Trophic strategies explain the ocean niches of small eukaryotic phytoplankton

Abstract

A large fraction of marine primary production is performed by diverse small protists, and many of these phytoplankton are mixotrophs that also consume bacterial prey. However, the mechanisms structuring this diversity and its biogeochemical consequences remain poorly understood. Here we use isolates from seven major taxa to demonstrate tradeoffs between phototrophic and phagotrophic abilities. We then show that trophic strategy along the autotrophy-mixotrophy spectrum correlates strongly with global niche differences, across depths and across gradients of stratification and chlorophyll a. A model of competition shows that community shifts can be explained by greater fitness of faster-grazing mixotrophs when nutrients are scarce and light is plentiful. Our results illustrate how basic physiological constraints and principles of resource competition can organize complexity in the surface ocean ecosystem.

Recommendations for advancing mixoplankton research through empirical-model integration

“…Efforts focusing on genetic commonalities among diverse mixoplankton may also reveal conserved traits retained from early phagotrophic ancestors, while instances of gene loss/acquisition can shed light on traits that have enabled the diversification of younger lineages. Ongoing culture work suggests a mixotrophy continuum, ranging from mostly photosynthetic groups (e.g., prasinophytes, haptophytes) to mostly phagotrophic (e.g., dictyochophytes) (Li et al, 2022;Edwards et al, 2023a), but with a high degree of variability within lineages and even among species (Calbet et al, 2011;Wilken et al, 2020).…”

“…For example, do mixoplankton with higher photosynthetic rates have lower affinity for prey uptake? Recent studies provide initial insights into how these trade-offs play out among nanoplankton (Edwards et al, 2023a(Edwards et al, , 2023bBarbaglia et al, 2024). When performed both across and within different mixotrophic species, these analyses can help to reveal the relative roles of intraspecific phenotypic plasticity on setting mixotrophy trade-offs.…”