The effect of ambient air pollution on bacterial and fungal communities in the phylloplane of Hedera helix and their implications in degradation of airborne pollutants

Abstract

Ambient air pollution is one of the serious problems our world is currently facing, exerting wide-ranging and deleterious effects on the environment and on our health. It is most pronounced in high-traffic environments such as urban areas where substantial quantities of airborne pollutants, including particulate matter (PM) and volatile organic compounds (VOCs), are generated. While technological improvements and stricter emission regulations have contributed to a decrease in traffic-related emissions of airborne pollutants, current emission abatement strategies are not sufficient to cope with the global threat of air pollution and there remains a strong demand for effective solutions to be developed. This doctoral research contributes to meet this demand by exploring the use of plants and their associated microorganisms to detoxify airborne pollutants. We provide an indepth analysis of the phylloplane microbiome associated with Hedera helix (ivy), an evergreen plant known for its hardiness and climbing ability, including a comprehensive study of the influence of ambient air pollution on the leaf communities. We present a thorough taxonomic description of the microbial communities living on H. helix leaves, which we show to be dominated by the phyla Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Ascomycota, and Basidiomycota, and the genera Hymenobacter, Sphingomonas, Pseudomonas, Methylobacterium, Massilia, Pantoea, Cryptococcus, Davidiella, Mycosphaerella, Sporobolomyces, Kabatiella, and Rhodotorula. We also evaluated the culturability of this habitat using five different growth media, and present a collection of microbial phylloplane isolates underrepresented in current databases, including the characterization of PGP profiles, trace metal resistance, and pollutant degradation potential. In a pilot phase, we started with investigating if ambient air pollution has the potential to influence the structure and function of bacterial communities in the phylloplane of H. helix. Our results indicated that ambient air pollution increases the metabolic versatility of microbial leaf communities and the prevalence of bacterial functional traits affiliated to an increased fitness in a polluted environment. Continuing on these results, we designed a field study with a duration of two years where we sampled a total of 4,320 H. helix leaves at six locations exposed to different ambient air pollution conditions. We performed eight sampling events, one in every season, from autumn 2017 until summer 2019. Daily we monitored ambient black carbon (BC), PM2.5, PM10, nitrogen dioxide, and ozone concentrations and found that ambient air pollution led to a 2- to 7-fold increase of BC that was present on the sampled leaves, the phylloplane BC load. Our results further indicated this phylloplane BC load strongly correlates with the diversity of bacterial and fungal H. helix leaf communities, impacting this diversity even more than seasonal effects and suggesting that ambient air pollution significantly contributes to the shaping of bacterial and fungal ivy leaf communities. Further, we focus on the phylloplane-associated Microbacteriaceae genera Curtobacterium, Rathayibacter, Frigoribacterium, and Frondihabitans in a genus-level pangenomic approach and propose updated phylogenies. We show that phylloplane bacteria have primarily invested in genomic features related to interactions with their environment, among which osmoregulation, ion transport, uptake and metabolism of various nutrients, and signal transduction, reflecting key adaptations to bacterial life in the harsh conditions of the phylloplane. Lastly, we present six phylloplane microorganisms that were isolated from an air-polluted environment and able to metabolize diesel fuel. Three of these isolates, Bacillus licheniformis VSD4, Pseudomonas sp. VS38, and Rhodotorula sp. VS67, also degraded the fossil fuel-related VOCs benzene, toluene, and/or xylene (BTX), and constitute promising candidates as inoculants towards (phyllo)remediation applications.