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.

中文翻译：

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混合营养型金藻植物 Ochromonas 的环境依赖性代谢投资

混合营养原生生物结合光合作用和吞噬作用来获取能量和营养。由于混合营养生物既可以充当初级生产者，也可以充当消费者，因此它们在海洋食物网和生物地球化学循环中发挥着复杂的作用。许多混合营养生物在表型上也是可塑的，可以根据资源的可用性调整其代谢投资。因此，单个物种的生态作用可能随环境条件而变化。在这里，我们量化了光照和食物供应如何影响八种混合营养金藻植物Ochromonas的生长率、能量获取率和代谢投资策略。所有八个Ochromonas菌株都通过随着光强度的增加而降低叶绿素含量来进行光适应。一些菌株是专性光养生物，需要光才能生长，而其他菌株则对猎物可用性表现出更强的代谢反应。当猎物可用性较高时，所有八个品系都表现出加速的生长速度，并减少了对光合作用和吞噬作用的投资。光合作用和吞噬作用通常会产生额外的好处：在低猎物环境中，随着光照的增加，红单胞菌的生长速率增加到最大，光饱和速率，但随着丰富的细菌猎物的增加而进一步增加。在红单胞菌中观察到的光合作用和吞噬作用之间的附加效益表明，这两种代谢模式提供了不可替代的资源，这可能解释了为什么在某些但不是所有菌株中出现吞噬和光养投资之间的权衡。