Rain­drops Keep Falling On Their Heads

(Thanks To Mush­room Spores, That Is)

by Elio

Fig­ure 1. Lac­tar­ius hy­grophoroides, a mush­­room in­cluded in this study. I fre­quently found spec­i­mens of this de­li­cious crunchy mush­room in a woods near where I lived in New­ton, MA. Source

I have two pur­poses in dis­cussing a re­cent pa­per from the lab of my­col­o­gist Nik Money. One is to present a no­tion of the way mush­room spores may con­tribute sig­nif­i­cantly to rain mak­ing. The other one is in­tro­duce a note­wor­thy wrin­kle in elec­tron mi­croscopy (pre­vi­ously un­known to me), namely the abil­ity to look at wet ob­jects un­der the elec­tron beam. These two top­ics com­bine in this pa­per.

Mush­room spores are re­leased into the at­mos­phere every day by the bil­lions. In weight, this amounts to 50 mil­lion tons per year or, if you wish, about 1000 spores per square mil­lime­ter of the earth's sur­face. To­gether with pollen, bac­te­ria, and other bi­o­log­i­cal par­ti­cles, they serve as nu­clei for cloud for­ma­tion and, there­fore, rain (we leave the in­trigu­ing story of at­mos­pheric bac­te­ria for an­other time). Fun­gal spores come in dif­fer­ent sizes but a di­am­e­ter of 5 ‒ 8 µm is rather typ­i­cal. This size is small enough for the spores to be car­ried aloft even by gen­tle waft­ing breezes. Mea­sure­ments made in Brazil show that fun­gal spores ac­count for about 35% of the to­tal par­ti­cles of this size range. The fig­ure is higher and varies less dur­ing the year for the heav­ily forested trop­ics than else­where.

Fig­ure 2. Dis­tri­b­u­tion of fun­gal phyla re­pre­sen­ted by DNA in sam­ples of air­borne fungi. Source

The at­mos­phere may be a thin soup all right, but enough ma­te­r­ial can be gath­ered for analy­sis us­ing high ca­pac­ity fil­tra­tion tech­niques and long sam­pling times (one week is typ­i­cal). In­ter­est­ingly, in the sam­ples so col­lected, man­ni­tol can be used as a suit­able bio­marker for the pres­ence of fun­gal spores and, of course, so can DNA. In fact, the DNA analy­sis shows that about 2/3 of the fun­gal spores are from Ba­si­dio­my­ce­tes (the ma­jor­ity of mush­rooms) and about 1/3 from As­comycetes (which in­clude morels and some other mush­rooms); see here. So, mush­rooms are in­deed the largest source of fun­gal spores in the at­mos­phere. The species found are quite di­verse and in­clude al­ler­genic ones (not a sur­prise for peo­ple suf­fer­ing from such al­ler­gies) and hu­man and plant pathogens (e.g., Can­dida, Puc­cinia or ce­real rust).

It doesn't make much dif­fer­ence here, but mush­room spores are made sex­u­ally, those of mold largely asex­u­ally. And their mech­a­nisms of re­lease into the air are dif­fer­ent. Asex­ual spores ‒ coni­dia they're called ‒ are re­leased pas­sively, that is, they sim­ply de­tach from the hy­phae that make them and are lim­ited in their dis­per­sal. Sex­ual spores, on the other hand, are forcibly ejected from their ma­ter­nal cells and are im­me­di­ately swept out by air cur­rents.

Fig­ure 3. The tra­jec­to­ries of spores be­ing ejec­ted from the gills of a Ba­si­dio­my­ce­te mush­room. No­tice the sharp break in the curve (so­me­thing a base­ball pitcher would dream of!). From AHR Buller, Re­searches on the Fungi, Vol. II, 1922.

The mech­a­nism of spore re­lease in both the Ba­si­dio­my­ce­tes and the As­comycetes de­fies the imag­i­na­tion. Ba­si­dio­spores, as they're called, are ejected from the "gills" or pores found on the un­der­side of mush­rooms with an as­tound­ing force of over 10,000 x g and an ini­tial ve­loc­ity of close to 2 meters/second! Of course, the ini­tial ve­loc­ity is sus­tained for a very brief time, or else the spore would hit the op­po­site gill. In fact, the tra­jec­tory is in­ter­est­ing: the spores travel hor­i­zon­tally for a short dis­tance, they then "hit a wall", when their mo­men­tum can­not over­come the vis­cos­ity of air. At this point, they drop ver­ti­cal­ly, to be caught by air cur­rent and car­ried over a long dis­tance. The mech­a­nism of re­lease de­pends on the for­ma­tion of a wa­ter droplet at the base of the still at­tached ba­sidiospore. When this droplet in­creases in size due to the se­cre­tion of hy­drophilic sub­stance such as man­ni­tol, it even­tu­ally col­lapses, trans­fer­ring its cen­ter of grav­ity out­wards. This im­pels the spore to take off at prodi­gious ‒ if sus­tained ex­tremely briefly ‒ speeds.

