel and food products. This comprehensive overview aims to shed light on the potential of C. oleaginosus in leveraging carbon source alternatives for sustainable SCO production for multifaceted, industrial applications of SCO.

A growing global population and climate change challenge conventional agriculture and global food safety. Microalgae are an emerging sustainable nutrient food resource, increasingly regarded as an important component for the human diet. The chlorophyte Chlorella sp. is regulated and commercialized for food applications. Commercial Chlorella preparations are either produced by heterotrophic fermentation or photoautotrophic cultivation in bioreactors or outdoor ponds. Products can differ significantly with regard to taxonomy of the production strain, changed genetic properties due to strain development, cultivation conditions, and downstream biomass processing methods. Notably, heterotrophic strains may be adapted to generate little or no photosynthetic pigments, resulting in yellow or white variant. In this study, eight different commercial Chlorella products from different EU suppliers were analyzed and compared regarding identity of the production strain, macro-and micronutrient profiles, sensory properties, as well as microbial load. The production processes featured different cultivation methods, production strains, genetic variants (yellow and white) and downstream processing methods. Data indicates significant variations between the Chlorella products. White and yellow Chlorella products showed different protein and taste profiles compared to green, photosynthetically competent wild type strains. We confirmed relatively high and stable protein concentrations but could detect variations in sugar and specifically lipid and vitamin profiles depending on strain, cultivation and downstream processing methods. The microbial load varied strongly between closed and open cultivation systems but was compliant with legislative (EFSA) specifications for all samples. The comparative nutrient- and sensory data set presented in this study will aid in selecting algae products for the development of innovative foods, thereby accelerating adoption and differentiation of algae food products.

Single-cell oil (SCO) produced by oleaginous microorganisms is potentially a more land-efficient and sustainable alternative to vegetable oil. The cost of SCO production can be reduced by value-added co-products like squalene, a highly relevant compound for the food, cosmetic, and pharmaceutical industry. For the first time, squalene in the oleaginous yeast Cutaneotrichosporon oleaginosus was analyzed, reaching 172.95 ± 61.31 mg/100 g oil in a lab-scale bioreactor. Using the squalene monooxygenase inhibitor terbinafine, cellular squalene was significantly increased to 2169 ± 262 mg/100 g SCO, while the yeast remained highly oleaginous. Further, SCO from a 1000 L scale production was chemically refined. The squalene content in the deodorizer distillate (DD) was found to be higher than that in DD from typical vegetable oils. Overall, this study demonstrates squalene as a value-added compound in SCO from C. oleaginosus for application in food and cosmetics without the use of genetic modifications.

Cutaneotrichosporon oleaginosus is a highly efficient single cell oil producer, which in addition to hexoses and pentoses can metabolize organic acids. In this study, fed-batch cultivation with consumption-based acetic acid feeding was further developed to integrate the transformation of an industrial paper mill lignocellulosic hydrolysate (LCH) into yeast oil. Employing pentose-rich LCH as a carbon source instead of glucose significantly improved both biomass formation and lipid titer, reaching 55.73 ± 5.20 g/L and 42.1 ± 1.7 g/L (75.5% lipid per biomass), respectively. This hybrid approach of using acetic acid and LCH in one process was further optimized to increase the share of bioavailable carbon from LCH using a combination of consumption-based and continuous feeding. Finally, the techno-economic analysis revealed a 26% cost reduction when using LCH instead of commercial glucose. In summary, we developed a process leading to a holistic approach to valorizing a pentose-rich industrial waste by converting it into oleochemicals.