Mixotrophs are organisms that combine two fundamentally different ways of feeding: they photosynthesize like plants and also consume other organisms like animals. Far from being biological oddities, they are among the most abundant and ecologically influential life forms in aquatic environments, shaping food webs, driving carbon cycling, and responding to environmental change in ways that purely photosynthetic or purely predatory organisms cannot. The dual lifestyle gives them a flexibility that researchers are only now beginning to appreciate, and incorporating mixotrophs into ecological models is changing how scientists understand everything from ocean productivity to the future of nutrient-poor waters under climate change.
Two Ways to Eat, One Organism
For a long time, biology drew a clean line between producers (organisms that make their own food from sunlight) and consumers (organisms that eat other things). Mixotrophs blur that line entirely. A single-celled mixotrophic protist might spend the morning photosynthesizing and the afternoon engulfing bacteria. The balance between these two modes shifts depending on conditions like light availability, nutrient concentrations, and prey abundance. This is not a fixed compromise but a dynamic, responsive strategy.
A widely used framework groups planktonic protists into four functional categories: organisms that only eat other things, organisms that only photosynthesize, constitutive mixotrophs that have their own built-in photosynthetic machinery while also eating prey, and non-constitutive mixotrophs that acquire their photosynthetic ability from the organisms they consume.1PubMed. Defining Planktonic Protist Functional Groups on Mechanisms for Energy and Nutrient Acquisition: Incorporation of Diverse Mixotrophic Strategies That last category is the strange one. Some of these non-constitutive mixotrophs are generalists, grabbing photosynthetic components from whatever prey happens to be available, while others are specialists that rely on particular prey species for their stolen light-harvesting equipment.
Constitutive Mixotrophs and the Art of the Tradeoff
Constitutive mixotrophs carry their own chloroplasts and can photosynthesize without help, but they also engulf bacteria and other small particles. Think of them as organisms that have kept the full photosynthetic toolkit of their ancestors while also retaining (or re-evolving) the ability to eat. Many common marine nanoflagellates fall into this group.
The tradeoff between photosynthesis and feeding is not uniform across species. Experiments on Antarctic mixotrophic nanoflagellates showed that different species responded to changes in light and nutrient levels in species-specific ways: some ramped up feeding when light dropped, while others ate more when nutrients ran low, and the pattern did not follow evolutionary lineage.2PubMed. Physiological responses of three species of Antarctic mixotrophic phytoflagellates to changes in light and dissolved nutrients In other words, being closely related does not predict how a mixotroph balances its two nutritional modes. Each species has its own playbook, shaped by the specific environmental pressures it faces.
This variability matters because it means you cannot model all mixotrophs as one thing. A blanket assumption that “mixotrophs eat more when it gets dark” might hold for one species and fail for its neighbor in the same water column.
Stolen Sunlight and Kleptoplasty
Non-constitutive mixotrophs do something conceptually wilder: they steal the ability to photosynthesize. The process called kleptoplasty involves an organism eating a photosynthetic cell and then keeping the chloroplasts alive and functional inside its own body, discarding the rest. This is relatively common among protists.3PubMed Central. Kleptoplasty: Getting away with stolen chloroplasts
Among animals, sea slugs were long considered the only known practitioners of kleptoplasty. That changed when researchers discovered that certain marine flatworms also steal functional plastids, in their case from diatoms. Ultrastructural analysis confirmed that the stolen chloroplasts sat inside the flatworm’s own cells, with no algal nuclei or other organelles present, just the isolated photosynthetic machinery.4PubMed Central. A new case of kleptoplasty in animals: Marine flatworms steal functional plastids from diatoms
The catch with kleptoplasty is that the stolen chloroplasts have an expiration date. Generalist non-constitutive mixotrophs that grab chloroplasts from whatever prey is available have no regulatory control over those acquired photosystems. When light is limiting, they cannot adjust pigment levels the way a true photosynthesizer would. As the organism divides, the stolen chloroplasts get diluted among daughter cells and eventually fail.5PubMed Central. Cutting the canopy to defeat the “selfish gene”; conflicting selection pressures for the integration of phototrophy in mixotrophic protists To keep photosynthesizing, the organism needs to keep eating photosynthetic prey. It is a treadmill of theft.
