University Institute of Water Research, University of Granada, C/Ramón y Cajal, 4, Granada, 18071, Spain.
Department of Ecology, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, Granada, 18071, Spain.
J Phycol. 2019 Oct;55(5):1028-1040. doi: 10.1111/jpy.12865. Epub 2019 May 25.
Mixotrophic protists combine phagotrophy and phototrophy within a single cell. Greater phagotrophic activity could reinforce the bypass of carbon (C) flux through the bacteria-mixotroph link and thus lead to a more efficient transfer of C and other nutrients to the top of the trophic web. Determining how foreseeable changes in temperature and UVR affect mixotrophic trade-offs in favor of one or the other nutritional strategy, along the mixotrophic gradient, is key to understanding the fate of carbon and mineral nutrients in the aquatic ecosystem. Our two main hypotheses were: (i) that increased warming and UVR will divert metabolism toward phagotrophy, and (ii) that the magnitude of this shift will vary according to the organism's position along the mixotrophic gradient. To test these hypotheses, we used two protists (Isochrysis galbana and Chromulina sp.) located in different positions on the mixotrophic gradient, subjecting them to the action of temperature and of UVR and their interaction. Our results showed that the joint action of these two factors increased the primary production:bacterivory ratio and stoichiometric values (N:P ratio) close to Redfield's ratio. Therefore, temperature and UVR shifted the metabolism of both organisms toward greater phototrophy regardless of the original position of the organism on the mixotrophic gradient. Weaker phagotrophic activity could cause a less efficient transfer of C to the top of trophic webs.
混养原生动物在单个细胞内同时结合吞噬作用和光合作用。更高的吞噬活性可以加强通过细菌-混养体连接的碳（C）通量的旁路，从而更有效地将 C 和其他营养物质转移到营养网的顶部。确定温度和紫外线辐射的可预见变化如何影响有利于一种或另一种营养策略的混养权衡，沿着混养梯度，是理解碳和矿物质营养物质在水生生态系统中的命运的关键。我们的两个主要假设是：（i）变暖加剧和紫外线辐射会将代谢转向吞噬作用，（ii）这种转变的幅度将根据生物体在混养梯度上的位置而有所不同。为了检验这些假设，我们使用了两种位于混养梯度不同位置的原生动物（球等鞭金藻和Chromulina sp.），使它们受到温度和紫外线辐射及其相互作用的影响。我们的结果表明，这两个因素的共同作用增加了初级生产力：细菌摄食比和化学计量值（N:P 比）接近 Redfield 比。因此，温度和紫外线辐射使两个生物体的代谢都偏向于更强的光合作用，而不管生物体在混养梯度上的原始位置如何。较弱的吞噬活性可能导致 C 向营养网顶部的转移效率降低。
