Collaborative Research: Phosphorus-driven metabolic switching in marine mixoplankton

Microscopic marine organisms play essential roles in maintaining healthy ocean ecosystems by recycling nutrients and supporting marine food webs that sustain fisheries and other ocean resources important to society. Among these organisms are mixoplankton, unique microbes that can both produce their own food through photosynthesis and consume other microorganisms as prey. It is currently not yet understood what causes these organisms to switch between these feeding strategies or how these changes influence ocean ecosystems. This project investigates how nutrient availability affects the balance between photosynthesis and prey consumption using laboratory experiments and modern molecular techniques. The resulting knowledge improves our understanding of the microorganisms that underpin productive marine ecosystems and provides data needed to better predict ecosystem function. This research advances fundamental scientific knowledge, trains the next generation of marine scientists, and creates hands-on educational resources for students, teachers, and the public that promote scientific literacy and interest in STEM careers. Mixoplankton, protists capable of both photosynthesis and phagotrophy, play critical but poorly understood roles in marine carbon and nutrient cycling. Current paradigms suggest that constitutive mixoplankton maintain photosynthetic carbon fixation while supplementing nutrient acquisition through prey ingestion under nutrient limitation. However, preliminary transcriptomic analyses of prasinophytes suggest a fundamentally different strategy: phosphorus limitation induces coordinated upregulation of phagotrophy-associated genes and down regulation of photosynthetic pathways, suggesting a metabolic shift from autotrophy to heterotrophy. This project tests the metabolic switch hypothesis by integrating radio tracer experiments, transcriptomics, and cellular and biochemical analyses to quantify carbon and phosphorus acquisition through photosynthetic and phagotrophic pathways. Specifically, the project (1) measures carbon and phosphorus assimilation under phosphorus-replete and phosphorus-depleted conditions, (2) identifies the molecular and cellular mechanisms underlying metabolic switching, (3) determines whether trophic switching is reversible following nutrient supply, and (4) evaluates whether this response is conserved across evolutionarily distinct prasinophyte lineages. By directly linking gene expression with metabolic function, this research establishes mechanistic relationships between transcriptional regulation and nutrient acquisition strategies. These findings improve understanding of carbon cycling in oligotrophic subtropical gyres, which cover more than 40% of the global ocean. The results also provide critical empirical data needed to improve representation of mixoplankton metabolism in marine biogeochemical and Earth system models, leading to more accurate predictions of ocean carbon cycling and ecosystem responses to changing environments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.