Modeling work suggests that kleptoplasty can boost an organism’s division rate by roughly 40%, which is enough to significantly accelerate population growth and even promote algal blooms.6PubMed Central. Modeling Unveils How Kleptoplastidy Affects Mixotrophy Boosting Algal Blooms That connection between stolen chloroplasts and blooms is something harmful algal bloom researchers are increasingly paying attention to.
Symbiotic Mixotrophy in the Open Ocean
Not all mixotrophy involves eating your photosynthetic partner. Some organisms form lasting symbioses instead. Planktonic foraminifera and radiolaria, amoeba-like creatures that drift in the open ocean, frequently harbor microalgae as intracellular symbionts.7Journal of Phycology. srDna‐based taxonomic affinities of algal symbionts from a planktonic foraminifer and a solitary radiolarian These associations are often obligate for the host, meaning the host organism cannot survive without its algal partners.8PubMed Central. Metabolic interdependence and rewiring in radiolaria-microalgae photosymbioses
This arrangement is functionally mixotrophic: the host gets photosynthetically produced carbon from its symbionts while also feeding heterotrophically on prey it captures. The most familiar large-scale version of this strategy is reef-building coral, which acquires nutrition both from photosynthate translocated by its endosymbiotic microalgae and from heterotrophic prey and particle capture.9PubMed Central. Selective nutrient incorporation may underestimate heterotrophy of a mixotrophic reef-building coral Even in corals, research suggests that the heterotrophic contribution is often underestimated, because the way corals selectively incorporate nutrients from different sources can mask how much they are actually eating.
An Evolutionary Stepping Stone
The existence of mixotrophs raises a fascinating evolutionary question. If some organisms can temporarily steal chloroplasts, could that temporary arrangement eventually become permanent? This is essentially the hypothesis behind how all plant and algal chloroplasts originated in the first place: an ancient cell engulfed a photosynthetic bacterium and, over evolutionary time, that engulfed organism became an organelle.
Eco-evolutionary modeling supports the idea that transient chloroplast acquisition, like the kleptoplasty described above, could provide the evolutionary raw material for this kind of transition. The conditions under which retaining stolen chloroplasts becomes advantageous can be studied by examining how environmental factors like light and nutrient availability shape selection pressure.10PubMed. Acquired Phototrophy as an Evolutionary Path to Mixotrophy In a sense, living mixotrophs may offer a window into the intermediate stages of one of the most transformative events in the history of life: the evolution of photosynthetic eukaryotes.
Why Mixotrophs Matter for Carbon Cycling
The reason marine scientists and climate modelers care so intensely about mixotrophs has a lot to do with carbon. In the ocean, the “biological carbon pump” moves carbon from the surface to the deep sea. Photosynthetic organisms at the surface fix carbon dioxide into organic matter, some of which sinks. The efficiency of that pump depends on the structure of the microbial food web at the surface.
When mixotrophs dominate, the food web looks fundamentally different. They short-circuit the traditional chain by combining primary production and grazing in a single organism. A mixotroph-dominated system has a shortened and more efficient path from nutrient recycling to primary production, and mixotrophy opens a direct channel for bacterial production to support photosynthesis.11Biogeosciences. The role of mixotrophic protists in the biological carbon pump Models that leave mixotrophs out misrepresent the dynamics of carbon flow entirely.
One ocean biogeochemical model that incorporated mixotrophy found striking results. Adding mixotrophic grazing on bacteria produced a three-fold increase in bacterivory, which accelerated the microbial loop, shifted biomass to higher trophic levels, and increased carbon export by more than 20%.12Journal of Geophysical Research: Biogeosciences. Mixo‐COBALT: Incorporating Plastic Mixotrophy Into a Coupled Biogeochemical Ocean Model in the Sargasso Sea A 20% boost in carbon export is a big deal. It suggests that models without mixotrophs systematically underestimate how much carbon the ocean pulls out of the atmosphere.
Global-scale modeling work reinforces this picture, finding that high investment in phagotrophy (eating) is the dominant mixotrophic strategy across ocean biomes, used primarily to acquire nitrogen rather than carbon.13PubMed Central. Predicting optimal mixotrophic metabolic strategies in the global ocean That is an important nuance: many mixotrophs eat not because they are hungry for energy, but because they need nutrients like nitrogen and phosphorus that are scarce in the water. Photosynthesis provides the carbon, and phagotrophy provides the limiting nutrients.
