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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 eight Ochromonas strains 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, Ochromonas growth 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 in Ochromonas suggest 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., (© 2023 The Authors. Journal of Phycology published by Wiley Periodicals LLC on behalf of Phycological Society of America.)

Kleptoplastidic, or chloroplast-stealing, lineages offer insight into the process of acquiring photosynthesis. By quantifying the ability of these organisms to retain and use photosynthetic machinery from their prey, we can understand how intermediaries on the endosymbiosis pathway might have evolved regulatory and maintenance mechanisms. Here, we focus on a mixotrophic kleptoplastidic ciliate, Mesodinium chamaeleon, noteworthy for its ability to retain functional chloroplasts from at least half a dozen cryptophyte algal genera. We contrasted the performance of kleptoplastids from blue-green and red cryptophyte prey as a function of light level and feeding history. Our experiments showed that starved M. chamaeleon cells are able to maintain photosynthetic function for at least 2 weeks and that M. chamaeleon containing red plastids lost chlorophyll and electron transport capacity faster than those containing blue-green plastids. However, likely due to increased pigment content and photosynthetic rates in red plastids, M. chamaeleon had higher growth rates and more prolonged growth when feeding on red cryptophytes. For example, M. chamaeleon grew rapidly and extensively when fed the blue-green cryptophyte Chroomonas mesostigmatica, but this growth appeared to hinge on high levels of feeding supporting photosynthetic activity. In contrast, even starved M. chamaeleon containing red plastids from Rhodomonas salina could achieve high photosynthetic rates and extensive growth. Our findings show that plastid origin impacts the maintenance and magnitude of photosynthetic activity, though whether this is due to variation in ciliate control or gradual loss of plastid function in ingested prey cells remains unknown., (© 2021 Phycological Society of America.)