Determinism of microbial community assembly by drastic environmental change

Microbial community assembly is shaped by deterministic and stochastic processes, but the relationship between these processes and the environment is not understood. Here we describe a rule for the determinism and stochasticity of microbial community assembly affected by the environment using in silico, in situ, and ex situ experiments. The in silico experiment with a simple mathematical model showed that the existence of essential symbiotic microorganisms caused stochastic microbial community assembly, unless the community was exposed to a non-adapted nutritional concentration. Then, a deterministic assembly occurred due to the low number of microorganisms adapted to the environment. In the in situ experiment in the middle of a river, the microbial community composition was relatively deterministic after the drastic environmental change caused by the treated wastewater contamination, as analyzed by 16S rRNA gene sequencing. Furthermore, by culturing microbial communities collected from the upstream natural area and downstream urban area of the river in test tubes with varying carbon source concentrations, the upstream community assembly became deterministic with high carbon concentrations while the downstream community assembly became deterministic with low carbon concentrations. These results suggest that large environmental changes, which are different from the original environment, result in a deterministic microbial community assembly.

Study of Rhizobia Impact on Nutritional Element Concentration in Legumes

The concentration of nitrogen in the Earth’s atmosphere is about 78%, but most plants are not able to acquire it directly from the atmosphere. One of the most common ways for binding atmospheric nitrogen is the development of an efficient symbiotic system between legumes and rhizobia. The aim of this study was to compare how different legumes and rhizobia symbiosis systems affect the concentrations of nutrients and other elements in soya and faba beans. Seeds of plants were inoculated with a preparation of rhizobia just before sowing. Plant samples were collected at the flowering stage (vegetative parts) and during harvest (seeds). Samples were air-dried and analysed with inductively coupled plasma mass spectrometry (ICP-MS). Total nitrogen and carbon concentrations were determined with an elemental analyser (EA). The obtained results showed that inoculation of legume plants with rhizobia not only affected nitrogen uptake by plants but also uptake of other elements. Inoculation had an effect on mineral element uptake for both faba bean and soybean leaves, where a significant increase in Mg, P, K, and Ca was observed. Treatment of legume plants with rhizobia caused a decrease of P and K concentrations in seeds, and there were changes in Fe and Mn concentrations.

Hydrologic Setting Dictates the Sensitivity of Ecosystem Metabolism to Climate Variability in Lakes

Global change is influencing production and respiration in ecosystems across the globe. Lakes in particular are changing in response to climatic variability and cultural eutrophication, resulting in changes in ecosystem metabolism. Although the primary drivers of production and respiration such as the availability of nutrients, light, and carbon are well known, heterogeneity in hydrologic setting (for example, hydrological connectivity, morphometry, and residence) across and within regions may lead to highly variable responses to the same drivers of change, complicating our efforts to predict these responses. We explored how differences in hydrologic setting among lakes influenced spatial and inter annual variability in ecosystem metabolism, using high-frequency oxygen sensor data from 11 lakes over 8 years. Trends in mean metabolic rates of lakes generally followed gradients of nutrient and carbon concentrations, which were lowest in seepage lakes, followed by drainage lakes, and higher in bog lakes. We found that while ecosystem respiration (ER) was consistently higher in wet years in all hydrologic settings, gross primary production (GPP) only increased in tandem in drainage lakes. However, interannual rates of ER and GPP were relatively stable in drainage lakes, in contrast to seepage and bog lakes which had coefficients of variation in metabolism between 22–32%. We explored how the geospatial context of lakes, including hydrologic residence time, watershed area to lake area, and landscape position influenced the sensitivity of individual lake responses to climatic variation. We propose a conceptual framework to help steer future investigations of how hydrologic setting mediates the response of metabolism to climatic variability.