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Molecular and ecophysiological diversity of phytoplankton-fungi associations.

Subject Area Ecology and Biodiversity of Plants and Ecosystems
Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Microbial Ecology and Applied Microbiology
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 347469280
 
Final Report Year 2021

Final Report Abstract

Fungal parasites (e.g. chytrids) of phytoplankton are ubiquitous in aquatic ecosystems. Despite the growing evidence that they have profound effects on ecosystem functioning via top-down control of phytoplankton blooms and by providing alternative nutrient flows, they remain largely understudied. This is mainly because they are difficult to identify and thus frequently overseen. As a consequence only very few phytoplankton parasitic chytrids have been isolated and sequenced which severely limits the use of next generation sequencing and other molecular approaches to integrate chytrids into plankton ecology. In this project we successfully applied and combined different isolation approaches, i.e. cultivation, single cell isolation and in situ baiting, with laboratory experiments, microscopy and DNA metabarcoding to connect chytrid sequence diversity and ecological interactions. By isolating and sequencing chytrid parasites associated with a wide variety of phytoplankton taxa (e.g. diatoms, green algae, cyanobacteria) we substantially increased our knowledge on the phylogenetic diversity of chytrids. We “re-discovered” previously known, but unsequenced, morphospecies but also discovered and described entirely novel species (Algomyces stechlinensis). We identified several family-, order- up to phylum-level novel lineages in the fungal tree of life and thereby significantly contributed to fill some of the existing gaps in chytrid taxonomy. Importantly we showed that cryptic diversity (diversity that is not distinguishable by morphology) in both host and chytrid can mask host-chytrid specificity and emphasize that molecular characterization of both phytoplankton host and parasitic chytrid is needed to accurately identify and compare host range and specificity of chytrids. With the sequences derived from the different isolation efforts we established a unique reference library that we applied to a DNA metabarcoding dataset fom mesotrophic Lake Stechlin to characterize seasonal dynamics and ecological interactions of chytrids. The sequence reference library greatly improved our ability to assign ecological function to NGS data. Most importantly, we were able to “illuminate” the ecological interaction of almost 70%, of the “dark matter” zoosporic fungal (ZF) community in the lake, demonstrating that parasitic phytoplankton-chytrid and saprophytic pollen-chytrid interactions were the dominant components of the zoosporic fungal community in a close to 1:1 ratio. The ZF community showed a clear seasonal dynamic with distinct winter-spring, summer and autumn community compositions, whose succession was largely affected by ecological shifts like algal blooms and allochthonous organic matter (i.e. pollen) inputs. Hereby we highlight the tight coupling of the ZF community to seasonal host/substrate dynamics and a high degree of resource specialization in zoosporic fungi. We explicitly demonstrate for the first time the dominance of parasitic chytrids during diatom springblooms. Different temporal dynamics between diatom-specific chytrid clades (e.g. Rhizophydiales and Zygophlyctis) sharing the same host species indicated different niche occupation and competitive strategies. In the time frame of the project we were not able to elucidate the mechanisms driving these different temporal dynamics. Nevertheless, in laboratory experiments we identified ecophysiological differences between chytrid species in response to light. Counteracting the common assumption that chytrids are unable to infect their host in the absence of light, we found examples of obligate parasitic chytrid species that can infect and spread under dark conditions. Interestingly the two diatom specific parasite species belonging to the clades which displayed different temporal dynamics in Lake Stechlin also showed a contrasting response in respect to their ability to infect in the dark. This finding is relevant in context of ongoing climate change and reduced light conditions in aquatic ecosystems via “brownification” as its effect on the impact of chytrid epidemics is more likely to result in species shifts, due to en- or dis-abling light host refugia. This project has provided deeper insights into the diversity, ecology, and spatio-temporal dynamics of chytrid fungi in temperate lakes. We have generated high quality reference sequences for various commonly observed phytoplankton-fungal pairs and advanced fluorescent staining methods which will further aid the integration of phytoplankton parasitic chytrids into plankton ecology and facilitate exploration of their spatial and temporal distribution patterns in future studies.

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