Bonner, John Tyler

Biblioteca Digital de les Illes Balears

SOCIETAT D'HISTORIA 
NATURAL DE LES BALEARS 
Editorial 
On fine 
Un futur per a l' ecologia del sol 
Ri ha una area relativament inexplorada de l'ecologia que tindril.,amb seguretat, un futur 
molt brillant. Actualment es tracta quasi literalment d'una capsa negra: qualque cosa que no 
podem veure per de dins i que esta, per tant, plena de misteri i de promeses. És I'ecologia del 
qué ocorr dintrad'un petit bocí de sol. Nosaltres podem veure, amb els uUs nus, auceUs, 
mamífers, reptils, amfibis, una gran diversitat d'invertebrats, i plantes de tots els tipus i formes 
que aporten I'energia que és, en darrer cas, la font de tots ells. En comparació,el que trovam 
a un petit cub de sol és un trencaclosques, donat que esta amagat de la nostra vista. No obstant 
aixo, sabem que conté rotífers, cil'liats, nematodes, acars i altres petits artropodes, amebes, fongs 
i grans quantitats de bacteris que, igual com les plantes verdes, juguenel paper d'aliment deis 
hervívors i camívors de major mida que els enrevolten. 
Encara que I'ecologia del sOl ha estat estudiada des de distintes vessants (veure per 
exemple, Coleman i Crossley, 1995) encara resten molts de fascinants problemes per descobrir. 
Per tal d'afermar el meu argument de que el sol conté molts de problemes fascinants, a 
continuació relataré tres histories a les que estan implicades amebes edafiques - fongs 
mucilaginosos uniceHulars (Dictiyosteliomycota) - pels quals m'he interessat durant molts 
d'anys. 
La primera historia és un estudi d'Edward Rom (1971) qui es va centrar en la questió 
de perque diferentes especies de fongs mucilaginosos poden coexistir en una petita superficie 
de sol; perque una especie no competeix fins a eliminar a totes les altres. Aquest estudi va ser 
la seva tesi doctoral, la qual va realitzar sota la direcció d'en Robert MacArthur i de mi mateix. 
MacArthur (1958) havia fet alguns estudis, avui coneguts, sobre com diferents especies de 
busquerets habitaven el mateix bosc de manera estable sense que una anuHa a totes les altres. 
Cada una d'elles tenia el seu nínxol alimentari especialitzat a diferents parts deis arbres, de ma-
nera que no violaven els deis altres. Rom va trobar que en una petita porció de sol coexistien 
pacíficament quatre especies de fongs mucilaginosos unicel·lulars. Va amar, partint d'una 
mateixa font, diferents soques de bacteris i els va fer creixer separadament en agar en plaques 
de cultiu. El que va trovar, és que diferents especies de fongs mucilaginosos tenien distintes 
preferencies alimentaries entre les varies soques de bacteris. Amb altres paraules, el sol alberga 
els seus nínxols alimentaris d'una manera similar a com ho fa el bosc amb les distintes especies 
de busquerets. 
El segon conte microecologic del sol procedeix d'un treball sobre l'orientació deis fongs 
mucilaginosos unicel'lulars a la seva etapa multiceHular segons el gradient de temperatura. 
Molts de milers d'amebes es junten per a formar un mucílag migrador que s'orienta cap a la 
llum i segons gradients de calor. La raó adaptativa d'aquestes orientacions sembla ser la recerca 
del lloc optim per a formar un petit cos fructífer (una massa d'espores ques'eleva a l'aire a 
I'extrem d'un brot fi; Fig. 1) per tal de dispersar les espores.Si estan a prop de la superficie 
del sol els hi sera més facil situar-se en el camí deis invertebrats que hi passin i als quals les 
espores s'adheriran, essent duites a noves taques de bacteris. Es ben conegut que aquests 
10 Boll. SOCo Hist. Nat. Balears, 42 (1999) 
mucílags migradors es dirigeixen cap a intensitats de llum extremadament baixes, i també que 
són extraodimiriament sensibles a petits gradients de temperatura - tan petits que poden detectar 
diferencies de menys de 0,0005 oC entre un extrem del mucílag i l'altre (Bonner et al., 1950). 
Els nostres primers resultats no només van ser confirmats per en Kenneth Poff sinó que, 
juntament amb un deis seus estudiants, va mostrar que si el rang de temperatura del gradient 
estava per damunt de la temperatura a la que creixien, els bacteris anirien cap a l'extrem més 
calent (termotilxia positiva); mentre que si es trovaven dins un rang de temperatures més fredes 
que aquella de creixement, s'orientaven cap al costat més fred (termotaxia negativa) (Whitaker 
i Poff, 1980). Ells defensaven que durant el període del dia amb llum, temperat, l'aire estaria 
més calent que el sol, de manera que el "llimac" (nom que se ti dóna a la colonia 
mucilaginosa) migrarien cap a la superficie; i que amb la frescor de la nit l'aire estaria més 
fred que el sol, de manera que el "llimac" també es desplac;aria cap amunt. Mitjanc;ant aquest 
mecanisme tan intrigant de signe invers a la termotaxia, els bacteris anirien cap a la superficie 
tant de dia com de nit. 
