Exploring functional traits and functional diversity of mixotrophic phytoflagellates in small browned forest lakes—mixotrophy, phagotrophy and osmotrophy

Abstract Mixotrophy is the combination of autotrophy and heterotrophy within a single organism. Heterotrophy in mixotrophs encompasses two main processes: ingestion of prey, termed phagotrophy, and nutrition by direct absorption and uptake of organic molecules, osmotrophy. Though osmotrophy is common in phytoplankton species it is often neglected in mixotrophic studies despite in some types of aquatic ecosystems, such as small humic lakes, obligate-osmotrophic organisms are especially important. This study was aimed at investigating the contributions of potential mixotrophs and examining the relationship between their functional traits (including osmotrophy) and functional diversity in response to environmental factors in small forest lakes. Through large-scale lake sampling, we found that light-availability and DIN concentration support potential mixotroph success. Lakes with high inputs of allochthonous organic material exhibited a greater influence of potential mixotrophs over autotrophs. This study indicates that obligate-osmotrophs may be a crucial metabolic trait in browned forest lakes, providing an adaptive advantage for mixotrophs and the inclusion of osmotrophy within mixotrophy studies appears to be promising. We also found that despite dominance, the homogeneous distribution of mixotrophs suggests functional redundancy.