Thriving Where Specialists Struggle
Mixotrophs tend to do particularly well in nutrient-poor environments, which makes intuitive sense. If you can photosynthesize for carbon and eat bacteria for nitrogen and phosphorus, you have an advantage over organisms locked into just one mode. Data from ocean transects show that mixotrophs increase in abundance at lower latitudes, where waters are warmer and more nutrient-poor, while the abundance of pure autotrophs and pure heterotrophs does not show the same pattern. This appears to be driven in part by increased light availability, which boosts the synergy between photosynthetically derived carbon and prey-derived nutrients.14PubMed Central. Mixotrophy in nanoflagellates across environmental gradients in the ocean
The same principle applies in freshwater systems, but with a twist. In boreal lakes, where dissolved organic matter turns the water brown and limits light penetration, you might expect mixotrophs to struggle because their photosynthetic side gets less light. Instead, mixotrophs actually thrive in browner lakes. Their success, measured as a percentage of total biomass, increases with dissolved organic matter and dissolved carbon dioxide.15Limnology and Oceanography. Environmental drivers of mixotrophs in boreal lakes The reason seems to be that strict autotrophs are even more hampered by the low light, and mixotrophs can compensate by eating. Lab work with a model alga confirmed this: dissolved organic carbon at concentrations mimicking brownified lake conditions enhanced growth, offering a mechanism to bypass reduced light availability.16PubMed Central. Metabolic plasticity of mixotrophic algae is key for their persistence in browning environments
This is especially relevant as northern lakes continue to “brown” due to increasing runoff of dissolved organic matter linked to land-use changes and warming. In a browning world, mixotrophs may increasingly dominate lake communities at the expense of strict photosynthesizers.
Climate Change and the Shifting Balance
Rising temperatures and ocean acidification are changing the rules of the game for mixotrophs, but not in a single predictable direction. Evolutionary modeling suggests that as temperatures rise, mixotrophs tend to evolve toward greater reliance on eating prey. Higher grazing rates reduce prey abundance, but if prey becomes too scarce, selection pressure reverses and favors greater reliance on photosynthesis.17PubMed Central. Modeling the metabolic evolution of mixotrophic phytoplankton in response to rising ocean surface temperatures This back-and-forth creates a kind of evolutionary thermostat, with the balance between feeding modes adjusting to match conditions.
At the community level, warming can tighten the coupling between grazers and their prey in productive systems. In warm, stratified, nutrient-poor waters, however, the trend favors communities dominated by smaller organisms, trophic flexibility, and nutrient recycling.18Limnology and Oceanography. Warming effects on marine microzooplankton and mixoplankton: Physiological responses, community reorganization, and implications for a warming ocean That description of “trophic flexibility and recycling-dominated pathways” is essentially a description of a mixotroph-friendly world.
Ocean acidification adds another layer of complexity. Experiments with different strains of mixotrophic protists exposed to elevated carbon dioxide showed variable responses. One strain nearly tripled its growth rate under high CO₂ conditions. Another became more autotrophic, increasing its carbon fixation. A third became more heterotrophic, increasing its grazing rate. The responses were strain-specific, not uniform.19ISME Communications. Variable responses to ocean acidification among mixotrophic protists with different lifestyles When warming and elevated CO₂ were combined in experiments on a single mixotrophic species, the two stressors had opposing effects on the balance between photosynthesis and grazing, keeping the organism’s net role in the carbon cycle roughly stable.20Ocean Acidification International Coordination Center. Changes in photosynthesis and grazing facilitate growth of a mixotrophic protist under ocean acidification and warming
The honest take is that predicting how mixotrophs will respond to future ocean conditions is hard, precisely because different species and strains react differently. Blanket predictions about “what mixotrophs will do” under climate change are almost certainly too simple. The diversity of responses is itself a defining feature of the group.
Mixotrophy on Land and in Reefs
Most research on mixotrophy focuses on planktonic protists, but the concept extends well beyond the open water. Coral reefs are built on mixotrophic partnerships, as reef-building corals rely on both their photosynthetic symbionts and heterotrophic feeding.9PubMed Central. Selective nutrient incorporation may underestimate heterotrophy of a mixotrophic reef-building coral Understanding the balance between these two nutritional sources is critical for predicting how corals respond to bleaching events, when the photosynthetic symbiont is expelled and the coral must rely more heavily on prey capture to survive.
