Coming to the dark side: How does nitrogen eutrophication reshape the mixotrophic trade-off of osmo-mixotrophy in Ochromonas?

Abstract

Increasing nitrogen level is one of the most serious environmental problems in global natural waters, disturbing the stability of function and structure of aquatic ecosystem. As important functional group, mixotrophs with plastic metabolism modes perform high adaptations under changing environments, potentially with positive biogeochemical consequences. Here we focus on the trophic plasticity of a model eukaryotic microorganism, mixotrophic Ochromonas under increasing nitrogen and tested the role of osmo-mixotrophy (= mixotrophy) on the physiology of Ochromonas. Results showed that nitrogen eutrophication significantly reduced the proportion of open PSII reaction centers of mixotrophic Ochromonas, and osmo-mixotrophic Ochromonas enhanced the relative contribution of organic carbon uptake with increasing nitrogen. Furthermore, genes involved in photosynthetic electron transfer and photosynthetic carbon fixation were down-regulated, and genes involved in energy metabolism were upregulated. These findings suggested that increasing nitrogen caused mixotrophic organisms to become more heterotrophic, which may bring unexpected impacts to the balance of photosynthesis and respiration within aquatic ecosystem.

Keywords: Autotrophy; Heterotrophy; Mixotrophy; Nitrogen; Nutritional strategy; Photosynthesis.