Establishing microbial communities to promote the growth of <i>Pleurotus ostreatus</i> through a top-down approach is hindered by the dominance of antagonistic interactions

Few attempts have been made to examine the diversity and functions that characterize microbial communities compatible with and beneficial to the biotechnologically relevant mushroom <i>Pleurotus ostreatus</i>. The quest for complementarity is complicated by the variable nature of beneficial traits, impairing the rational assembly of synthetic communities to improve bioprocesses. This study investigated whether the compatibility between <i>P. ostreatus</i> and microbial enrichments is lessened in conditions favoring saprophyte metabolism and is enhanced when a combination of recalcitrant and labile carbon is integrated in the enrichment. The microbial diversity of enrichment cultures and substrates colonized by <i>P. ostreatus</i> was analyzed through PCR amplicon sequencing, and the proliferation of <i>P. ostreatus</i> was assessed by quantitative image analysis. In general, co-inoculation of lignocellulosic substrate with microbial enrichment reduced the growth of <i>P. ostreatus</i>. The saprophytic enrichment conditions were more conducive to the development of antagonistic communities inhibiting <i>P. ostreatus</i> growth than enrichment cultures integrating labile carbon substrates. Both microbial community analyses and <i>in vitro</i> assays led to the identification of a single phylotype affiliated with <i>Brevundimonas</i> spp., which displayed neutral interaction with <i>P. ostreatus</i>. Recalcitrant and labile carbon degradation functions were not primary factors driving beneficial microbial communities for <i>P. ostreatus</i>. Additional functions beyond carbon metabolism are likely to promote beneficial interactions. Directed enrichment cultures, integrating the mushroom in the earliest stage of the procedure, are expected to promote more beneficial interactions than top-down approaches.IMPORTANCELignocellulosic biomass upcycling biotechnologies integrating solid-state fermentation by fungi are aligned with sustainable development perspectives. While the recalcitrance of this biomass imposes a challenge for the implementation of these bioprocesses converting the lignocellulosic feedstock into bioenergy and bioproducts, pretreatment of lignocellulose biomass with fungi is efficient and generates fewer by-products than chemical approaches. Optimization and stabilization of this bioprocess by integrating microbial consortia has received little attention. The significance of our research is to bridge that knowledge gap by examining how interactions between the biotechnologically relevant basidiomycete <i>Pleurotus ostreatus</i> and microbial communities influence fungal growth in lignocellulosic substrate. Directed enrichment cultures integrating <i>Pleurotus ostreatus</i> as a selective agent are expected to trigger more beneficial interactions promoting mushroom growth than our top-down approaches, due to a dominance of antagonistic mushroom-bacteria interactions.