Department of Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland.
Spectral Imaging Laboratory, Faculty of Information Technology, University of Jyväskylä, Jyväskylä, Finland.
Mol Ecol. 2022 Sep;31(18):4726-4738. doi: 10.1111/mec.16619. Epub 2022 Aug 7.
Light availability is the main regulator of primary production, shaping photosynthetic communities and their production of ecologically important biomolecules. In freshwater ecosystems, increasing dissolved organic carbon (DOC) concentrations, commonly known as browning, leads to lower light availability and the proliferation of mixotrophic phytoplankton. Here, a mixotrophic algal species (Cryptomonas sp.) was grown under five increasing DOC concentrations to uncover the plastic responses behind the success of mixotrophs in browning environments and their effect in the availability of nutritionally important biomolecules. In addition to the browning treatments, phototrophic, heterotrophic and mixotrophic growth conditions were used as controls. Despite reduced light availability, browning did not impair algal growth compared to phototrophic conditions. Comparative transcriptomics showed that genes related to photosynthesis were down-regulated, whereas phagotrophy gene categories (phagosome, lysosome and endocytosis) were up-regulated along the browning gradient. Stable isotope analysis of phospholipid fractions validated these results, highlighting that the studied mixotroph increases its reliance on heterotrophic processes with browning. Metabolic pathway reconstruction using transcriptomic data suggests that organic carbon is acquired through phagotrophy and used to provide energy in conjunction with photosynthesis. Although metabolic responses to browning were observed, essential fatty acid content was similar between treatments while sterol content was slightly higher upon browning. Together, our results provide a mechanistic model of how a mixotrophic alga responds to browning and how such responses affect the availability of nutritionally essential biomolecules for higher trophic levels.
光是初级生产力的主要调节因子，塑造了光合群落及其生态重要生物分子的产生。在淡水生态系统中，溶解有机碳（DOC）浓度的增加，通常被称为褐变，会导致光可用性降低和混合营养型浮游植物的增殖。在这里，一种混合营养藻类（Cryptomonas sp.）在五种不断增加的 DOC 浓度下生长，以揭示混合营养体在褐变环境中成功的可塑性反应及其对营养重要生物分子可用性的影响。除了褐变处理外，还使用了光合、异养和混合营养生长条件作为对照。尽管光可用性降低，但褐变并没有像光合条件那样损害藻类的生长。比较转录组学表明，与光合作用相关的基因下调，而吞噬作用基因类别（吞噬体、溶酶体和内吞作用）沿着褐变梯度上调。磷脂分数的稳定同位素分析验证了这些结果，突出表明所研究的混合营养体随着褐变增加了对异养过程的依赖。使用转录组数据进行代谢途径重建表明，有机碳通过吞噬作用获得，并与光合作用一起提供能量。尽管观察到对褐变的代谢反应，但处理之间的必需脂肪酸含量相似，而褐变时固醇含量略高。总之，我们的结果提供了一种机制模型，说明了一种混合营养藻类如何对褐变做出反应，以及这种反应如何影响营养必需生物分子对更高营养水平的可用性。
