You are not an individual. You are a committee.
Every cell in your body contains the descendants of bacteria that were swallowed and never digested. The mitochondria—the organelles that generate your ATP, regulate your apoptosis, and maintain your cellular homeostasis—were once free-living α-proteobacteria that an ancestral eukaryotic cell engulfed approximately two billion years ago (Origin and Evolution of Organelle Genomes, 2004). The chloroplasts in every plant cell are the descendants of cyanobacteria that a eukaryotic lineage acquired roughly 500 million years later (Symbiosis and Evolution, 2022). These are not metaphors. They are mergers. And they are the most successful mergers in the history of life.
The mitochondrion and the chloroplast are the original engineered symbiotes—natural experiments in integrating foreign genomes into a host organism. They retain their own DNA, their own ribosomes, their own membranes. They are, in the most literal sense, guests that became organs. And the question that follows is the one this entire series has been building toward: if nature can do this, what can we do? The answer is already visible in the literature, and it is stranger than most people expect.
Tier 1: What Is Already Working in Mammalian Cells
Photosynthetic Neo-Organelles. The human eye is exposed to visible light every waking moment. Plants use that same light to generate metabolites. A 2026 paper in Cell asked the obvious question: can mammalian cells be given photosynthetic functionality? The answer is yes—with the right delivery system.
The researchers extracted thylakoid grana—the stacks of discs inside chloroplasts that are the site of the light-dependent reactions—from spinach leaves and engineered them into a nanoscale system called LEAF (light-reaction enriched thylakoid NADPH-foundry). The key innovation was removing the chloroplast components that consume NADPH while retaining the thylakoid grana and their photoreaction function (Transplanting Light-Dependent Reactions, 2026). When LEAF was delivered to mammalian corneal cells, it enabled light-driven production of NADPH and ATP—the same photosynthetic output that plant leaves generate (Borrowing Sunlight, 2026). Intracellularly, LEAF integrates with host cells to supply NADPH and ATP via intact photosynthetic electron transport, restoring redox balance. Extracellularly, photosynthesized NADPH enhances endogenous antioxidant enzyme activity and reduces reactive oxygen species in the local environment (Transplanting Light-Dependent Reactions, 2026).
The paper’s own framing is precise: this establishes “a possible cross-kingdom, endosymbiosis-like interaction in which animal cells derive functional benefits from plant-derived photosynthetic neo-organelles” (Transplanting Light-Dependent Reactions, 2026). The word “neo-organelle” is doing real work here. LEAF is not a drug. It is not a gene therapy. It is an organelle transplant—a temporary symbiote that the host cell tolerates and benefits from.
The LEAF work has already been translated to human dry eye patients, where it increased NADPH levels in tear samples (Borrowing Sunlight, 2026). It is not permanent, and it is not inherited. But it is the first demonstration that mammalian cells can run photosynthesis.
Algal Chloroplasts in Mammalian Cells. A parallel line of work has taken a different approach: instead of extracting thylakoid grana, researchers have incorporated intact, photosynthetically active chloroplasts from the primitive red alga Cyanidioschyzon merolae directly into cultured mammalian cells via co-cultivation (Incorporation of Photosynthetically Active Algal Chloroplasts, 2024). The incorporated chloroplasts retained their thylakoid structure inside intracellular vesicles, were maintained in the cytoplasm surrounded by mitochondria near the nucleus, and maintained electron transport activity of photosystem II for at least two days after incorporation (Incorporation of Photosynthetically Active Algal Chloroplasts, 2024). The authors frame this as a “top-down synthetic biology-based approach” that “may serve as a foundation for creating artificially photosynthetic animal cells” (Incorporation of Photosynthetically Active Algal Chloroplasts, 2024).
Two days is not a lifetime. But two days is proof of principle. The chloroplast did not rupture. The host cell did not reject it. The electron transport chain kept running. The machinery of endosymbiosis works in mammalian cells.
Mitochondrial Transplantation. If chloroplasts can be transplanted into animal cells, what about mitochondria? Mitochondrial transfer and transplantation (MTT) is an emerging therapeutic strategy for diseases associated with mitochondrial dysfunction—neurodegeneration, cardiovascular disorders, ischemia-reperfusion injury. Preclinical studies suggest that exogenous mitochondria can successfully integrate into recipient cells, leading to enhanced ATP production, restoration of redox balance, and improved cellular survival under stress conditions (Biotechnological Approaches and Therapeutic Potential, 2025).
