Activity throughout the lichen phylogeny indicates a focus on regulation of specialized metabolites

Abstract

Lichens are complex multi-microorganismal communities that have evolved the ability to share their thalli with a variety of microorganisms. As such, the lichenized fungus becomes a scaffold for a variety of microbes and occasionally insects. Lichens are known to produce a plethora of unique specialized (secondary) compounds that demonstrate biological activities, including antibacterial, antifungal, antiviral, and antioxidant, that may provide protection from harmful microbes. The longevity of lichens and their robustness, despite a close association with diverse microbes, provides an interesting study system to view the role of specialized metabolites in managing a microbial community. The objective of this study was to identify the effects lichens may have on basic functions of fungi in and on the lichens. We tested chemical extracts from lichen species across the phylogenetic tree for their effects on sporulation, hyphal growth and specialized metabolite production, using two well-studied mycotoxigenic fungi (Aspergillus parasiticus (aflatoxin) and Fusarium graminearum (trichothecenes) whose functions are easily observed in culture. By far the most prevalent activity among the 67 lichens we tested were effects on accumulation of fungal specialized metabolites, which appeared in 92% of the lichen species analyzed across the phylogeny, although the lichen extracts were also active against fungal sporulation (31%) and growth (12%). The consistent presence of this regulatory activity for specialized metabolism indicates this is an important aspect of lichen integrity. Interestingly, inhibition of accumulation of products of the aflatoxin biosynthetic pathway was the predominant activity, whereas increased accumulation versus decreased accumulation of the production of trichothecenes were about equal. This suggests multiple mechanisms for addressing fungal processes. We performed microbiome analysis of four lichen species and identified oomycetes as members of the microbiomes. Although a small sample size was used for comparing microbiomes, the lichen species exhibiting lower effects on the test fungi had a higher number of OTUs. Members of the lichen community may manipulate specialized metabolism of the essential and transient fungal members and thus attenuate negative interactions with the incumbent fungi or, alternatively, may support the production of compounds by beneficial fungal partners. The ability to control the microbiome by specialized metabolites as opposed to controlling by reducing sporulation of growth, can be effective, discerning, and energetically thrifty, allowing the microbiome members to be controlled without being invasive. Elucidating the role of specialized metabolites in the mechanisms underlying lichen assembly and function has important implications for understanding not only lichen community assembly but for revealing the fundamental processes in microbiota in general.