Red Microalga Outgrows Rivals in Internally Lit Photobioreactor, Yielding Pigments, Protein and Starch

A thermoacidophilic red microalga has delivered its best performance yet in a reactor that shines light from the inside. In a systematic comparison of cultivation platforms, the unicellular species Cyanidioschyzon merolae reached a final cell dry weight of 11.5 ± 0.6 grams per litre in a custom-designed internally illuminated photobioreactor, the highest value among the systems tested. The same platform achieved a space–time yield of 1.07 ± 0.06 grams of cell dry weight per litre per day, roughly 1.5-fold higher than a flat-panel photobioreactor and a multi-cultivator, a difference the authors report as statistically significant at p < 0.01 using Tukey’s HSD after a one-way ANOVA.

The study, published in Microbial Biotechnology, set out to close a gap in comparative data on how reactor design shapes both biomass formation and the biochemical profile of extremophilic microalgae. The researchers benchmarked a small-scale multi-cultivator against two larger, scalable configurations: the internally illuminated photobioreactor, in which immersed glass rods distribute light directly into the culture, and a flat-panel reactor characterised by a high surface-to-volume ratio. Reactor choice matters because it strongly influences biomass and pigment productivity as well as operational cost, making platform selection a critical step in process development.

Before scaling up, the team compared C. merolae with two reference organisms: the unicellular red microalga Galdieria javensis and the filamentous cyanobacterium Limnospira platensis, the current industrial source of the blue pigment C-phycocyanin. In multi-cultivator runs at 42 °C for the two red algae and 30 °C for L. platensis, with 5% CO2 and 400 μmol photons per square metre per second, C. merolae posted the strongest figures of the three strains. It grew at 0.034 ± 0.001 per hour with a doubling time of 20.4 ± 0.9 hours, nearly twice the rate of G. javensis, and reached a final optical density of 28.8 ± 1 and a final cell dry weight of 8.3 ± 0.3 grams per litre. Its overall space–time yield of 0.75 ± 0.03 grams per litre per day after 266 hours peaked at about 1.25 grams per litre per day around the 100-hour mark. G. javensis managed 0.51 ± 0.02 grams per litre per day over the same period, hampered by a slower growth rate of 0.022 ± 0.001 per hour and a lag phase of roughly 70 hours, while L. platensis, which entered stationary phase after about 167 hours, recorded the lowest final cell dry weight at 4.3 ± 0.17 grams per litre.

The authors caution that the different cultivation temperatures limit a direct comparison of the two red algae with L. platensis and preclude attributing the observed differences exclusively to strain-specific traits. To sharpen the analysis, the team turned to microfluidic growth chambers 1.72 micrometres in height, which track division at single-cell resolution. C. merolae proliferates by binary fission and was seen shifting to an irregular, fluid-like shape during division before rounding again on completion. G. javensis stayed spherical and reproduced by successive divisions, forming four to 32 daughter cells inside a mother cell. L. platensis, a multicellular filamentous organism with a spiral structure, did not fit the monolayer chambers; even after the multicellular structure was broken up, the inoculated single cells were not viable. Growth rates from the microfluidic experiments matched the reactor data well for the two red algae, and the authors state this is the first microfluidic growth characterisation of both species.

Condition screening in the multi-cultivator showed that growth was optimal under the standard 5% CO2 and 400 μE conditions. Lower light intensities reduced growth, higher intensities caused saturation, 1% CO2 consistently lowered growth, and 10% CO2 produced extended lag phases and oscillatory behaviour. Light–dark cycles of 14 hours light and 10 hours dark hurt biomass accumulation, with optical density at 720 nanometres falling during dark phases, whereas continuous illumination supported more stable and efficient growth.

In the reactor benchmark, the internally illuminated platform told a nuanced story. Its maximum growth rate of 0.023 ± 0.003 per hour was slightly below that of the flat-panel reactor at 0.03 ± 0.002 and the multi-cultivator at 0.034 ± 0.001, yet it delivered significantly higher final optical density, 40.2 ± 2.1, and final cell dry weight, 11.5 ± 0.6 grams per litre. Total biomass at the end of cultivation also favoured the new design: 18.4 ± 0.6 grams from the 1.6-litre working volume, against 3.5 ± 0.05 grams from the 0.45-litre flat panel and 0.7 ± 0.02 grams from the 0.08-litre multi-cultivator. The integrated light rods keep light distribution favourable even at larger volumes, which the authors identify as the basis for combining effective scale-up with consistent production. The growth rates measured across the study exceeded previously reported values of roughly 0.006 and 0.011 per hour but remained below the approximately 0.075 per hour achieved under further optimised medium conditions, suggesting that medium optimisation is a promising route to still better performance.

