Protists play a fundamental role in all ecosystems, but we are still far from estimating the total diversity of many lineages, in particular in highly diverse environments, such as freshwater. Here, we survey the protist diversity of the Paraná River using metabarcoding, and we applied an approach that includes sequence similarity and phylogeny to evaluate the degree of genetic novelty of the protists' communities against the sequences described in the reference database PR². We observed that ~28% of the amplicon sequence variants were classified as novel according to their similarity with sequences from the reference database; most of them were related to heterotrophic groups traditionally overlooked in freshwater systems. This lack of knowledge extended to those groups within the green algae (Archaeplastida) that are well documented such as Mamiellophyceae, and also to the less studied Pedinophyceae, for which we found sequences representing novel deep-branching clusters. Among the groups with potential novel protists, Bicosoecida (Stramenopiles) were the best represented, followed by Codosiga (Opisthokonta), and the Perkinsea (Alveolata). This illustrates the lack of knowledge on freshwater planktonic protists and also the need for isolation and/or cultivation of new organisms to better understand their role in ecosystem functioning.

中文翻译：

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淡水原生生物：在大型洪泛区系统中揭开未探索的面纱

原生生物在所有生态系统中都发挥着重要作用，但我们仍远未估计许多谱系的总多样性，特别是在淡水等高度多样化的环境中。在这里，我们使用元条形码调查了巴拉那河的原生生物多样性，并应用了一种包括序列相似性和系统发育的方法来评估原生生物群落相对于参考数据库 PR ^{2中描述的序列的遗传新颖程度}. 我们观察到约 28% 的扩增子序列变体根据其与参考数据库中的序列的相似性被归类为新的；其中大多数与淡水系统中传统上被忽视的异养群落有关。这种缺乏知识扩展到绿藻（Archaeplastida）中的那些有充分记录的群体，例如Mamiellophyceae，以及研究较少的Pedinophyceae，我们发现了代表新的深分支簇的序列。在具有潜在新型原生生物的群体中，Bicosoecida (Stramenopiles) 代表最好，其次是Codosiga(Opisthokonta) 和 Perkinsea (Alveolata)。这说明缺乏对淡水浮游原生生物的了解，以及需要隔离和/或培养新生物以更好地了解它们在生态系统功能中的作用。