Who is who in litter decomposition: Metaproteomics reveals major microbial players and their biogeochemical functions

Background/Question/Methods

The decomposition of organic matter in natural ecosystems is strongly influenced by the microorganisms present. Conversely, bacteria and fungi are limited by the amount and type of organic matter available in a given environment, most of which is ultimately derived from plants. Changes in the nutrient content (stoichiometry) and biochemical constituents of plant litter may therefore alter microbial composition and elicit changes in the activities of the present microbial communities. The identification of the microbial proteins of a given habitat together with the analysis of their phylogenetic origin are expected to provide fundamentally new insights into the role of microbial diversity in biogeochemical processes. To relate structure and functionality of microbial communities involved in leaf-litter decomposition we analyzed the protein complement of beech litter collected in winter and spring 2009 at various forest sites in Austria with differences in litter stoichiometry. A semi-quantitative proteomics approach by one-dimensional polyacrylamide gel electrophoresis (1-D-SDS-PAGE) combined with liquid chromatography/tandem mass-spectrometry (LC-MS/MS) was performed. A newly developed bioinformatics workflow was used to assign acquired spectra information to phylogenetic and functional groups and to validate the obtained results. Furthermore, determined phylogenetic structure and functional assignments of the decomposer community were validated with complementary approaches like phospholipid fatty acid (PLFA) analyses, culture based approaches and enzyme activity measurements.

Results/Conclusions

We were able to show that litter decomposition rates as well as decomposer community structure were affected by litter nutrient stoichiometry. Highest decomposition rates were observed at sites with the highest nutrient availability in the litter. A flow of carbon from the litter material to fungi and bacteria was observed with a higher increase of microbial biomass at sites with higher nutrient, in particular phosphorus (P) and micronutrient (K, Mn, Fe) availabilities. In our approach, exclusively extracellular hydrolytic enzymes produced by fungi were detected. Cellulases were the most abundant group of enzmyes at all sites. Interestingly, the phylogenetic origin of cellulases changed over time. While cellulases from plant pathogenic and endophytic fungi which might inhabit the leaf material already prior to litter fall dominated in winter, cellulases from soil derived fungi came up in spring. Bacteria clearly relied on the presence of the enzymes produced by fungi. Our findings indicate a dominant role of fungi in the decomposition of recalcitrant litter material while bacteria behave as cheaters that profit from the supply of breakdown products produced from the litter material by fungal extracellular hydrolytic enzymes.