Contribution of fungi to primary biogenic aerosols in the atmospherewet and dry discharged spores, carbohydrates, and inorganic ions

Picture a tropical rainforest at night. The canopy is dripping, the air is thick with humidity, and invisible to anyone standing there, a blizzard of fungal spores is launching itself into the dark. Not drifting, but launching. Propelled by a surface-tension catapult, each spore is flung free by a single merging droplet of liquid. Elbert, Taylor, Andreae, and Pöschl found that those spores may account for nearly half of all coarse airborne particles over the Amazon during the wet season. The atmosphere above a rainforest is alive with fungal traffic, and until this work, almost none of it had been measured. Primary biogenic aerosol particles, or PBA, are particles emitted directly from living organisms rather than formed chemically in the air. This category includes pollen, bacteria, plant fragments, viruses, and fungal spores, along with the carbohydrates and ions those particles carry. They matter because they scatter and absorb sunlight, seed clouds, affect rainfall, carry allergens and pathogens across continents, and transport living organisms to new habitats. Despite this reach, Elbert and colleagues stressed that the abundance and origins of many PBA components were poorly quantified. Fungi, they argued, were the most conspicuous gap. Two groups sit at the center of this study: actively wet spore-discharging Ascomycota, which the paper calls AAM, and actively wet spore-discharging Basidiomycota, referred to as ABM. These are not marginal organisms. Ascomycota include the molds and cup fungi living on dead wood, soil, and lichens. Basidiomycota include the roughly thirty thousand known species of mushrooms, bracket fungi, rusts, smuts, and basidiomycetous yeasts. Both groups share a remarkable trick: they shoot their spores into the air using liquid. The mechanism is known as Buller's drop, and it is worth picturing carefully. On the surface of a basidiospore, a tiny droplet forms at the base while a thin liquid film forms at the opposite end. At high relative humidity, both grow by pulling water vapor from the air. When the drop and the film meet, surface tension snaps them together. That merger converts surface energy into kinetic energy — a catapult — and the spore is launched, sometimes enveloped in liquid, distances of about 0.1 to 1.5 millimeters in still air. For Ascomycota, the mechanism is different but equally forcible: the ascus, a pressurized sac, bursts through a narrow aperture and ejects spores and droplets distances of one to several hundred millimeters. Both mechanisms work without wind. That is the key point. These fungi put particles into calm air, which means they are especially effective at night when humidity rises and winds drop. To measure exactly how effective, Elbert and colleagues drew on a field campaign at Balbina, in the Brazilian Amazon state of Amazonas, carried out in July 2001. Graham and colleagues collected air samples with rotating impactors and jet impactors mounted two meters above the ground, as well as stacked filter units and high-volume samplers. Two detailed microscopy samples were taken on consecutive nights and days and examined at up to one thousand five hundred times magnification, with spore types identified morphologically across two hundred fields of view per sample. The counts were striking. Actively wet-discharged basidiospores reached about one thousand eight hundred per cubic meter during the day and roughly twelve thousand eight hundred per cubic meter at night. Ascospores went from about three thousand by day to around seven thousand four hundred at night. Total wet-discharged spores averaged more than twelve thousand per cubic meter across the campaign. Converting those counts to mass and using literature values of sixty-five picograms per basidiospore and two hundred picograms per ascus, the team calculated spore mass concentrations of roughly seven hundred ten nanograms per cubic meter during the day and around two thousand three hundred nanograms per cubic meter at night. Compared to total measured coarse particulate matter at Balbina, wet-discharged fungal spores account for about twenty-five percent of coarse particles by day, forty-five percent at night, and thirty-five percent on average. That nocturnal doubling is exactly what Buller's drop biology predicts: high humidity at night drives liquid-dependent discharge, while dry wind-dispersed spores peak during the warm, breezy day. But counting spores under a microscope is labor-intensive and spatially limited. What Elbert and colleagues needed was a molecular fingerprint — a chemical measurable in bulk aerosol samples that could stand in for a spore count. The sugar alcohol mannitol turned out to be that fingerprint. Mannitol is abundant in