The challenges are real. Naked mitochondria achieve less than 5% transplantation efficiency, and the immune system recognizes them as foreign entities (Organelle Therapy, 2026; Biotechnological Approaches and Therapeutic Potential, 2025). But the solutions are being developed. Researchers have encapsulated isolated mitochondria within vesicles derived from erythrocyte plasma membranes—”mitochondrial capsules”—to enhance delivery and biocompatibility (Organelle Therapy, 2026). Others have developed storable mitochondria that can be delivered to human fibroblasts and vascular endothelial cells, where they are rapidly internalized as independent puncta without fusing with the endogenous mitochondrial network (Bioenergetic Benefits and Therapeutic Potential, 2026).
The clinical implication is already here: mitochondrial replacement therapy (MRT) has been licensed in the UK since 2015 and in Australia since 2022 for the explicit purpose of reducing the risk of transmitting severe mitochondrial disease (Mitochondrial Replacement Techniques, 2025). Eight babies have been born in the UK after MRT treatment (Eight Babies Born After Mitochondrial Donation, 2025). This is not enhancement. It is therapy. But the boundary is regulatory, not biological.
Tier 2: Engineered Microbial Symbionts
The human body already hosts trillions of microbial symbiotes—the gut microbiome is a genomic reservoir that influences physiology, cognition, stress responses, and physical performance (The Gut Microbiome is a Malleable Genomic Reservoir, n.d.). The question is whether those symbiotes can be engineered to do things they were not designed to do.
Engineered Probiotics for Performance. Researchers have engineered Escherichia coli Nissle 1917—a well-characterized human probiotic—to produce guanidinoacetic acid (GAA), which is converted to creatine in the liver. The goal is to mitigate the cognitive effects of sleep deprivation. The work demonstrates that a probiotic bacterium can be engineered to produce sustained GAA titers known to impact cognitive performance (Design of a Continuous GAA-Producing Probiotic, n.d.). This is not a drug. It is a living organism that colonizes the gut and continuously produces a metabolite that crosses the blood-brain barrier.
The military implications are not lost on the people funding this research. Multiple efforts are underway to develop engineered probiotics for enhancing resiliency to fatigue and improving cognitive performance in sleep-deprived warfighters (The Gut Microbiome is a Malleable Genomic Reservoir, n.d.). The delivery mechanism is the gut. The target is the brain. The organism is a symbiote.
Clostridia as a Therapeutic Chassis. The MiStiC project, funded by the European Union’s Horizon programme, is pioneering the use of Clostridia—naturally abundant and stable human gut symbionts—as a next-generation therapeutic chassis. Unlike E. coli and Lactococcus lactis, which do not stably colonize the human gut, Clostridia are permanent residents. The project aims to engineer Clostridium strains that can detect disease-associated biomarkers, respond by producing natural product-based anticancer agents, and self-eliminate when treatment is complete (Microbial Synthetic In Vivo Cell Therapy Systems, 2026). The framing is explicit: “using engineered beneficial bacteria as ‘living medicines’” (Microbial Synthetic In Vivo Cell Therapy Systems, 2026).
The same logic applies to enhancement. A bacterium that can detect a biomarker and release a therapeutic molecule can be reprogrammed to detect a physiological state and release a performance-enhancing metabolite. The chassis is the same. The payload changes. The regulatory boundary is the only thing standing between a “living medicine” and a “living enhancement.”
Artificial Microbial Consortia. Single engineered strains are limited by metabolic burden and genetic instability. The next step is engineered artificial microbial consortia—precisely designed communities of microorganisms that divide labor across multiple strains (Engineering Artificial Microbial Consortia, 2025). A consortium can perform functions that no single organism can perform alone: one strain senses, another produces, a third regulates. The consortium is the symbiote. The human is the host ecosystem.
Tier 3: Speculative—But Not Crazy
Permanent Photosynthetic Symbiotes. LEAF is temporary. The thylakoid grana degrade. The NADPH production stops. The next step is permanence—a chloroplast or thylakoid system that stably integrates into the host cell’s genome and replicates with it. This would require the transfer of chloroplast genes to the nuclear genome, a process that occurred naturally over hundreds of millions of years of endosymbiosis (Origin and Evolution of Organelle Genomes, 2004). Doing it artificially would require solving the intron-splicing incompatibility between plant and animal systems, which remains an obstacle that “hinders the advancement of this concept” (Cross-Species Implantable ATP Battery, 2026). The mechanism is understood. The engineering is not done.
