Cellulose decomposers in complex ecosystems
Environmental conditions such as frost, fire, and water availability strongly affect the activity of soil microorganisms and soil carbon cycling in ecosystems. Seasonal freezing or burning events affect microbial activity substantially although the consequences for the soil carbon flow and the microbial community are often unclear. Although microorganisms can assimilate small carbon compounds present in frozen soils, soil organic matter (SOM) is dominated by large polymeric C molecules that require exoenzymatic breakdown before they can be used as substrate by micro-organisms. Many fungal taxa have the ability to produce exocellulases and accumulate cellulose-derived C in their biomass in high quantities (Vĕtrovský et al, 2016), but what is the capacity under extreme circumstances?
Stable Isotope Solution : 13C-cellulose
The use of stable isotopes helps to clarify the contribution of polymers such as cellulose to the total soil respiration or soil carbon storage in complex ecosystems containing a large background of 12C-polymers, as well as to identify the microbial species or genera involved in C-cycling processes.
Results from publications using IsoLife‘s U-13C Cellulose
The effect of frost on mineralisation of cellulose was investigated by Segura et al (2017). Using 13C Heteronuclear Single Quantum Coherence NMR spectroscopy, they showed that atoms originating from U-13C Cellulose had been incorporated into cell membrane lipids (Figure 1).
Figure 1. 13C-enriched membrane lipids after addition of 13C-cellulose to frozen soil (Segura et al, 2017).
Bastias et al (2009) have used 13C-cellulose to study the effects of prescribed burning of forests since 1972 on the occurrence of cellulose decomposers. The DGGE showed that all 12C- and 13C-fractions in the unburned plots had complex profiles, but in the 2-yr-burn plots the 13C-fraction contained only a few discrete bands (Figure 2a, b).
Figure 2. DGGE patterns from soil RNA extracts following incubation with 13C-cellulose (A unburned plots, B 2-yr burn plots; Bastias et al, 2009).
The small number of bands in the DDGE-profiles for the 13C fractions reveal that fewer active fungi incorporated 13C from the labelled cellulose. Some fungi that were able to use the 13C-cellulose were identified as Cryptococcus podzolicus, Crypotococcus humicolus, and Metarhizium anisopliae. Repeated burning apparently significantly altered the soil fungal community.
López-Mondéjar et al (2018) investigated the food web of cellulose decomposers in forest soil showing that this network has a high level of recycling of microbial products, rather than hierarchical structures that maintain a unidirectional carbon flow through the web. In contrast to current knowledge, they showed that both fungi ánd bacteria were involved in mineralisation of complex carbon sources (Figure 3), confirming an observation by Štursová et al (2012), although cellulose decomposition was 10 times faster in the fungi‐dominated litter. Taubert et al (2019) showed that the occurrence of groundwater selects the soil microbes involved in decomposition of plant polymers. Using stable isotope probing, they found a complete different set of microbial species responsible for degrading cellulose in groundwater than in soil.
Figure 3. The share of fungal and bacterial operational taxonomic units (OTUs) accumulating carbon from one, two, or three substrates (López-Mondéjar et al, 2018).
Bastias BA, IA Anderson, J Ignacio-Castro, PI Parkin, JI Prosser, JWG Cairney. 2009.
Influence of repeated prescribed burning on incorporation of 13C from cellulose by forest soil fungi as determined by RNA stable isotope probing.
Soil Biology & Biochemistry 41: 467-472.
López-Mondéjar R, V Brabcová, M Štursová, A Davidová, J Jansa, T Cajthaml, P Baldrian. 2018.
Decomposer food web in a deciduous forest shows high share of generalist microorganisms and importance of microbial biomass recycling.
NATURE – The ISME Journal 12: 1768–1778.
Segura JH, MB Nilsson, M Haei, T Sparrman, JP Mikkola, J Gräsvik, J Schleucher, MG Öquist. 2017.
Microbial mineralization of cellulose in frozen soils.
NATURE Communications 8: 1154.
Štursová M, L Žifčáková, MB Leigh, R Burgess, P Baldrian. 2012.
Cellulose utilisation in forest litter and soil: identification of bacterial and fungal decomposers.
FEMS Microbiology Ecology 80: 735-746.
Taubert M, J Stähly, S Kolb, K Küsel. 2019.
Divergent microbial communities in groundwater and overlying soils exhibit functional redundancy for plant-polysaccharide degradation.
PLoS ONE 14: e0212937.
Vĕtrovský T, M Štursová, P Baldrian. 2016.
Fungal communities in soils: Soil organic matter degradation.
Methods in Molecular Biology (Microbial Environmental Genomics (MEG)) 1399: 89-100.