(Reprill ted from Nature, Vol. 246, No. 5433, pp. 416-417, December 14, 1973)

Distribution of Microorganisrns in Hailstones LARGE numbers of microorganisms a re present in the residues of hailstones, together wi th otber insoluble par­ticles . As the hailstone is tbe surviving testimony of the life history of a storm a nd as the organic ma teria l can be considered a na tu ral 'tracer' of the ingested ai r mass, it is natural to investigate whether any information a bout the conditions in which hails tones grow can be deduced from the internaI distribution of microorganisms.

Cloud physicists a re in fact in search of new parameters with which to determine the growth conditions in the cloud solely from an examination of the hailstones collected. Aerobiologists, on the ot her ha nd , a re interested in the aerosolisation, diffusion in tbe a tmosphere, transport and removal of aerospora; in this cycle the process of in-cloud scavenging a nd washout by precipitation elements plays an important role for particles of low fall veloc ity (~0.3 cm S-'

for a lO fLm particle of density l g cm-a). Thus there a re tWO go od reasons for investigating the presence of micro­organisms in precipitations. Surprisingly, however , the subject remained almost unstudied in detail even though it wa discovered a long time ago"

As former work dealt with the melting of complete hailstones' our interest centred on the local distribution of germina ting microorganisms ins ide hailstones relative to the internai features. N on-germinati ng particles (pollen a nd fungal spores) have also been identified a nd counted with a microscope. We chose for the test a giant ellipsoidal ha ilstone, largest dimension about 7 cm, whicb was collected during the summer of 1969 near Garden Ci ty, Ka nsas.

Fig. t Hailstone slice, as seen in reflected lighl, showing the sequence of opaque-bubbly and transparent layers.

I ° \ ..

o •

\ ••• o

\ • °

\ '\, o ~ 0 ,::1 o .

"o oOo ,o n oo , 0 o -., o

......... \0 O • o~ ,/ ........ f ____

-- - -- -lc m

Fig, 2 Distributi on of baclerial and fungal co lonies grown on lhe concentric ser ies of nllers : lhe black disks represent colonies grown 24 h later than those represenled by empty circ1es. The map has been superimposed on the oUllines drawn from Fig. l and shows layers of opaque (M) and Iran parent (T) ice. The halched circumference dennes lhe area thal has been examined.

A slice 2 mm thick was sawn off the mai n section and prepa red with a microtome. All operations, including working, melting a nd filter prepa ration, were ca rried out in sterile conditi on . The slice was then photographed in crossed polaroids and reftected light. The external layer was washed with sterile water to eliminate possible con­tamina tion and the slice was placed on a concentric a rray of membrane filters (pore size 0.45 fLm) of different dia meters in a sterilised filte r holder (diameter 142 mm) connected to a vacuum p ump.

The particles deposited on the filters mainta ined thei r originai locations on account of the very slow melting of the bottom of the slice, The filters were then separated a nd placed in different Petri dishes containing a nutritive medium' a nd incubated a t 24 0 C. Oth'er s lices of the same hai lstone were melted and filtered, a nd the residues observed with the microscope. The res ults are shown in Figs I to 3. Fi gure I is a photograph of the s lice in reftected light , which indica te the alternating sequence of opaque-bubbly and transparent layers. A map of the colony distribution has been superimposed (Fig. 2) on the outline of the interna i features of the ha il tone as deduced from Fig. I. A graph of the radiai distribution of micro­organism concentration for the different layers of the stone is shown in Fig. 3. There is a generai decrease in the concentra tion of colonies towards the centre of the slice. More surpri si ng, however, i lhe rema rka ble correspondence with the interna i fea tures: in the opaque layers there is only one-third to one-half of the colonies found in the transparent layers.

