Microbial response to nitrogen addition and prolonged drought is phylogenetically patterned

Background/Question/Methods

Microbes such as Bacteria and Fungi play a critical role in the degradation of organic matter. Different combinations of microbial taxa result in different rates of decomposition, so predicting how climate change will affect microbial composition and function is fundamental to understanding future environments.  However, the high number of microbial species in any given location constrains our ability to predict the fate of individuals following perturbation. This complexity may be reduced, theoretically, if changes in community composition are non-random with respect to phylogeny and are "clustered" at taxonomic ranks higher than species. In this experiment we address the question of whether microbial response is phylogenetically patterned following climate change disturbance.  We present the results of a long-term manipulation experiment in which replicated grassland plots were subjected to drought and nitrogen fertilization within the range of predicted future conditions. Fungal and bacterial communities were enumerated using high-throughput DNA sequencing of phylogenetically informative markers. To test whether phylogenetic patterning correlates with decomposition rates, leaf litter fungi were isolated and deployed in microcosms as mock communities along a gradient of community clustering.  

Results/Conclusions

Our global change simulations produced clear "winners" and "losers" and a microbe’s quantitative response to nitrogen fertilization was correlated with its response to drought. Nearly all bacteria and fungi replicate communities were significantly more clustered than a random selection process would predict. Proportional changes in abundance were highly co-correlated amongst individuals that were most closely related, with a significant correlation detectable among relatives containing 92% ribosomal sequence similarity or greater in the case of fungal community response to drought. Using consenTRAIT, a simple measure of 90% consensus within evolutionary lineages, we circumscribe clusters of taxa with the same binary response to climate change disturbance. Dispersion of these clusters throughout the fungal and bacterial phylogenies suggests multiple pathways to climate change sensitivity. In microcosm experiments we found no significant correlation between phylogenetic community composition and decomposition rates. We show how a phylogenetic framework may be used to aggregate individuals into a finite number of response groups from whom changes in community composition may be inferred, although it remains unclear whether or how this may affect ecosystem function.