Kyle Marcia, Haande Sigrid, Ostermaier Veronika, Rohrlack Thomas
Norwegian University of Life Sciences (NMBU), Environmental Sciences, Ås, Norway.
Norwegian Institute for Water Research (NIVA), Oslo, Norway.
PLoS One. 2015 Mar 20;10(3):e0118738. doi: 10.1371/journal.pone.0118738. eCollection 2015.
Parasitic chytrid fungi (phylum Chytridiomycota) are known to infect specific phytoplankton, including the filamentous cyanobacterium Planktothrix. Subspecies, or chemotypes of Planktothrix can be identified by the presence of characteristic oligopeptides. Some of these oligopeptides can be associated with important health concerns due to their potential for toxin production. However, the relationship between chytrid parasite and Planktothrix host is not clearly understood and more research is needed. To test the parasite-host relationship over time, we used a sediment core extracted from a Norwegian lake known to contain both multiple Planktothrix chemotype hosts and their parasitic chytrid. Sediment DNA of chytrids and Planktothrix was amplified and a 35-year coexistence was found. It is important to understand how these two antagonistic species can coexistence in a lake. Reconstruction of the time series showed that between 1979-1990 at least 2 strains of Planktothrix were present and parasitic pressure exerted by chytrids was low. After this period one chemotype became dominant and yet showed continued low susceptibility to chytrid parasitism. Either environmental conditions or intrinsic characteristics of Planktothrix could have been responsible for this continued dominance. One possible explanation could be found in the shift of Planktothrix to the metalimnion, an environment that typically consists of low light and decreased temperatures. Planktothrix are capable of growth under these conditions while the chytrid parasites are constrained. Another potential explanation could be due to the differences between cellular oligopeptide variations found between Planktothrix chemotypes. These oligopeptides can function as defense systems against chytrids. Our findings suggest that chytrid driven diversity was not maintained over time, but that the combination of environmental constraints and multiple oligopeptide production to combat chytrids could have allowed one Planktothrix chemotype to have dominance despite chytrid presence.
寄生壶菌（壶菌门）已知会感染特定的浮游植物，包括丝状蓝藻浮游颤藻。浮游颤藻的亚种或化学型可通过特征性寡肽的存在来识别。其中一些寡肽因其潜在的毒素产生能力而可能与重要的健康问题相关。然而，壶菌寄生虫与浮游颤藻宿主之间的关系尚未完全明确，还需要更多的研究。为了长期测试寄生虫与宿主的关系，我们使用了从挪威一个湖泊提取的沉积物岩芯，该湖泊已知同时含有多种浮游颤藻化学型宿主及其寄生壶菌。对壶菌和浮游颤藻的沉积物DNA进行了扩增，发现它们共存了35年。了解这两种拮抗物种如何在湖泊中共存很重要。时间序列重建表明，在1979年至1990年期间，至少存在2种浮游颤藻菌株，壶菌施加的寄生压力较低。在此之后，一种化学型变得占主导地位，但对壶菌寄生的易感性仍然较低。浮游颤藻持续占主导地位可能是由环境条件或其内在特征导致的。一种可能的解释是浮游颤藻向温跃层转移，温跃层通常是一个光照低且温度降低的环境。浮游颤藻能够在这些条件下生长，而壶菌寄生虫则受到限制。另一个潜在的解释可能是由于浮游颤藻化学型之间细胞寡肽变异的差异。这些寡肽可以作为对抗壶菌的防御系统。我们的研究结果表明，随着时间的推移，壶菌驱动的多样性并未得到维持，但环境限制和多种寡肽产生以对抗壶菌的组合可能使一种浮游颤藻化学型在壶菌存在的情况下仍占主导地位。
