Mixotrophy emerges as the optimal strategy in mature waters of the Amazon River plume

Current evidence shows that phytoplankton are mixotrophs, combining photoautotrophy with osmotrophy (i.e., uptake of dissolved organic matter). Additionally, some unicellular eukaryotes are also capable of phagotrophy, representing an intermediate step between autotrophs and herbivores named mixoplankton. Mixoplankton seem to provide higher-quality food for metazoans, and to improve energy transfer to higher trophic levels. However, field studies on all aspects of mixotrophy are difficult due to the challenge of distinguishing their activity from that of autotrophs. Our April/May 2018 and 2021 cruises focused on the base of the surface planktonic food web in the distinct Amazon River plume habitats, where we used nitrogen stable isotopes of amino acids (CSIA AA) in seston within a multidisciplinary framework for resolving trophic function. Mixotrophy dominates in the Outer Plume Margin, a region with mature waters aged ca. 27 days. Mixotrophy seems the optimal strategy for growth in these heterogeneous outer margins as part of the succession of phytoplankton functional diversity along the plume. Our study supports the growing evidence for the cosmopolitan distribution of mixotrophy among unicellular aquatic organisms, underscores the urgent need to study it in situ, and paves the way for a novel application of the CSIA AA in field research.