Biodiversity-function relationships in methanogenic communities

Abstract

Methanogenic communities play a crucial role in carbon cycling and biotechnology (anaerobic digestion), but our understanding of how their diversity, or composition in general, determines the rate of methane production is very limited. Studies to date have been correlational because of the difficulty in cultivating their constituent species in pure culture. Here, we investigate the causal link between methanogenesis and diversity in laboratory anaerobic digesters by experimentally manipulating the diversity of cultures by dilution and subsequent equilibration of biomass. This process necessarily leads to the loss of the rarer species from communities. We find a positive relationship between methane production and the number of taxa, with little evidence of functional saturation, suggesting that rare species play an important role in methane-producing communities. No correlations were found between the initial composition and methane production across natural communities, but a positive relationship between species richness and methane production emerged following ecological selection imposed by the laboratory conditions. Our data suggest methanogenic communities show little functional redundancy, and hence, any loss of diversity-both natural and resulting from changes in propagation conditions during anaerobic digestion-is likely to reduce methane production.

Figure 1

(a) Nonmetric multidimensional scaling (NMDS) plot of unweighted Bray–Curtis distances between ancestral (blue) and descendant (red) communities; (b) OTU number of ancestral (blue) and descendant (red) communities; (c) Mean frequency of the most common phyla in ancestral and descendant communities [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

(a) Relationship between the counts of Archaea and biogas production; (b) Power curve of natural logarithms of gas production and richness of the descendant sample (OTU). b (slope) of the fitted linear trend = 4.07, R ² = 0.47, F _1,11 = 8.8; p = 0.0012 [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Relationship between the percentage of acetoclastic methanogen reads in total methanogen reads and gas production of a sample [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 4

(a) Power curve of ln of cumulative biogas production [mL] and ln of the number of OTUs in the diluted communities. b exponent = 0.43, R ² = 0.27, linear fit: F _1,35 = 12.5, p < 0.001. (b) Relationship between the biogas production and Archaea [cells/g sample]. Archaeal cell numbers plotted on log₁₀ scale; (c) NMDS plots of Bray–Curtis based on the dilution treatment (see legend). Stress score is 0.14 [Colour figure can be viewed at http://wileyonlinelibrary.com]