Frenemies: Interactions between Rhizospheric Bacteria and Fungi from Metalliferous Soils

Stefano Rosatto,Andrea Di Cesare,Mirca Zotti,Simone Di Piazza,Mauro Giorgio Mariotti,Grazia Cecchi,Luigi Vezzulli,Enrica Roccotiello

doi:10.3390/life11040273

Stefano Rosatto, Andrea Di Cesare + Show 6 more

Open Access

https://doi.org/10.3390/life11040273

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Abstract

Is it possible to improve the efficiency of bioremediation technologies? The use of mixed cultures of bacteria and fungi inoculated at the rhizosphere level could promote the growth of the associated hyperaccumulating plant species and increase the absorption of metals in polluted soils, broadening new horizons on bioremediation purposes. This work investigates interactions between Ni-tolerant plant growth-promoting bacteria and fungi (BF) isolated from the rhizosphere of a hyperaccumulating plant. The aim is to select microbial consortia with synergistic activity to be used in integrated bioremediation protocols. Pseudomonas fluorescens (Pf), Streptomyces vinaceus (Sv) Penicillium ochrochloron (Po), and Trichoderma harzianum group (Th) were tested in mixes (Po-Sv, Po-Pf, Th-Pf, and Th-Sv). These strains were submitted to tests (agar overlay, agar plug, and distance growth co-growth tests), tailored for this aim, on Czapek yeast agar (CYA) and tryptic soy agar (TSA) media and incubated at 26 ± 1 °C for 10 days. BF growth, shape of colonies, area covered on plate, and inhibition capacity were evaluated. Most BF strains still exhibit their typical characters and the colonies separately persisted without inhibition (as Po-Sv) or with reciprocal confinement (as Th-Sv and Th-Pf). Even if apparently inhibited, the Po-Pf mix really merged, thus obtaining morphological traits representing a synergic co-growth, where both strains reached together the maturation phase and developed a sort of mixed biofilm. Indeed, bacterial colonies surround the mature fungal structures adhering to them without any growth inhibition. First data from in vivo experimentation with Po and Pf inocula in pot with metalliferous soils and hyperaccumulator plants showed their beneficial effect on plant growth. However, there is a lack of information regarding the effective co-growth between bacteria and fungi. Indeed, several studies, which directly apply the co-inoculum, do not consider suitable microorganisms consortia. Synergic rhizosphere BFs open new scenarios for plant growth promotion and soil bioremediation.

Highlights

Bacteria and fungi are simultaneously present in a wide variety of environments [1].These organisms are capable of coexisting, and of actively interacting and the bacteria-fungal interactions (BFIs) range from antagonism to mutualism [2,3]
The screening tests of co-growth showed that Czapek yeast agar (CYA) and tryptic soy agar (TSA) buffered at pH 6.0 resulted the most suitable agar substrates for the growth of both bacterial and fungal strains
Most of the bacterial and fungal strains used in mixes showed typical macro- and micromorphology where the two strains separately persisted without inhibition, or with reciprocal confinement sometimes developing slower than expected

Summary

Introduction

Bacteria and fungi are simultaneously present in a wide variety of environments [1]. These organisms are capable of coexisting, and of actively interacting and the bacteria-fungal interactions (BFIs) range from antagonism to mutualism [2,3]. Bacteria and fungi affect their microenvironment by producing secretes and metabolites to compete with each and/or other organisms [2,3]. They can live in synergistic communities, cooperating to survive and thrive together [1,4,5,6]. BFIs play a key role in most of natural ecosystems, driving biogeochemical cycles, contributing to both the health and diseases of plants and animals [2], as well as degrading and inactivating pollutants [7]

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