Print PageRecent studies of bacterial communities across contrasting terrestrial environments have recorded soil moisture as an edaphic variable strongly regulating community structure. Phylogenetic analyses demonstrated a non-random distribution of microbial groups across ‘wet’ and ‘dry’ environments. While a large number of taxa were associated with ‘wet’ conditions, many gram-positive bacteria, particularly Actinobacteria consistently demonstrated a strong negative correlation with soil moisture. Such grouping by moisture preference suggests a functional basis selected for on evolutionary timescales. This study examines the mechanistic rationale for such distribution and the genetic capacity for adaptation to decreasing precipitation. Rainfall exclusion plots were set-up in a subtropical rainforest in Puerto Rico in June 2009, of the ten excluded plots, half were experiencing this perturbation for the second time, and half were de novo exclusion plots. These exclusions reduced soil moisture by 20 – 30 %. The effect of decreased rainfall microbial diversity was measured through 16S rRNA sequencing prior to exclusion and 3 and 11 months post-exclusion. Furthermore, a mass-spectrometry approach was taken to detail isolate biochemical response to osmotic stress and also whole community metabolic response to rainfall exclusion.
Results/Conclusions
Rainforest soil microbial communities decreased in taxonomic diversity under conditions of rainfall exclusion. This decrease was more pronounced in de novo exclusion plots than in those experiencing exclusion for a second time, suggesting drought-induced plasticity. In these plots ‘dry-adapted’ taxa, e.g. Actinobacteria, increased under rainfall exclusion. Correlations between community diversity and edaphic variables suggests that elevated solute concentrations due to decreased water content are primary variables explaining differences in bacterial community structure. The metabolic profiles of Actinobacterial rainforest isolates exposed to osmotic stress (to mimic increasing solute concentration under decreased rainfall) demonstrated elevated concentrations of amino acids by which Actinobacteria alleviate osmotic stress. Additionally, the profiles of metabolites extracted from soil pore water successfully discriminated and clustered the sites based on treatment. We will discuss the metabolomic results with respect to how stress can regulate biochemical pathways that might alter carbon flow through perturbed ecosystems.
