Colonization and degradation of chitin by aquatic fungi

MASIGOL, H. ; VAN DEN WYNGAERT, S. ; BIANCALANA, F.; GROSSART, H-P.

Congreso; Symposium of Aquatic Microbial Ecology (SAME) 16; 2019

Chitin is one of the most abundant biopolymers in aquatic systems and plays a crucial role in biogeochemical cycles of carbon and nitrogen. The precise role of aquatic fungi in chitin degradation remains still unclear, but there is evidence that fungi hold an important role in production and also degradation of chitin in aquatic systems. The main objective of this work was to determine the capability of aquatic fungi to colonize and their potential to degrade chitin in situ. We performed an in situ field baiting experiment in three habitats (pelagic, littoral and sediment) in Lake Stechlin (Germany), using purified shrimp shell and dried zooplankton carcasses as chitin baits. Spatio-temporal dynamics of colonization was studied by isolation and molecular characterization of fungi and oomycetes colonizing the chitin baits, and by SEM (scanning electron microscopy). Chitin degradation potential of all obtained fungal isolates was tested with a colorimetric chitinase assay and an in vitro degradation experiment. In total, we obtained 36 isolates belonging to Basidiomycota (Heterobasidium sp.), Ascomycota (Penicillium sp., Cladosporium sp., Fusarium sp., among others), Chytridiomycota (Rhizoclosmatium globosum, Avachytrium platense) and fungi-like Oomycota (Saprolegnia sp.). SEM images corroborated the presence of different fungal structures on the shrimp shell substratum. The Ascomycota, Penicillium sp. and Cladosporium sp. were the most frequently isolated genera. The oomycete Saprolegnia sp. and chytrids, however, were only isolated from zooplankton carcasses and shrimp shells, respectively. All tested Ascomycota showed chitinase activity; particularly Penicillium sp. However, Saprolegnia sp. did not show any chitinase activity. In the in vitro experiment with two chytrid species, chitin degradation was observed after three and ten days, depending on the type of chitin substrate (Daphnia or shrimp, respectively). This study demonstrates the ability of a diverse community of aquatic fungi and oomycetes to efficiently colonize and degrade chitin substrates in aquatic ecosystems and hence should be regarded as important components of the aquatic carbon cycle.