Pigment analysis added a second advantage. Because accurate quantification of the phycobiliproteins R-phycocyanin and R-allophycocyanin requires strain-specific absorbance maxima, the team adapted an anion-exchange chromatography method and confirmed a maximum of 624 nanometres for R-phycocyanin, matching an earlier report, while resolving a previously undocumented maximum of 652 nanometres for R-allophycocyanin. The R-phycocyanin concentration was highest in the internally illuminated reactor, but the content per unit biomass was similar across platforms, ranging from 0.07 to 0.09 grams per gram of cell dry weight, indicating that under these conditions R-phycocyanin production depends primarily on biomass concentration rather than the reactor itself. R-allophycocyanin followed the same pattern at consistently lower levels. Chlorophyll a and carotenoid concentrations were comparable between the two larger platforms, at approximately 55 milligrams per litre and 13 milligrams per litre respectively. Because of its higher biomass, however, the internally illuminated reactor produced about four times more R-phycocyanin than the flat panel and 26 times more than the multi-cultivator, about five times and 24 times more R-allophycocyanin, and roughly five-fold higher total yields of chlorophyll a and carotenoids.

Fatty acid profiling showed a composition dominated by palmitic acid at 35%, linoleic acid at 31% and oleic acid at 20%, with stearic acid near 5%, and the relative abundance of these major fatty acids was similar across all systems, pointing to only a minor influence of reactor configuration. Distinct differences emerged among minor components: heptadecanoic and pentadecanoic acids, each about 4% of total fatty acids, were detected exclusively in the internally illuminated reactor. Elemental analysis revealed carbon contents of 47.1% ± 0.2% for biomass from the internally illuminated reactor and 46.8% ± 0.5% from the flat panel, with the multi-cultivator slightly lower at 45.2% ± 0.6%, consistent with reports of up to 50% carbon in this species. Under identical conditions, G. javensis and L. platensis reached only 40.9% ± 0.1% and 30.0% ± 0.4%, which the authors interpret as significantly lower carbon fixation efficiency. Nitrogen contents were three-fold and two-fold higher in C. merolae than in G. javensis and L. platensis respectively, and its carbon-to-nitrogen ratios of 5.4, 6.8 and 4.5 across the three platforms were consistently below the roughly 13 seen for G. javensis, suggesting a larger carbohydrate share in the latter, likely tied to its thick, rigid cell wall.

Starch data reinforced the feedstock case. Biomass from the internally illuminated reactor contained 21.5% ± 1.7% starch by cell dry weight, or 215 ± 17 milligrams per gram, exceeding the flat panel at 14.8% ± 1.6% and the multi-cultivator at 7.8% ± 0.3%. The authors note that starch accumulation in this species can be pushed further, with TOR-pathway inactivation reported to induce up to a 10-fold increase, though such conditions typically reduce protein content and impair growth. Cell wall analysis complicated the picture of a simple architecture: the destarched, alcohol-insoluble residue contained only about 6% cell wall polysaccharides, but glycosidic linkage analysis revealed 1,4-linked glucopyranosyl residues as the dominant motif, indicating cellulose, alongside branched glucan structures and diverse mannopyranosyl and galactopyranosyl linkages suggestive of mannoproteins and a complex wall matrix. Monosaccharide profiles also shifted with the platform, with glucose dominating in the internally illuminated and flat-panel systems while the multi-cultivator biomass was dominated by galacturonic acid. Together, the authors conclude, the cultivation platform decisively shapes both biomass formation and metabolic profile, supporting the case for C. merolae as a versatile chassis whose lack of a rigid cell wall eases product recovery while its protein- and starch-rich biomass offers a renewable feedstock for heterotrophic processes.

Subject of Research: Comparative evaluation of Cyanidioschyzon merolae cultivation across photobioreactor systems linking reactor design to biomass productivity and biochemical composition

Article Title: Systematic Evaluation of Cyanidioschyzon merolae Across Photobioreactor Systems: Linking Reactor Design to Biomass Production and Biochemical Composition

Article References: Ernst, P., Vanselow, J., Denter, M., Li, W., Witting, L., Gätgens, J., Pauly, M., Kohlheyer, D., Urlacher, V. B., Feldbrügge, M., & Frunzke, J. (2026). Systematic Evaluation of Cyanidioschyzon merolae Across Photobioreactor Systems: Linking Reactor Design to Biomass Production and Biochemical Composition. Microbial Biotechnology, 19(11), Article e70468. https://doi.org/10.1111/1751-7915.70468

Image Credits: AI Generated

DOI: 10.1111/1751-7915.70468

Keywords: Cyanidioschyzon merolae, photobioreactor, R-phycocyanin, floridean starch, microfluidics, biomass productivity, red microalgae, carbon capture, cell wall polysaccharides, Galdieria javensis, Limnospira platensis, biotechnology