Relative importance of bacterivorous mixotrophs in an estuary-coast environment

Abstract

Mixotrophic eukaryotes are important bacterivores in oligotrophic open oceans, but their significance as grazers in more nutrient-rich waters is less clear. Here we investigated the bacterivory partition between mixotrophs and heterotrophs in a productive, estuary-influenced coastal region in East China Sea. We found ubiquitous, actively feeding phytoplankton populations and taxa with mixotrophic potential, by identifying ingestion of fluorescent prey surrogate and analyzing community 18S rRNA gene amplicons. Potential and active mixotrophs accounted for 10-63% of total eukaryotic community and 17-69% of bacterivores observed, respectively, contributing 6-48% of estimated in situ bacterivory. The much higher mixotroph fitness outside of the turbid plume were potentially driven by increased light and decreased nutrients availability. Our results suggest that, although heterotrophs dominated overall in situ bacterivory, mixotrophs were abundant and important bacterivores in this low-latitude mesotrophic coastal region.Scientific Significance StatementMixotrophy, the combing of photosynthetic and phagotrophic nutrition, can dramatically increase primary production and alter material movements in aquatic food webs. Understanding the distribution and fitness of phagotrophic mixotrophs in a range of habitats are therefore critical to properly understand the biogeochemical cycles of the ocean. We developed a robust method to estimate active mixotrophs in situ and demonstrate rapid shifts of bacterivory partition between mixotrophs and heterotrophs across an estuary-coast environment in East China Sea. Our results help fill the knowledge gap of mixotroph niches in mesotrophic coastal environments that are impacted by riverine discharge, shedding light on relationships among mixotrophy, light and nutrient conditions.

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

“…In this study, we focus on the trophic strategy potentials among different eukaryotic lineages, rather than the in situ active trophic activities (are partially associated with instantaneous conditions). Therefore, we assigned each lineage/species (when possible) into three trophic groups by searching published database and references [5,37,38], including (potential) mixotrophs (pigmented eukaryotes with phagocytosis capability), heterotrophs (eukaryotes with no inherent chloroplasts) and autotrophs (pigmented eukaryotes with no available evidence for phagocytosis). When trophic annotation was impossible at lineage or species level, their affiliated genus or family was referred to for annotation.…”