On land, carnivorous plants represent an interesting parallel. Plants like Venus flytraps and sundews photosynthesize but also capture and digest insects, gaining nutrients (especially nitrogen and phosphorus) from their prey. Whether carnivorous plants should formally be called mixotrophs is a question researchers are actively debating, since the mechanisms differ from aquatic protist mixotrophy in important ways. But the underlying logic is the same: combining photosynthesis with heterotrophic nutrient acquisition gives an advantage in nutrient-poor habitats.
Biotechnology and Growing Algae
The dual metabolism of mixotrophs has practical applications in algal biotechnology. Growing microalgae for biofuel, nutritional supplements, or wastewater treatment requires maximizing biomass production. Pure autotrophic cultivation (light only) tends to produce lower yields, while pure heterotrophic cultivation (organic carbon only, no light) is expensive because you have to supply all the carbon. Mixotrophic cultivation, which combines light with an organic carbon source, offers a middle path that can outperform both.21International Journal of Hydrogen Energy. Mixotrophic cultivation, a preferable microalgae cultivation mode for biomass/bioenergy production, and bioremediation, advances and prospect
In mixotrophic cultivation, the alga photosynthesizes during the light phase and simultaneously assimilates dissolved organic carbon. This means it is not entirely dependent on either light or external carbon, and growth rates and final biomass tend to be higher than either strategy alone. For applications like wastewater bioremediation, where the water already contains dissolved organic matter, mixotrophic algae can treat the waste while also producing usable biomass. The approach is still being scaled up, but the economics look promising compared to the alternatives.
Why They Have Been Overlooked and How That Is Changing
Given how important mixotrophs appear to be, it is reasonable to ask why they were not prominent in ecological models until recently. Part of the answer is methodological. Traditional approaches to studying plankton relied on microscopy and pigment analysis, which classified organisms as either “phytoplankton” (photosynthetic) or “zooplankton” (heterotrophic). A cell that did both was hard to categorize and easy to misclassify. If you counted chlorophyll-containing cells as phytoplankton, you missed the fact that many of them were also eating bacteria.
New techniques are closing this gap. Single-cell approaches that combine imaging with stable isotope probing allow researchers to measure both photosynthesis and grazing rates within an individual cell. For instance, nano-scale secondary ion mass spectrometry can track uptake of isotope-labeled bacteria (to measure eating rates) and isotope-labeled bicarbonate (to measure photosynthesis) in the same protist.22PubMed Central. Mixoplankton and mixotrophy: future research priorities These tools are revealing mixotrophic behavior in species that were previously assumed to be strict autotrophs or strict heterotrophs, which means the true prevalence of mixotrophy has almost certainly been underestimated.
There is also a terminological shift underway. Some researchers now prefer the term “mixoplankton” for planktonic mixotrophs, to give them their own identity rather than forcing them into the phytoplankton or zooplankton bin. Whether the new name catches on broadly remains to be seen, but the underlying push is to make mixotrophy a first-class concept in ecology rather than an afterthought.
Mixotrophy and Harmful Algal Blooms
One practical consequence of ignoring mixotrophy is that we may misunderstand what fuels harmful algal blooms. Many bloom-forming species are mixotrophic. They do not just sit at the surface waiting for nutrients to show up; they actively eat bacteria and other small organisms, gaining nutrients that way. If kleptoplasty can boost an organism’s division rate by 40%, as modeling work suggests, then the ability to steal chloroplasts could be a meaningful accelerant during bloom formation.6PubMed Central. Modeling Unveils How Kleptoplastidy Affects Mixotrophy Boosting Algal Blooms
This has implications for bloom prediction and management. Models that treat bloom-forming species as pure photosynthesizers might miss the contribution of heterotrophic feeding and kleptoplasty to bloom initiation and persistence. A bloom that can feed on bacteria is harder to starve by reducing nutrient inputs alone, because the bacteria themselves represent a nutrient source independent of dissolved inorganic nutrients in the water. Getting the mixotrophic component right is not just an academic exercise; it could affect how coastal managers decide to allocate resources for bloom prevention.