Mixotrophs are organisms that combine two nutritional strategies to acquire energy and carbon. Unlike organisms that rely solely on sunlight (phototrophs) or consume other organisms (heterotrophs), mixotrophs integrate both methods. This dual approach allows them to adapt to varying environmental conditions. They represent a broad spectrum of life, from microscopic plankton to some larger plants and animals, blurring traditional classifications of how organisms obtain sustenance.
The Dual Nature of Mixotrophy
Mixotrophs perform both phototrophy and heterotrophy. Phototrophy involves generating energy from light, similar to plants, through photosynthesis. Heterotrophy means obtaining nutrients by consuming organic matter or other organisms, much like animals.
Mixotrophs integrate these methods in various ways. Some, known as constitutive mixotrophs, possess their own chloroplasts and can photosynthesize while also ingesting prey. Others, termed non-constitutive mixotrophs, acquire photosynthetic capabilities by consuming phototrophic prey and retaining their chloroplasts, a process called kleptoplasty.
The balance between phototrophy and heterotrophy varies among mixotrophs and is influenced by environmental factors like light availability and nutrient concentrations. Some primarily rely on photosynthesis but can supplement their diet by consuming prey when light or nutrients are limited. Conversely, some are primarily heterotrophic, but can use photosynthesis to supplement their nutrition or delay starvation when prey is scarce.
Ecological Importance of Mixotrophs
Mixotrophs are important in various ecosystems, particularly in aquatic environments, where they are estimated to comprise over half of all microscopic plankton. Their dual nature allows them to influence food webs by acting as both producers and consumers. This flexibility can lead to more efficient energy transfer within microbial food webs.
They also contribute to nutrient cycling, including carbon fixation and nutrient regeneration. Mixotrophs can acquire inorganic nutrients through photosynthesis and organic nutrients by ingesting other organisms. This allows them to thrive in environments where nutrients are limited or variable, such as oligotrophic (nutrient-poor) waters.
The presence of mixotrophs can regulate the availability of nutrients for other organisms and influence the stoichiometry of nutrient uptake and release. For example, mixotrophic phytoplankton can release excess nitrogen and phosphorus, making them available to other organisms. Their adaptability allows them to maintain a stable elemental composition, even when faced with stoichiometric imbalances in their environment.
Diverse Examples in Nature
Mixotrophy is observed in many organisms, from single-celled organisms to some multicellular species. Many planktonic protists, such as dinoflagellates and chrysophytes, are examples of mixotrophs in aquatic environments. These organisms can photosynthesize while also consuming bacteria or smaller plankton.
The ciliate genus Mesodinium, particularly Mesodinium rubrum, is another example of a mixotrophic protist, which gains photosynthetic capabilities from ingested prey. Some amoebae and ciliates can even sequester only the plastids of their algal prey for photosynthesis, though they must continuously replace them as they do not replicate within the host.
Among plants, mixotrophy is seen in carnivorous plants like the Venus flytrap, which photosynthesizes but also obtains nutrients by trapping and digesting insects. Some hemi-parasitic plants also exhibit mixotrophy, photosynthesizing while drawing nutrients from host plants. Myco-heterotrophic plants form symbiotic relationships with fungi to acquire organic carbon and nutrients from other plants or soil.
Mixotrophy is less common among animals, but some invertebrates and even one known vertebrate exhibit this trait. For instance, some sea slugs, like Elysia chlorotica, can perform kleptoplasty, retaining chloroplasts from consumed algae for photosynthesis. The spotted salamander, Ambystoma maculatum, has been found to host symbiotic algae within its embryonic cells, representing a rare example of a vertebrate with an endosymbiotic microbe.