Engineered Mitochondria with Enhanced Genomes. Mitochondrial transplantation currently delivers wild-type mitochondria. The next step is engineered mitochondria—organelles whose genomes have been edited to enhance ATP production efficiency, reduce reactive oxygen species emission, or increase resistance to stress. The challenges are formidable: mitochondrial genomes are difficult to edit, the organelle is recognized by the immune system, and the integration of exogenous mitochondria into the host’s endogenous network is poorly understood (Biotechnological Approaches and Therapeutic Potential, 2025). But the conceptual framework is established. The mitochondria is a symbiote. Symbiotes can be redesigned.
Synthetic Endosymbionts. The most radical version of this idea is the creation of synthetic endosymbionts—engineered organisms that live inside human cells and perform functions that no natural organelle performs. A synthetic bacterium that produces a non-native metabolite. A synthetic archaeon that repairs DNA damage. A synthetic alga that generates ATP from light and exports it to the host. The “biocyborg” framework describes this as “artificial chimerism”—a “complete fusion of human and technology” through synthetic biology (The Rise of the Biocyborg, n.d.). The symbiote is not a tool. It is a component.
Tier 4: Honestly Crazy, But Theoretically Possible
Chloroplast Genomes in Human Cells. The ultimate version of photosynthetic enhancement is not thylakoid transplantation. It is the introduction of the complete chloroplast genome into human cells through genetic engineering, creating a stable, self-replicating photosynthetic organelle. This would make human tissues partially energy-independent from oxygen, a property with obvious applications for spaceflight, wound healing, ischemia, and the metabolic decline of aging. The intron-splicing incompatibility is the current bottleneck (Cross-Species Implantable ATP Battery, 2026). The concept is not absurd. It is just very far away.
Inheritable Symbiotes. If a symbiote is engineered into the germline—if the chloroplast or the synthetic bacterium is present in every cell from conception—then the symbiote becomes inheritable. The enhancement is no longer a therapy. It is a lineage. This is the endosymbiotic equivalent of germline editing, with the same ethical architecture and the same prohibitions. No one has done it. No one has proposed a protocol. But the logic of the technology points in that direction.
The Uncomfortable Conclusion
The mitochondrion and the chloroplast are proof that symbiogenesis works. They are the most successful mergers in the history of life, and they were not designed. They were accidents that stuck. The next generation of symbiotes will not be accidents. They will be engineered—organelles transplanted, genomes edited, consortia assembled, chassis reprogrammed. The distinction between “human” and “symbiote” will become as meaningless as the distinction between “eukaryote” and “bacterium” became two billion years ago.
The enhancement literature has focused on editing the human genome. But the human genome is not the only genome in the body. It is not even the majority. The microbiome contains 100 times more genes than the human genome, and the mitochondria and chloroplasts carry their own. The next frontier of human enhancement is not the editing of our own DNA. It is the engineering of our guests.
References
Bioenergetic benefits and therapeutic potential of storable mitochondria. (2026). ScienceDirect.
Biotechnological approaches and therapeutic potential of mitochondria transfer and transplantation. (2025). Nature Communications, 16, 5709.
Borrowing sunlight: Photosynthetic bioengineering as an emerging therapeutic paradigm. (2026). Wiley Online Library.
Cross-Species Implantable ATP Battery Inspired by Photosynthesis for Application in Diseases. (2026). International Journal of Nanomedicine.
Design of a Continuous GAA-Producing Probiotic as a Potential Mitigator of the Effects of Sleep Deprivation. (n.d.). ACS Publications.
Eight babies born after Mitochondrial Donation treatment to reduce transmission of mitochondrial DNA disease. (2025, July 16). EurekAlert.
Engineering artificial microbial consortia for personalized gut microbiome modulation and disease treatment. (2025). Annals of the New York Academy of Sciences.
Incorporation of photosynthetically active algal chloroplasts in cultured mammalian cells towards photosynthesis in animals. (2024). Proceedings of the Japan Academy, Series B, 100(9), 524–536.
Microbial Synthetic in vivo Cell Therapy Systems (MiStiC). (2026). CORDIS, European Commission.
Mitochondrial replacement techniques: The perspectives of mitochondrial disease specialists in Germany. (2025). ScienceDirect.
Organelle Therapy: Transplantation of encapsulated mitochondria for mitochondrial and degenerative diseases. (2026). ScienceDirect.
Origin and evolution of organelle genomes. (2004). ScienceDirect.
Symbiosis and Evolution: At the Origin of the Eukaryotic Cell. (2022). HAL Sorbonne Université.
The gut microbiome is a malleable genomic reservoir influencing physiology. (n.d.). DTIC.
The rise of the biocyborg: synthetic biology, artificial chimerism and human enhancement. (n.d.). Taylor & Francis.
Transplanting light-dependent reactions for mammalian eye photosynthesis. (2026). Cell.