Unveiling the Microbial Diversity and Associated Secondary Metabolism on Black Apples

Black apples are the late-stage microbial decomposition of apples after having fallen to the ground. This phenomenon is highly comparable from year to year, with the filamentous fungus Monilinia fructigena most commonly being the first invader, followed by Penicillium expansum. Motivated by the fact that only little chemistry has been reported from apple microbiomes, we set out to investigate the chemical diversity and potential ecological roles of secondary metabolites (SMs) in a total of 38 black apples. Metabolomics analyses were conducted on either whole apples or small excisions of fungal biomass derived from black apples. Annotation of fungal SMs in black apple extracts was aided by cultivation of 15 recently isolated fungal strains on 9 different substrates in an OSMAC approach, leading to identification of 3319 unique chemical features. Only 6.8% were attributable to known compounds based on analysis of HPLC-HRMS/MS data using spectral library matching tools. Of the 1606 features detected in the black apple extracts, 32% could be assigned as fungal-derived, due to their presence in the OSMAC-based training dataset. Notably, the detection of several antifungal compounds clearly indicates the importance of such compounds for invasion of and control of other microbial competitors on apples. In conclusion, the diversity and abundance of microbial SMs on black apples was found to be much higher than that typically observed for other environmental microbiomes. Detection of SMs known to be produced by the six fungal species tested also highlights a succession of fungal growth following the initial invader M. fructigena. ImportanceMicrobial secondary metabolites constitute a significant reservoir of biologically potent and clinically valuable chemical scaffolds. However, their usefulness is hampered by rapidly developing resistance, resulting in reduced profitability of such research endeavours. Hence, it is vital that the ecological role of such microbial secondary metabolites be considered to understand how best to utilise such compounds as chemotherapeutics. Here, we explore an under-investigated environmental microbiome in the case of black apples; a veritable "low-hanging fruit", with relatively high abundances and diversity of microbially produced secondary metabolites. Using both a targeted and untargeted metabolomics approach, the interplay between metabolites, other microbes and the apple host itself was investigated. This study highlights the surprisingly low incidence of known secondary metabolites in such a system, highlighting the need to study the functionality of secondary metabolites in microbial interactions and complex microbiomes.