Membrane lipid adaptation of soil Bacteroidetes isolates to temperature and pH.

3-hydroxy fatty acids (3-OH FAs) are characteristic components of the Gram-negative bacterial membrane, recently proposed as promising temperature and pH (paleo) proxies in soil. Nevertheless, to date, the relationships between the 3-OH FA distribution and temperature/pH are only based on empirical studies, with no ground truthing work at the microbial level. This work investigated the influence of growth temperature and pH on the lipid composition of three strains of soil Gram-negative bacteria belonging to the Bacteroidetes phylum. Even though non-hydroxy FAs were more abundant than 3-OH FAs in the investigated strains, our results suggest that 3-OH FAs are involved in the membrane adaptation of these bacteria to temperature. The strains shared a common adaptation mechanism to temperature, with a significant increase in the ratio of anteiso vs. iso or normal 3-OH FAs at lower temperature. In contrast with temperature, no common adaptation mechanism to pH was observed, as the variations in the FA lipid profiles differed from one strain to another. We suggest that models reconstructing environmental changes in soils should include the whole suite of 3-OH FAs present in the membrane of Gram-negative bacteria, as all of them could be influenced by temperature or pH at the microbial level.

Responses of active soil microorganisms facing to a soil biostimulant input compared to plant legacy effects.

Agriculture is changing to rely on agroecological practices that take into account biodiversity, and the ecological processes occurring in soils. The use of agricultural biostimulants has emerged as a valid alternative to chemicals to indirectly sustain plant growth and productivity. Certain BS have been shown to select and stimulate plant beneficial soil microorganisms. However, there is a lack of knowledge on the effects and way of action of the biostimulants operating on soil functioning as well as on the extent and dynamic of these effects. In this study we aimed to decipher the way of action of a seaweed and amino-acids based biostimulant intended to be applied on soil crop residues to increase their microbial mineralization and the further release of nutrients. By setting-up a two-phase experiment (soil plant-growing and soil incubation), our objectives were to (1) determine the effects of the soil biostimulant over time on the active soil bacteria and fungi and the consequences on the organic carbon mineralization in bare soils, and (2) assess the biostimulant effects on soil microorganisms relatively to plant legacy effects in planted soils. We demonstrated that the soil biostimulant had a delayed effect on the active soil microorganisms and activated both plant growth promoting bacteria and saprophytes microorganisms at the medium-term of 49 days. However, the changes in the abundances of active microbial decomposers were not associated to a higher mineralization rate of organic carbon derived from soil and/or litter. The present study assessed the biostimulant beneficial effect on active soil microbial communities as similar as or even higher than the legacy effects of either A. thaliana or T. aestivum plants. We specifically showed that the biostimulant increased the active fungal richness to a higher extent than observed in soils that previously grew the two plants tested.

Specific recruitment of soil bacteria and fungi decomposers following a biostimulant application increased crop residues mineralization.

Agriculture is undergoing important changes in order to meet sustainable soil management with respect to biodiversity (namely agroecology). Within this context, alternative solutions to mineral fertilizers such as agricultural biostimulants are thus promoted and being developed. The mechanisms by which some soil biostimulants sustain soil biological functioning and indirectly increase crop yields are still unknown. Our goal in the present study was to demonstrate if and to what extent the application of a soil biostimulant affects the soil heterotrophic microbial communities that are involved in organic matter decomposition and carbon mineralization. We hypothesized that the addition of a biostimulant results in changes in the composition and in the biomass of soil microbial communities. This in turn increases the mineralization of the organic matter derived from crop residues. We performed soil microcosm experiments with the addition of crop residues and a biostimulant, and we monitored the organic carbon (orgC) mineralization and the microbial biomass, along with the microbial community composition by sequencing 16S rRNA gene and ITS amplicons. The addition of a soil biostimulant caused a pH neutralizing effect and simultaneous enhancement of the orgC mineralization of crop residues (+ 400 µg orgC g-1 dry soil) and microbial biomass (+ 60 µg orgC g-1 dry soil) that were linked to changes in the soil microbial communities. Our findings suggest that the soil carbon mineralization enhancement in the presence of the biostimulant was supported by the specific recruitment of soil bacteria and fungi. Whereas archaea remained stable, several operational taxonomic units (OTUs) of indigenous soil bacteria and fungi were enriched and affiliated with known microbial decomposers such as Cytophagaceae, Phaselicystis sp., Verrucomicrobia, Pseudomonas sp., Ramicandelaber sp., and Mortierella sp., resulting in lower soil microbial richness and diversity.