Desulfurization · Microbial organo-sulfur desulfurization in the mycorrhizosphere

Microbial organo-sulfur desulfurization in the mycorrhizosphere

Introduction. Sulfur is a macro-nutrient that is essential for plant growth, but over 90% of the sulfur in soils is bound to organic molecules, not directly available to plants that are almost entirely dependent on sulfate. In contrast, many bacteria and fungi are capable to mobilize sulfur from sulfate esters, and specific bacterial guilds can utilize sulfonates as a sulfur source. Over 80% of land plants receive a significant proportion of their nutrients largely through mycorrhizal fungal networks. Mycorrhizal fungi are living in symbiosis with the plant host where the plant supplies the fungus with sugars. In return, the fungus supplies the plant host with soil nutrients. Up to now, the involvement of mycorrhiza in plant sulfur supply is largely elusive. This is of special interest in agricultural systems where a sustainable sulfur supply through arbuscular mycorrhiza (AM) could substitute inorganic fertilizer applications. The hypothesis of this fellowship is that sulfate esters and sulfonates are mobilised by certain bacteria in the mycorrhizosphere and arbuscular mycorrhiza transport the mobilised form to the plant host (Figure 1). Aims. i) Determine the direct and indirect involvement of AM fungi, saprophytic fungi and bacteria in mobilization of organically bound sulfur (organo-S) in the mycorrhizosphere and ii) identify and quantify the activity of organo-S desulfurizing microorganisms. Methods. (i) Direct sampling of mycorrhizal hyphae, (ii) bi-compartmental microcosms and (iii) microcosms with rotating mesh cores were established to prevent mycorrhizal fungi directly accessing organically bound 34S labelled sulfur. Desulfonating bacteria were quantified via a Most Probable Number approach while microbial communities were studied cultivation independently via Denaturing-Gradient-Gel Electrophoresis (DGGE), cloning and sequencing and next generation sequencing. 34S in plant shoots was quantified to establish S transfer rates from the mesh cores. Results and discussion. The abundance of the cultivable hyphosphere bacteria was 1-2 order of magnitudes higher when compared to bulk soil (p ≤ 0.05). A more diverse community of desulfonating bacteria was associated with AM hyphae belonging to the Gamma- and Beta-proteobacteria, Bacilli, Bacteroidetes and Actinobacteria. Cultivation independent community analysis indicated a greater bacterial diversity on hand-picked AM hyphae when compared to bulk soil. Likewise, AM fungi diversity was greater in the hand-picked selection when compared to bulk soil. AM inoculation (Glomus) in bi-compartmental microcosms significantly increased percentage root colonisation of Lolium, Agrostis and Plantago and the quantity of cultivable desulfonating bacteria in the mycorrhizosphere. Mycorrhizosphere community analysis revealed significantly different bacterial and fungal communities following AM inoculation for all plants. The results demonstrate that increased AM root colonisation impacts bacterial and fungal community dynamics in the mycorrhizosphere and corresponding increased abundance of desulfonating bacteria that may be beneficial for plant-S supply. Root colonisation with AM fungi, arylsulfatase activity, and abundance of cultivable heterotrophic and desulfonating bacteria was greater in microcosms with static 35 µm mesh cores over respective rotating cores (mycorrhiza free controls). Higher 34S content in plant material of Plantago and Agrostis was reported for the microcosms with static cores for the first 3-6 months, suggesting that AM mobilisation from organo-S may be important at early stages of vegetative growth when plant S requirement was the highest. The capability to attach to AM with a putative type three secretion system was identified in hyphospheric desulfonating bacteria of the genus Pseudomonas, Stenotrophomonas and via a clone library also for Burkholderia. This type of attachment has been described before for bacteria including Burkholderia associated with ectomycorrhizal hyphae. While attachment is commonly facilitated via fimbriae, secretion systems have been mostly attributed for transfer of pathogenicity factors into the host cell. Therefore, this type of putative attachment might also include an additional transfer of molecules from the attaching bacteria to the AM hyphae. Contact details: Achim Schmalenberger, University of Limerick, Life Sciences, Schrodinger Building, Limerick, Ireland. Further information about this project can be found under https://sites.google.com/site/environmicro/

Data: CORDIS, © European Union