Environment and microbiome drive different microbial traits and functions in the macroscale soil organic carbon cycle.

Soil microbial traits and functions play a central role in soil organic carbon (SOC) dynamics. However, at the macroscale (regional to global) it is still unresolved whether (i) specific environmental attributes (e.g., climate, geology, soil types) or (ii) microbial community composition drive key microbial traits and functions directly. To address this knowledge gap, we used 33 grassland topsoils (0-10 cm) from a geoclimatic gradient in Chile. First, we incubated the soils for 1 week in favorable standardized conditions and quantified a wide range of soil microbial traits and functions such as microbial biomass carbon (MBC), enzyme kinetics, microbial respiration, growth rates as well as carbon use efficiency (CUE). Second, we characterized climatic and physicochemical properties as well as bacterial and fungal community composition of the soils. We then applied regression analysis to investigate how strongly the measured microbial traits and functions were linked with the environmental setting versus microbial community composition. We show that environmental attributes (predominantly the amount of soil organic matter) determined patterns of MBC along the gradient, which in turn explained microbial respiration and growth rates. However, respiration and growth normalized for MBC (i.e., specific respiration and growth) were more linked to microbial community composition than environmental attributes. Notably, both specific respiration and growth followed distinct trends and were related to different parts of the microbial community, which in turn resulted in strong effects on microbial CUE. We conclude that even at the macroscale, CUE is the result of physiologically decoupled aspects of microbial metabolism, which in turn is partially determined by microbial community composition. The environmental setting and microbial community composition affect different microbial traits and functions, and therefore both factors need to be considered in the context of macroscale SOC dynamics.

Effects of metal contamination with physicochemical properties on the sediment microbial communities in a tropical eutrophic-hypereutrophic urban reservoir in Brazil.

We investigated the effects of metals and physicochemical variables on the microbes and their metabolisms in the sediments of Guarapiranga reservoir, a tropical eutrophic-hypereutrophic freshwater reservoir located in a highly urbanized and industrialized area in Brazil. The metals cadmium, copper, and chromium showed minor contribution to changes in the structure, composition, and richness of sediment microbial communities and functions. However, the effects of metals on the microbiota are increased when taken together with physicochemical properties, including the sediment carbon and sulfur, the bottom water electrical conductivity, and the depth of the water column. Clearly, diverse anthropic activities, such as sewage discharge, copper sulfate application to control algal growth, water transfer, urbanization, and industrialization, contribute to increase these parameters and the metals spatially in the reservoir. Microbes found especially in metal-contaminated sites encompassed Bathyarchaeia, MBG-D and DHVEG-1, Halosiccatus, Candidatus Methanoperedens, Anaeromyxobacter, Sva0485, Thermodesulfovibrionia, Acidobacteria, and SJA-15, possibly showing metal resistance or acting in metal bioremediation. Knallgas bacteria, nitrate ammonification, sulfate respiration, and methanotrophy were inferred to occur in metal-contaminated sites and may also contribute to metal removal. This knowledge about the sediment microbiota and metabolisms in a freshwater reservoir impacted by anthropic activities allows new insights about their potential for metal bioremediation in these environments.