fungal tissue, measurable with standard analytical chemistry, and concentrated in the coarse particle fraction where spores live. In Amazonian air, measured mannitol concentrations ran from about ten to sixty-eight nanograms per cubic meter. Using spore counts and literature values for mannitol content per spore, the team estimated that basidiospore emissions alone could account for all of the nighttime mannitol, about thirty-five percent of the daytime mannitol, and roughly eighty percent of the twenty-four-hour average. The diurnal pattern matched: mannitol peaked at night, just as basidiospore counts did. Potassium told the complementary story for Ascomycota. Measured potassium in the one to ten micrometre size range ran from about seventeen to forty-three nanograms per cubic meter in the Balbina samples. The ascospore-based emission estimates yielded potassium contributions of seventeen nanograms per cubic meter by day, forty-three by night, and about thirty on average — accounting for roughly sixty percent of ambient potassium at Balbina and, at some Amazonian sites, essentially all of the coarse-fraction potassium. Again, the diurnal cycle aligned: potassium peaked at night, consistent with nocturnal ascospore discharge. Glucose and fructose introduced a puzzle. The amounts suggested by spore chemistry implied a significant fungal source, but the diurnal pattern was wrong — these hexose sugars peaked sharply during the day, up to fifty times higher than at night in some datasets. Budget calculations showed that basidiospore-related hexose emissions would exceed measured nighttime hexose concentrations by a factor of about ten, yet would account for only about ten percent during the day. Something else is producing daytime glucose and fructose. Mannitol and potassium work as tracers; hexose sugars, at least for now, do not. With those tracers validated, Elbert and colleagues scaled up. They took average mannitol concentrations reported for extratropical continental boundary layer air — about twenty-five nanograms per cubic meter — divided by five picograms of mannitol per basidiospore, and arrived at roughly five thousand basidiospores per cubic meter as a global average. Multiplying by spore mass and working through a column height of about one thousand meters and a residence time of roughly one day, they derived an emission flux of around sixty spores per square meter per second. Applied to the global land surface area, that gives a first estimate of about seventeen teragrams per year for actively wet-discharged basidiospores alone. Adding other fungal spore types, wet and dry discharged, the total sits at roughly fifty teragrams per year. To put that in context: contemporary estimates for anthropogenic primary organic aerosol ran to about forty-seven teragrams per year, and biogenic secondary organic aerosol — the kind formed chemically in the atmosphere — ranged from twelve to seventy teragrams per year. Fungal spores, previously treated as background noise, sit in the same league. The marine fungal source, by contrast, is negligible — on the order of ten tonnes per year. This is a continental land-surface phenomenon, and it is consistent with basidiospore concentrations of one thousand to ten thousand per cubic meter measured at other locations around the world, providing independent validation of the global estimate. The climate implications are real but still tentative, and Elbert and colleagues are careful to flag them as such. Coarse biogenic particles in the one to ten micrometre range are the right size to act as cloud condensation nuclei and ice nuclei — the seeds around which cloud droplets and ice crystals form. If fungal spore emissions influence cloud formation and precipitation, there is a potential feedback: forests produce spores, spores seed clouds, clouds water forests. Changes in land use, warming temperatures, or rising carbon dioxide could alter fungal emissions in either direction, tightening or loosening that loop in ways that are currently impossible to quantify. To get there, the authors call for vertical profiles, long-range transport measurements, and advanced molecular biological analyses that can identify fungal contributions without relying on manual spore counts. What this study established, clearly and for the first time, is the scale. The atmosphere above a tropical rainforest is not a passive medium through which spores occasionally drift. It is a recipient of continuous, mechanically driven, biologically timed fungal emissions — emissions that carry recognizable chemical signatures, follow predictable diurnal rhythms, and collectively rival industrial aerosol sources in annual mass. We have been breathing this for as long as there have been forests. We are only now beginning to measure it properly. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.