CRISPR-Edited Microalgae Boost Carotenoids but Hit Hard Limits on Carbon Capture

Microalgae have long been touted as a dual-purpose biological platform: organisms that can pull carbon dioxide out of the air through photosynthesis while simultaneously accumulating high-value pigments such as lutein and astaxanthin. A new review published in Bioresources and Bioprocessing takes a hard look at whether CRISPR/Cas genome engineering can push this platform to industrial maturity, and its conclusion is deliberately sobering. Targeted genetic interventions can significantly enhance specific metabolic pathways, the authors find, but these improvements are often not proportionally reflected in whole-cell carbon fixation, revealing constraints imposed by energy allocation, redox balance, and regulatory network coupling.

The review, authored by researchers including Nalinakshan Sreevidya Shruthy, Cheng-Di Dong, and Anil Kumar Patel, frames the core problem as a trade-off. CO2 fixation and carotenoid biosynthesis are connected through intracellular carbon partitioning, so strategies that enhance pigment accumulation may alter carbon allocation without proportionally increasing net CO2 fixation. In plain terms, what improves is usually the amount of a specific product, not the total amount of carbon being fixed by the cells. The authors argue that carotenoid enhancement often reflects carbon redistribution rather than new fixation, a distinction with major consequences for anyone hoping to market engineered algae as a climate mitigation technology.

The technical toolkit itself has expanded considerably. Beyond conventional Cas9 cutting, CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) use catalytically inactive dCas9 as a programmable DNA-binding scaffold for reversible gene regulation without permanent DNA breaks. In CRISPRi, dCas9 alone or fused to repressor domains such as KRAB blocks transcription; in CRISPRa, activator fusions like VP64 recruit transcriptional machinery to boost expression. Multiplexed systems employing multiple guide RNAs, ribozyme-based processing, or Cas12a arrays allow simultaneous editing or regulation of several genomic loci, enabling coordinated pathway rewiring that single-target editing cannot achieve.

The reported experimental results are striking on the product side. In Chlamydomonas reinhardtii, targeted disruption of the ZEP gene yielded dramatic pigmentation shifts, with zeaxanthin levels increasing 56-fold and productivity rising 47-fold relative to wild type. Joint knockdown of LCYE and ZEP redirected carotenoid flux toward the beta-carotene/zeaxanthin branch, producing roughly 60% higher zeaxanthin content than the ZEP single knockout after three days of cultivation. Under nitrogen constraints, simultaneous CRISPR-Cas9 deletion of ZEP and AGP increased oil and macular pigment formation by over 80%. In Phaeodactylum tricornutum, removal of the blue-light-responsive CryP gene significantly increased fucoxanthin accumulation and fucoxanthin chlorophyll a/c-binding proteins, clarifying CryP’s regulatory role in light-responsive pigment metabolism.

Carbon-side engineering shows more modest, condition-dependent gains. CRISPRi-mediated repression of the carbon partitioning gene PEPC1 in C. reinhardtii increased CO2 assimilation efficiency from 0.536 to 1.087 g-CO2 per g dry cell weight under mixotrophic conditions, alongside enhanced biomass, lipid, and lutein production. CRISPR activation of the g1248 gene in Nannochloropsis oceanica, using a dCas9-VP64 platform that upregulated the target roughly 2- to 6-fold, produced a 23% increase in growth and a 12% rise in the Fv/Fm photosynthetic efficiency parameter. Truncated light-harvesting antenna mutants of Chlorella sorokiniana displayed a 30-68% increase in biomass yield within dense cultures representative of industrial photobioreactor conditions.

But the review also documents failures that underscore how carefully targets must be chosen. CRISPR/Cas9 knockout of the Cia5 gene, a master regulator of the carbon concentrating mechanism, significantly reduced CO2 sequestration in C. reinhardtii: removal efficiency dropped from 27%, 37%, and 21% in the wild type to 16%, 23%, and 9% in the knockout strain under 0.04%, 0.5%, and 1% CO2, respectively. The lesson is that suppressing negative or limiting regulators can improve CO2 uptake in some contexts, while disrupting essential CCM regulators cripples it.

