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Abstract Mixotrophic protists combine photosynthesis and phagotrophy to obtain energy and nutrients. Because mixotrophs can act as either primary producers or consumers, they have a complex role in marine food webs and biogeochemical cycles. Many mixotrophs are also phenotypically plastic and can adjust their metabolic investments in response to resource availability. Thus, a single species's ecological role may vary with environmental conditions. Here, we quantified how light and food availability impacted the growth rates, energy acquisition rates, and metabolic investment strategies of eight strains of the mixotrophic chrysophyte,Ochromonas. All eightOchromonasstrains photoacclimated by decreasing chlorophyll content as light intensity increased. Some strains were obligate phototrophs that required light for growth, while other strains showed stronger metabolic responses to prey availability. When prey availability was high, all eight strains exhibited accelerated growth rates and decreased their investments in both photosynthesis and phagotrophy. Photosynthesis and phagotrophy generally produced additive benefits: In low‐prey environments,Ochromonasgrowth rates increased to maximum, light‐saturated rates with increasing light but increased further with the addition of abundant bacterial prey. The additive benefits observed between photosynthesis and phagotrophy inOchromonassuggest that the two metabolic modes provide nonsubstitutable resources, which may explain why a tradeoff between phagotrophic and phototrophic investments emerged in some but not all strains.
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Predicting optimal mixotrophic metabolic strategies in the global ocean
Mixotrophic protists combine photosynthesis with the ingestion of prey to thrive in resource-limited conditions in the ocean. Yet, how they fine-tune resource investments between their two different metabolic strategies remains unclear. Here, we present a modeling framework (Mixotroph Optimal Contributions to Heterotrophy and Autotrophy) that predicts the optimal (growth-maximizing) investments of carbon and nitrogen as a function of environmental conditions. Our model captures a full spectrum of trophic modes, in which the optimal investments reflect zero-waste solutions (i.e., growth is colimited by carbon and nitrogen) and accurately reproduces experimental results. By fitting the model to data forOchromonas, we were able to predict metabolic strategies at a global scale. We find that high phagotrophic investment is the dominant strategy across different oceanic biomes, used primarily for nitrogen acquisition. Our results therefore support empirical observations of the importance of mixotrophic grazers to upper ocean bacterivory.
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- Award ID(s):
- 2237017
- PAR ID:
- 10579639
- Publisher / Repository:
- Science
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 10
- Issue:
- 50
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1126/sciadv.adr0664
