Prevalence and preferred niche of small eukaryotes with mixotrophic potentials in the global ocean

Abstract

Single-celled eukaryotes that are capable of phago-mixotrophy in the ocean compete inorganic nutrients and light with autotrophs and bacterial prey with heterotrophs, yet the role these factors play in shaping the success of mixotrophs remain ambiguous. In this study, we ask what environmental variables could impact the prevalence of small eukaryotic mixotrophs in vast open ocean, and what role does nutritional resource play in these interactions. We used the Tara Oceans eukaryotic 18S rRNA gene dataset and environmental variables to conduct large-scale field based analysis, and isolates based culture experiments to verify growth and nutritional resource relationships for representative mixotorphs. The field observation showed that comparing to autotrophic and heterotrophic eukaryotes, mixotrophs were favored by environments with lower nutrients and higher light availability. Bacterial prey and abiotic conditions including temperature and depths of nitracline, maximal chlorophyll and mixed layer also presented somewhat correlations with mixotrophic prevalence but to less significant extent. The culture experiments demonstrated that most mixotrophs could maintain similarly high growth rates when receiving sufficient light and nutrients, either as inorganic form or organic bacterial prey, but not necessarily with low light and high nutrients conditions. This study revealed close relationship between abiotic and biotic nutritional resource and prevalence of mixotrophy, shedding light on the importance of light and nutrients for determining competitive success of mixotrophs versus autotrophic and heterotrophic eukaryotes in the ocean.