The increase with radi us of the number of germinating colonies is in agreement with the results of Rosi nsk i' for gia nt ae rosol particles in different hailstones. Possible explanations of this re ult a re in terms of the increase of

80

\ 60 u

~

" '" o "O u 40

20

o

....--

-

-

-

40

T

....-

~ -

""- '--

O 30 20 lO I lO 20 30 40 - - --M T M T

Dislance (mm) Fig. 3 Histogram or lhe distribution or colonies grown on the

areas M and J;.represenled in Fig. 2. ./:.:ç~::.:- .~ .... _~ ..

l ~ ~ '< .. ' r -~.. " ~'.\~

particle coocenùa tiQn ~{ ·. (h~ .accretipg droplets a nd the intense evaporatioli" bf· thé h-ailstone, w,bich is much wa rmer than its envìro~mèn.t durifig th,e . fili;l stage of growth. [t seems more -ditITçUlt>',however, (o ·illterpret the correspond­ence of the colori'y ·. é:òAcenttatio"rr I with the sequence of layers. T he lowei ; cli::_nsi~y ' of,~ (he opaque-bubbly layers cannot alone expla in the lower content of colonies. In fact measurements of local density in hailstonesS show, for this type of hailstone, a maximum reductioo of 10 % in density due to the presence of ai r bubbles. 00 the other hand , the high colIision efficiency of large cloud droplets for particles of the size of ai rborne microorganisms suggests that organic material is transferred to the hailstone more through capture of droplete than through direct aero­dyna mic capture. If so, the opaq ue layer and the lower par~cle content would both have been consequences of

accret ion with a narrower spectrum of cloud droplets. The possibil ity that the variat ions in pa rticle concentratioo are due to varia tion in the evaporation rate during the ha ilstone growth cannot, however, be excluded. Rejecti on of particles by the advancing ice-water interface during the final freezing is important only in very spongy growth , which is not the case fo r the hailstone examined.

lt seems, however , that a necessary fea ture of further investiga tions wilI be the measurement of the aerospora profile in the environmental a ir together wi th knowledge of the circulation pattern of the storm, especially concerning the updra ft levels.

As fa r as the identificat ion of germi na ted colonies is concerned, only 3 % a re fungi (two colonies of Ascomycetes, one of which is a Pell icillilll11 sp., a nd three colonies of Moniliales, one of which is a Cladosporium sp.); the remai nder is made up of a single type of Gram-posit ive st reptococcus.

The aerospora composition seen only at the microscope of other sli ces from the same hai lstone is quite different : AllemariCl, 30 % : Ciadosporilllll , 9 0/,, ; TondCl , 1% ; olher Dematiaceous fungi, 26 % : basidi ospores, 4 0/,, : olher spores, 23 % : Pinlls pollen , 5°:, ; a nd other pollen grai ns, 2%. The concenlration of.l he total aerospora is four limes that of Ihe germinating colo nies. Apart from morlality due to stress inside the hailstorm , some of these organisms cou ld have been damaged dur ing long storage in the freezer, including the ai r Iravel lo Eu rope in a dry-ice con tai ner. This is con­firmed by the observalion thal the incubation time before germinalio n was longer for this tha n for other 'fresh' hail­stones (4 IO 5 d instead of 1 lo 2 d).

h 'Ii/1I1O di BO/CInica, Via Imerio, 42, Unive rsi/ii di Bologna

Osserva/orio Scien/ifìco Sperill1enlale, IFA (CNR) Torricella No.2, Verona

Received AuguSI 13, 1973 .

I Bujwid, O., Zenrbl. Bakl., 3, 1 (1888).

P. MANDRIOLI

G. L. PUPPI

N. BAGN I

F. PRODI

Dubois, R., AI/I/Is Soc. lil/I/. LYOII, 64, 45 (1918). Bagni, N., Mandrioli, P.,'and Slabellini, G., Ecologio , 7,23 (1972).

• Rosinski, l ., J . Appl. Mel ., 5, 481 (1966). 5 Prodi, F., J . Appl. Mel., 9, 903 (1970).

Printed in Orcat Bdraio by Henry Ling Lld . . al (h e Do rset Press , Do rchester. Dorset