Abstract
The contribution of fungi to the degradation of plant litter and transformation of dissolved organic matter (humic substances, in particular) in freshwater ecosystems has received increasing attention recently. However, the role of Saprolegniales as one of the most common eukaryotic organisms is rarely studied. In this study, we isolated and phylogenetically placed 51 fungal and 62 Saprolegniales strains from 12 German lakes. We studied the cellulo-, lignino-, and chitinolytic activity of the strains using plate assays. Furthermore, we determined the capacity of 10 selected strains to utilize 95 different labile compounds, using Biolog FF MicroPlates™. Finally, the ability of three selected strains to utilize maltose and degrade/produce humic substances was measured. Cladosporium and Penicillium were amongst the most prevalent fungal strains, while Saprolegnia, Achlya, and Leptolegnia were the most frequent Saprolegniales strains. Although the isolated strains assigned to genera were phylogenetically similar, their enzymatic activity and physiological profiling were quite diverse. Our results indicate that Saprolegniales, in contrast to fungi, lack ligninolytic activity and are not involved in the production/transformation of humic substances. We hypothesize that Saprolegniales and fungi might have complementary roles in interacting with dissolved organic matter, which has ecological implications for carbon cycling in freshwater ecosystems.
Highlights
According to the blast results of the obtained internal transcribed spacer (ITS) sequences, we predominantly isolated strains from Saprolegniales (59 out of 62) that belonged to Saprolegnia, Achlya, and Leptolegnia (34, 21, and 18%, respectively), and isolated two Pythium strains and one Phytophthora strain from the order
We contributed to the knowledge of fungal and oomycete communities associated with floating plant litter in the littoral zone of freshwater ecosystems
Our results suggest that they occupy the same habitat, Saprolegniales contrasts with fungi by showing no ligninolytic activity
Summary
Dissolved organic matter (DOM) mainly constitutes a heterogeneous mixture of aliphatic and aromatic polymers, and plays a pivotal role in shaping aquatic ecosystems [1]. It consists of labile autochthonous DOM (DOMauto ), generally produced by phytoplankton, and more refractory allochthonous DOM (DOMallo ), which mainly originates from plant litter degradation [2]. The production of polymer-degrading enzymes [5] and reactive oxygen species (ROS) [6] facilitates the interaction between fungi and recalcitrant organic matter which eventually contributes to the aquatic microbial loop and transform or mineralize organic carbon [7]
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