Mixotrophy

Mixotrophy is a nutritional strategy in which an organism combines autotrophic energy capture through photosynthesis with heterotrophic feeding on organic matter. In mixotrophs, carbon and energy can be obtained from both inorganic sources via light-driven primary production and from organic sources through ingestion or uptake of dissolved compounds. The arrangement can be facultative, where an organism mainly photosynthesizes but can feed when needed, or obligate, requiring both modes for growth.

Mechanisms fall into several forms. Phagotrophy involves ingesting prey such as bacteria or small algae; osmotrophy uses dissolved organic compounds; and some organisms employ kleptoplasty or retain photosynthetic organelles acquired from prey. This versatility allows mixotrophs to exploit variable environments, shifting between phototrophy and heterotrophy as light, nutrients, or prey availability change.

Ecological significance is considerable. Mixotrophy blurs the line between traditional producers and consumers and affects carbon and nutrient cycling. In nutrient-poor (oligotrophic) or light-limited habitats, mixotrophs can sustain growth by combining photosynthesis with grazing, influencing microbial food webs and energy flow. They can also contribute to nutrient turnover by releasing inorganic nutrients through grazing and by altering prey populations.

Occurrence is widespread among microbial eukaryotes in both marine and freshwater systems. Notable groups include various flagellates, ciliates, chrysophytes, and some dinoflagellates; the model organism Euglena gracilis is a classic example. Researchers study mixotrophy using isotopic tracers, microscopy, flow cytometry, and molecular methods to understand its prevalence and ecological roles.