Ecological implications of microscopic leaf wetness on the phyllosphere

Microscopic wetness occurs in many microbial habitats including the phyllosphere – the above ground parts of plants – which is a huge microbial habitat of global importance. There is increasing evidence that during daytime, microscopic wetness in the form of thin films and droplets, invisible to the naked eye, persist on leaf surfaces worldwide. This wetness results from deliquescence of hygroscopic aerosols that are ubiquitous on leaves. Little is known on the impact of such microscopic wetness on the ecology of leaf bacteria. Here we study how such microscopic wetness affects central features of microbial life, including cells’ survival. We developed an experimental system that create microscopic wetness on artificial surfaces. Using advanced microscopy and image processing we analyzed microscopic wetness, bacterial self-organization and survival. We revealed that stable microscopic droplets are formed around bacterial aggregates on a drying surface under moderate relative humidity, due to pinning and deliquescence. Notably, droplet size correlated with aggregate size, and cell survival was higher within larger droplets. Similar results were observed for >15 species, two of them – Pseudomonas fluorescens and P. putida – studied in depth here. The formation and retention of microscopic droplets around bacterial aggregates during daytime are likely central features of the leaf surface as a microbial habitat and key for bacterial survival on plant leaf surfaces.