Microbial community dynamics under periodic perturbations.

Ecosystems supply economically valuable goods and services on which human society depends. Anthropogenic modifications to the communities in ecosystems as well as the introduction of perturbations can alter ecological functions and life support services that are vital to the well-being of human societies. Most biodiversity and conservation research has focused on the value and importance of large organisms despite the fact that microbial communities are very important in providing ecosystems services due to their major role on biogeochemical cycles. Since ecosystem processes are highly dependent on those cycles, they may be directly affected by changes in microbial communities. That will not be the case if during perturbations, microbial communities are resistant or resilient to changes, or even if the microbial composition changes, the new community might be functionally similar to the original (functional redundancy). To date, it is not clear how microbial communities respond during perturbations. For example, some studies suggest that terrestrial communities show a great degree of resistance, but other works showed that in aquatic systems present annual dynamics in communities often triggered by environmental disturbances. Many factors like temperature, pH, and nutrient concentrations, trophic relationships, have been identified shaping freshwater bacterial communities.
We use four replicated microcosms (MCs) under the controlled environmental conditions of the lab and parallel sequencing of the 16S rRNA gene, to study how microbial communities change through time under both periodic perturbations in the carbon and energy source (methane) and a pH constrain, and test if the observed changes on microbial communities correspond to alterations on ecosystem services. We have observed that under a severe and well known environmental constrain (low pH) the perturbed communities and the control communities changed similarly (fig. a), loosing biodiversity, evenness and cell abundances. The strong influence of pH is not an unexpected finding as it has been reported previously. After fixing the pH to neutral conditions, the communities change following different paths becoming very from each other. During this period of time perturbed communities are not more similar to each other than to the control ones.
All communities showed a highly dynamic species composition, constantly changing and with very few bacteria present through all the time, an indication of the low degree of resistance and resilience of these communities. The microcosms are dominated by methanotrophic bacteria as expected, but the main methanotroph successively changes through time. First, in all communities Methylomonas sp. is displaced by Methylosoma sp. that dominates the microcosms under pH limitation. Once the pH is neutral, the succession of main players is more stochastic and differs for each microcosm. We also study the ecosystem responses measuring diverse biogeochemical parameters such us the methane oxidation rate. In general the ecosystem functions remain constant through the experiment despite of the observed dynamics on microbial community structure and composition, indicating functional redundancy on these microbial ecosystems. Although the analysis is not finish, these results support the idea that microorganisms are interchangeable and the idea that biogeochemistry of ecosystems is not dependent on microbial composition.
Figure1: a) non-metric multidimensional scaling plot showing the temporal development of community composition in perturbed (numbers 1 and 4) and control (2 and 3) microcosms (MC). The analysis is based on Morisita-Horn dissimilarities. The lines connect consecutive sampling occasions indicated by dots. Days indicated by numbers and grayscale. The dotted line marks the change on pH conditions. b) Succession on more abundant microbial phylotypes (>5%) through time for a perturbed and a control community. Taxonomy assignment of the main phylotypes has been included. The red line marks the change of pH conditions from acidic to neutral.