OCES Departmental Seminar: Relative importance and competitive advantages of mixotrophs vs. autotrophic and heterotrophic protists in various ocean environments  

Single cellular protists in the ocean possess three major trophic strategies, autotrophy, heterotrophy, and mixotrophy. Small mixotrophic protists (<5 µm) integrate photosynthesis and phagocytosis in the same cell, resulting a multidimensional interaction among three trophic groups, i.e., mixotrophs and autotrophs compete for inorganic nutrients and light and mixotrophs and heterotrophs compete for bacterial prey. Understanding the relative fitness and importance among those three groups in various ocean environments can help us gain insight into relevant biogeochemical and ecological consequences. In this talk, I will first present results of our recent study conducted in estuary-coastal East China Sea targeting mixotrophic populations using flow cytometry, microscopic observation and 18S rRNA gene metabarcoding. Secondly, I will discuss niche partitioning among mixotrophic, autotrophic and heterotrophic protists on a global scale, based on trophic annotation and statistical analysis of the Tara Oceans 18S rRNA gene dataset across eight ocean habitats. Environmental variables that could control competitive advantages/disadvantages of mixotrophs were of particular interests, and the competitiveness index of mixotrophs, CIMixo were established as abundance ratios between Mixotroph:Autotroph, Mixotroph:Heterotroph and Mixotroph:Total protists. Our results demonstrated varying fitness among three trophic protistan groups under different environments and promoted mixotrophic fitness in warm, sunlit ocean with lower nutrients and productivity, shedding light on the importance of studying protists based on their trophic mode within the ocean ecosystems.