What are mixotrophic protists? and why are they important?


Mixotrophic protists are tiny organisms that play an intriguing double role in ecosystems. Much like corals, these organisms live in symbiosis – in a mutually beneficial relationship – with tiny photosynthetic algae. While on their own the protists feed on other organisms to acquire carbon (C) and energy, they can also delegate the job to the micro-algae they harbor, which, using sunlight and photosynthesis, can transform atmospheric carbon into sugar.

Mixotrophic protists are increasingly recognized for their significant contribution to C cycling in aquatic ecosystems because they can either respire CO2 as they digest their food, or ‘breath in’ the gas, transforming it, and storing the carbon in their own biomass. Shifts in this function (photosynthesis vs. predation) or the abundance of mixotrophs may thus influence the magnitude and direction of the carbon flux of ecosystems.

Mixotrophic protists can be very abundant in peatlands, but despite their abundance, their contribution to peatland C cycling has been almost entirely ignored. The world’s peatlands store tremendous amounts of carbon – up to 20 years’ worth of human and natural emissions. While today they sequester more carbon than they release, research suggests that in a warmer world, they could release additional CO2 into the atmosphere.

Some examples of mixotrophic protists (here testate amoebae) we can find in Sphagnum mosses.: Hyalosphenia papilio, Archerella flavum, and Amphitrema wrightianum. Tiny green cells in the shell are endosymbiotic algae that can photosynthesize and fix atmospheric carbon.

Peatlands accumulate C when input through photosynthesis exceeds C losses through respiration. Any changes in either rate in response to climate changes may thus modify the capacity of peatlands to sequester and store C. Mixotrophs can employ either strategy, thus their per-capita switching may drive the entire peatland C dynamic from C sink to source, or vice versa.

MIXOPEAT will use a combination of mathematical models, lab and field experiments to figure out how mixotrophic protists change the dynamics of carbon in peatlands and understand what climatic pressures lead to the evolution and/or loss of these organisms!