A de­tour into the As­comycetes. Al­though they do not tend to make as many large fruit­ing bod­ies – i. e., mush­rooms – their spore dis­charge mech­a­nisms can be even more spec­tac­u­lar. Im­pelled by pres­sur­iza­tion, the spores of Pi­lobo­lus can travel up to 2.5 me­ters. Those of Sphaer­obo­lus, aptly called the can­non­ball or ar­tillery fun­gus, im­pel their spher­i­cal spores to travel over a 6 m. hor­i­zon­tal and 2 m ver­ti­cal dis­tance. The force is sup­plied by tur­gor built up by an in­crease in os­motic pres­sure in the spore-bear­ing tis­sue by con­ver­sion of glyco­gen into sug­ars. But over­all, spore dis­charge over dis­tances is more com­mon among the ba­sidios than the as­cos.

Nice, but what does this have to do with rain for­ma­tion? The pro­posal from Money's lab is an ex­pla­na­tion for the way fun­gal spores make for nu­clei ef­fec­tive in the for­ma­tion of rain­drops. Be­fore con­sid­er­ing the pos­si­ble mech­a­nism, let's do some num­bers. Are there suf­fi­cient spores in the at­mos­phere to make a dif­fer­ence? The an­swer is yes. Fun­gal spores are dis­persed every year to the tune of 50 mil­lion tons, enough to cover each square mm of the planet with 1000 spores. They are not dis­trib­uted evenly, with forested re­gions ac­count­ing for the greater share of the pro­duc­tion. So, the num­bers seem to add up.

Fig­ure 4. Schematic of the busi­ness end of an ESEM. Source

How to show how fun­gal spores make rain? The in­ves­ti­ga­tors mea­sured the amount of mois­ture that ac­cu­mu­la­tes on fun­gal spores at dif­fer­ent lev­els of mois­ture. For this, they used a not so novel but rel­a­tively un­der­used (and to me, pre­vi­ously un­known) vari­a­tion on the elec­tron mi­cro­scope, namely one that al­lows to look at wet ob­jects. Called the En­vi­ron­men­tal Scan­ning Elec­tron Mi­croscopy (ESEM), this ma­chine over­comes the prob­lem of con­ven­tional EMs, namely that the ob­jects have to be de­hy­drated. A web­page by its main de­vel­oper, GD Da­ni­la­tos is found here. In the ESEM, the spec­i­mens are placed in a small cham­ber that can be vented with gases, in­clud­ing wa­ter va­por. For this pur­pose, the EM needs con­sid­er­able mod­i­fi­ca­tion with re­gard to pumps and the de­tec­tion sys­tem. It is com­mer­cially avail­able, ap­par­ently not cheap. Mi­crobes have been stud­ied with the ESEM (see here) but best I can tell, it has only had lim­ited use in mi­cro­bi­ol­ogy. But this pa­per does it jus­tice.

Fig­ure 5. Droplet for­ma­tion on ba­sidiospore of Suil­lus bre­vipes in the ESEM at 101% RH.(A‑D) Spores are ori­en­tated with adax­ial sur­face fa­cing away from sur­face of the spec­i­men stub, al­low­ing ob­ser­va­tion of droplets grow­ing from this hy­gro­scopic re­gion of the spore. Note mer­ger of droplets in sec­ond and third pan­els. Scale = 5 μm. Source

The au­thors col­lected 8 species of mush­rooms and placed the spores in the ESEM and al­tered the rel­a­tive hu­mid­ity (RH) in the spec­i­men cham­ber. Droplets of wa­ter formed on the spores at RH val­ues of 101 and 102%. In­ter­est­ing is that these droplets formed pre­cisely on the sites on the spore where they are seen be­fore be­ing dis­charged, namely the pe­dun­cle where the spore sticks to its mother cell and on its ad­ja­cent (adax­ial) face. The drops evap­o­rated when RH was low­ered be­low 100%, only to reap­pear when RH was once again increa­sed. In time, the wa­ter droplet on the face of the spore reached large di­men­sions (13 µm). The au­thors also looked at the be­hav­ior of spores of ba­sidios such as puff­balls that do not forcibly dis­charge them. Here, the wa­ter makes a thin shell around the spores with no spe­cific lo­cal­iza­tion, sug­gest­ing that there are dif­fer­ent ways whereby spores in­duce wa­ter droplets to form.

Does this ben­e­fit the mush­rooms? The au­thors state: "There is no adap­tive sig­nif­i­cance to the pu­ta­tive ef­fect of spores on cloud for­ma­tion. It is a con­se­quence of the dis­per­sal mech­a­nism that hap­pens to ben­e­fit the fun­gus be­yond its ef­fec­tive­ness at dis­trib­ut­ing spores. If changes in cli­mate re­duce rain­fall in trop­i­cal ecosys­tems, the re­sult­ing in­hi­bi­tion of fun­gal growth and spore re­lease may ex­ac­er­bate the fre­quency of droughts through this un­ex­pected feed­back loop". I guess one could ar­gue whether this feed­back loop has adap­tive sig­nif­i­cance. Let me put it this way: No rain, no mush­rooms!