Phagotrophic protists are a key component of microbial communities processing leaf litter under contrasting oxic conditions
Authors: Risse‐Buhl Ute , Schlief Jeanette , Mutz Michael 
Published: August 2015

The transformation of leaf litter, a key process in aquatic systems, is known to be reduced with decreasing oxygen concentrations, mainly due to lower abundance of and/or less active shredding macroinvertebrates. Aquatic fungi and phagotrophic protists involved in leaf litter processing can tolerate low oxygen, but little is known about their role in leaf litter processing under these conditions.

We aimed to unravel the importance of phagotrophic protists within microbially mediated leaf litter processing under contrasting oxic conditions. We hypothesised that respiration, abundance of aquatic bacteria and biomass of aquatic fungi, and thus leaf litter processing, are enhanced in the presence of phagotrophic protists, both under normoxic and low oxic conditions. Lower leaf processing was expected under low oxic than under normoxic conditions.

In microcosms, oxygen concentration was adjusted to either normoxic or low. For a total of 105 days, leaf litter was cocultivated with three microbial communities: (i) a multispecies bacterial community enriched from stream leaf litter, (ii) the aquatic fungusHeliscus lugdunensisadded to the bacterial community and (iii) the phagotrophic protistGlaucoma scintillansadded to the bacteria–fungi community.

Oxic condition had no significant effect on microbially mediated leaf mass loss. The leaf mass loss was faster in the presence of the aquatic fungus and further accelerated by adding the phagotrophic protist. After 105 days, leaf mass remaining approximated 64–79%, 57–68% and 55–61% of initial leaf mass in the bacteria, bacteria–fungi, and bacteria–fungi–phagotrophic protist communities, respectively.

Under both oxic conditions, the lower leaf toughness indicated that the aquatic fungus had the potential to process leaf structural components more efficiently than bacteria alone. The combination of lower ergosterol concentrations and enhanced leaf mass loss indicated that phagotrophic protists stimulated the efficiency of leaf processing by the microbial community. Under normoxic conditions, leaf‐associated respiration increased when successively adding an aquatic fungus and a phagotrophic protist to bacterial communities, which matches the faster leaf litter processing in these treatments. Under low oxic conditions, respiration of all three microbial communities was comparable. Thus, enhanced leaf litter processing in treatments with aquatic bacteria, fungi and phagotrophic protists was presumably caused either by other metabolic pathways such as fermentation and anaerobic respiration, or by changes in community composition and growth efficiency of the microbial community.

Microbial communities composed of aquatic bacteria, fungi and phagotrophic protists play a crucial role in leaf litter processing under a range of oxic conditions. Although processing of leaf litter by microbial communities alone is slower than in the presence of shredding invertebrates, the interaction of aquatic bacteria, fungi and phagotrophic protists can modulate leaf quality and subsequently carbon flow within the microbial food webs of aquatic ecosystems.