ABSTRACT: Host-parasite interactions between phytoplankton and fungi (chytrids) are key processes in aquatic ecosystems. However, individual-level heterogeneity in these interactions remains unexplored, although its importance in predicting the spread of diseases has been demonstrated in epidemiology. In this study, we experimentally tested whether individual-level heterogeneity could be a good indicator of phytoplankton-chytrid interactions, using a freshwater green alga Staurastrum sp., the diatoms Ulnaria sp. and Fragilaria crotonensis, and chytrid fungi. The number of attached fungi per host cell showed a non-random clumped parasite distribution on Ulnaria sp. and F. crotonensis, but a random Poisson distribution on Staurastrum sp. To explore the potential mechanisms of these patterns, we developed a mathematical model describing sequential encounters between chytrid zoospores and host cells. The statistical fits of the model explained the parasite distributions for Ulnaria sp. and F. crotonensis well, indicating that the clumped parasite distributions may result from an infection rate, increasing with the number of infections that already occurred on each host cell. Simultaneous analysis of volatile organic compounds (VOCs) from uninfected and infected host populations revealed that, among 13 VOCs detected, 6 components characterized the differences in VOC compositions between species and infection status. In particular, the level of beta-ionone, potentially acting against fungal activities, was significantly reduced in the presence of chytrid infection of Staurastrum sp. These VOCs are targets for future studies, which potentially act as chemical signals influencing chytrid zoospores’ behaviors. The combination of mathematical and chemical analyses represents a promising approach to better understand the individual-level processes of phytoplankton-chytrid interactions.

中文翻译：

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浮游植物宿主细胞壶菌感染的非随机模式：数学​​和化学生态学方法

摘要：浮游植物和真菌（壶菌）之间的寄主-寄生虫相互作用是水生生态系统中的关键过程。然而，这些相互作用中个体水平的异质性仍未得到探索，尽管它在预测疾病传播方面的重要性已在流行病学中得到证实。在这项研究中，我们使用淡水绿藻Staurastrum sp.，硅藻Ulnaria sp. ，通过实验测试了个体水平的异质性是否可以成为浮游植物 - 壶菌相互作用的良好指标。和Fragilaria crotonensis和壶菌属真菌。每个宿主细胞附着的真菌数量在Ulnaria sp上显示出非随机的成簇寄生虫分布。和F. crotonensis，但是Staurastrum sp上的随机泊松分布。为了探索这些模式的潜在机制，我们开发了一个数学模型，描述了壶菌游动孢子和宿主细胞之间的连续相遇。该模型的统计拟合解释了Ulnaria sp.的寄生虫分布。和F. crotonensis好吧，这表明成群的寄生虫分布可能是由感染率引起的，随着每个宿主细胞上已经发生的感染数量的增加而增加。对来自未感染和感染宿主种群的挥发性有机化合物 (VOC) 的同时分析表明，在检测到的 13 种 VOC 中，有 6 种成分表征了物种和感染状态之间 VOC 成分的差异。特别是，在存在Staurastrum壶菌感染的情况下，可能对抗真菌活动的 β-紫罗兰酮的水平显着降低sp. 这些 VOC 是未来研究的目标，它们可能充当影响壶菌游动孢子行为的化学信号。数学和化学分析的结合代表了一种有前途的方法，可以更好地了解浮游植物 - 壶菌相互作用的个体水平过程。