Abstract
Millions of tons of fungal spores are dispersed in the atmosphere every year. These living cells, along with plant spores and pollen grains, may act as nuclei for condensation of water in clouds. Basidiospores released by mushrooms form a significant proportion of these aerosols, particularly above tropical forests. Mushroom spores are discharged from gills by the rapid displacement of a droplet of fluid on the cell surface. This droplet is formed by the condensation of water on the spore surface stimulated by the secretion of mannitol and other hygroscopic sugars. This fluid is carried with the spore during discharge, but evaporates once the spore is airborne. Using environmental electron microscopy, we have demonstrated that droplets reform on spores in humid air. The kinetics of this process suggest that basidiospores are especially effective as nuclei for the formation of large water drops in clouds. Through this mechanism, mushroom spores may promote rainfall in ecosystems that support large populations of ectomycorrhizal and saprotrophic basidiomycetes. Our research heightens interest in the global significance of the fungi and raises additional concerns about the sustainability of forests that depend on heavy precipitation.
Highlights
Million of tons of fungal spores are dispersed in the atmosphere every year [1]
Water condensing on the spore forms large droplets when RH is increased to 102% RH, and shrinks when it is dropped below 100% RH, appearing as a thin film on the spore surface before complete evaporation
Drop expansion on the hilar appendix of spores in the Environmental scanning electron microscopy (ESEM) resembles the process that creates the Buller’s drop in the seconds before spore discharge from mushroom gills (Fig 3). The regrowth of this discrete drop is only visible in spores that are oriented with the hilar appendix pointing away from the surface of the stub on which the spores are deposited
Summary
Million of tons of fungal spores are dispersed in the atmosphere every year [1]. An individual gilled mushroom can release 30,000 basidiospores every second, corresponding to a daily output of billions of microscopic particles [2]. Basidiospores are discharged from the gill surfaces by a catapult mechanism powered by the rapid movement of a drop of fluid over the spore surface (Fig 1). This fluid is called Buller’s drop in tribute to “the Einstein of Mycology,” A. The merger of Buller’s drop with this second volume of fluid (the adaxial drop) causes a rapid displacement of the center of mass of the spore. This fluid motion, driven by surface tension, imparts momentum to the spore and it is PLOS ONE | DOI:10.1371/journal.pone.0140407. This fluid motion, driven by surface tension, imparts momentum to the spore and it is PLOS ONE | DOI:10.1371/journal.pone.0140407 October 28, 2015
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