Underlying all of this are fundamental biochemical constraints that no amount of gene expression can bypass. RuBisCO, the enzyme that fixes CO2, has low catalytic turnover and significant oxygenase activity, so increasing its abundance cannot overcome intrinsic kinetic limitations. The carbon concentrating mechanism that raises CO2 levels around RuBisCO carries an energetic cost: modeling studies of pyrenoid-based CCMs estimate an additional requirement of approximately 1.3 ATP per CO2 fixed, with CO2 or bicarbonate leakage pushing demand higher. Strengthening CCM activity may therefore divert energy from growth and biomass production. Photosynthetic efficiency itself remains bounded; while microalgae show estimated solar conversion potentials of 8-10% with theoretical biomass productivity up to 77 grams per square meter per day, actual efficiency under realistic cultivation is generally closer to 4.5%.

Delivery remains a stubborn bottleneck. Electroporation works well for C. reinhardtii but is limited by stiff cell walls in diatoms and Nannochloropsis. Ribonucleoprotein delivery of preassembled Cas9-guide RNA complexes enables transgene-free editing, achieving up to 90.1% mutation efficiency in Euglena gracilis and, via biolistic delivery, up to 100% editing for specific multiplex knockout combinations in P. tricornutum, though efficiencies were initially lower in C. reinhardtii. Homology-directed repair, needed for precise knock-ins, is generally below 1% across most microalgae, though it can reach roughly 5-10% in C. reinhardtii with NHEJ suppression and cell cycle synchronization. Haematococcus pluvialis, the commercially important astaxanthin producer, is particularly recalcitrant: peer-reviewed evidence for CRISPR/Cas editing in this species remains limited, with a recent gene-gun RNP delivery described only in a patent lacking independent validation.

Scale-up adds another layer of difficulty. Engineered strains that perform well in flasks often show reduced stability in large photobioreactors, where light penetration, self-shading, oxygen accumulation, gas exchange, and temperature, nutrient, and pH gradients all degrade performance. Reduced antenna size can improve light distribution but may increase photoinhibition sensitivity under fluctuating light. Most studies remain limited to shaking flasks and small photobioreactors, with a notable lack of pilot or industrial-scale validation, meaning current yield values should be treated as ideal upper limits. Regulatory hurdles compound the problem: in the European Union, even strains with only endogenous genes altered are categorized as GMOs.

The review’s overall verdict is that CRISPR-enabled microalgal engineering offers significant potential for sustainable carbon management and high-value bioproduct generation, but the two goals are not equivalent. These strategies mainly improve how carbon is distributed and utilized within the cell rather than overcoming the fundamental biological limitations of photosynthesis itself. Quantitative evaluations of CO2 fixation rates, carbon conversion efficiency, and long-term sequestration performance remain scarce, and direct comparisons between studies are difficult due to differing cultivation conditions and metrics. Future progress, the authors argue, will depend on combining CRISPR engineering with systems biology, quantitative metabolic analysis, multi-omics target selection, machine-learning-guided strain design, and synthetic regulatory circuits capable of dynamic flux control, alongside bioprocess optimization that addresses both biological and reactor-scale limitations.

Subject of Research: CRISPR engineering of microalgae for CO2 sequestration and carotenoid production

Article Title: CRISPR-enhanced microalgae for carbon sequestration and carotenoid production: opportunities and fundamental constraints

Article References: Shruthy, N. S., Dong, C.-D., Chen, Y.-T., Hsieh, S.-L., Singhania, R. R., & Patel, A. K. (2026). CRISPR-enhanced microalgae for carbon sequestration and carotenoid production: opportunities and fundamental constraints. Bioresources and Bioprocessing, 13(1), Article 147. https://doi.org/10.1186/s40643-026-01136-y

Image Credits: AI Generated

DOI: 10.1186/s40643-026-01136-y

Keywords: microalgae, CRISPR, carbon sequestration, carotenoids, astaxanthin, photosynthesis, RuBisCO, carbon concentrating mechanism, CRISPRi, CRISPRa, biotechnology, photobioreactors