La tercera historia és un recent treball de laboratori d'en Richard Kessin, (Kessin et al., 
1996). EH i el seu equip va investigar les interaccions entre els nematodes - altres organismes 
molt comuns al sol - i els fongs mucilaginosos uniceHulars. Van mesclar els dos en plaques 
de cultiu i van trovar que els nematodes eran vorac;os predadors d'amebes, a les quals engotien 
i digerien. Pero, les espores travessaven tots els intestins deIs nematodes sense patir cap mal 
i així eran transportades fins a una certa distancia, és a dir, eran dispersades. Endemés, una 
vegada que les amebes s'havien agregat dins una massa, es mantenien protegides per una beina 
gelatinosa, de tal forma que els nematodes foren incapac;os de menjar-les. Doncs, aquí podem 
veure un predador vertader que també actua com a dispersor, mentre que les amebes de fongs 
reptants estan protegides tant quan són espores com quan passen a ser una massa multicel·lular. 
1 encara hi ha més. Els nematodes compten amb el seu propi sistema de dispersió: tenen petits 
individus que mai no es nodreixen que suposen la fase dispersora del seu cicle vital. Per a 
mi llorar les seves possibilitats de dispersió, aquestes larves escalen el brot d'un cos fructífer 
de fongs mucilaginós i s'arrombollen entom de la massa esferica d'espores (igual com les serps 
del cap de Medusa) aprofitant el fet que els cosos fructífers estan a l'aire i suposen llocs 
millors per a la dispersió. Hi ha un estira i arronc;a entre aquests dos oponents. 
Si es considera quants d'organismes diferents conviuen en un petit grapat de sol, apart 
de bacteris, fongs mucilaginosos i nematodes, les possibilitats de riques i fascinants interaccions 
semblen enormes. Pero, per tal d'arrivar a composar el trencaclosques, a més a més que la 
simple vista o un parell de binoculars, fa falta un microcopi i una certa feina de detectiu 
intel·ligent. 
Agrai'ments 
Voldria donar les "gracies al meu col, lega, Professor Henry Hom, per la seva ajuda i 
suggeriments a I'hora de realitzar les primeres notes d'aquest manuscrit. 
Traduft per la Junta de Publicacions 
Editorial 11 
A future for soil ecology 
There is one relatively unexplored area of ecology that will certainly have a very bright 
future. At the moment it is almost literally a black box--something we cannot see within, and 
therefore full of mystery and promise. It is the ecology of what goes on within a small bit of 
soi!. We can easily see with our unaided eyes birds, marnmals, reptiles, amphibians, a great 
diversity of invertebrates, and plants of every variety and description that provide the energy, 
the ultimate fodder for them al!. By comparison what we find in a small cube of soil is a 
puzzle, for it is shielded from our view. Yet we know that it contains rotifers, ciliates, 
nematodes, mites and other small arthropods, amoebae, fungi and vast amounts of bacteria that, 
just like green plants, play the role of fodder for the larger herbivores and carnivores that 
surround them. 
Although much has been done on the ecology of the soil (see for example, Coleman and 
Crossley, 1995), there are many more fascinating problems. To make my point that the soil 
contains many fascinating problems, I shall relate three stories that involve soil amoebae----
cellular slime molds--that have been my interest for many years. 
The first is a study by Edward Hom (1971) who addressed the question of why different 
species of slime molds can exist together in a small patch of soil; why does one species not 
out compete all the others. This was his doctoral thesis which he did under the aegis of Robert 
MacArthur and myself. MacArthur (1958) had done sorne now famous studies on how different 
species of warblers lived in the same forest in a stable state without one swamping out all the 
others; they each had their specialized feeding niche in different parts of the trees that did not 
infringe upon those of others. Hom found that in a small portion of soil. four species of cellular 
slime molds were peacefully co-existing. He isolated all the different strains of bacteria from 
the same source and grew them separately on agar in culture dishes. What he found was that 
different species of slime molds had distinct food preferences among the various stráins of 
bacteria; in other words the soil harbors its food niches in a way that is parallel to the forest 
with its co-existing warblers. 
The second micro-ecological tale in the soil comes from work on the orientation of the 
multicellular stage of cellular slime molds in temperature gradients. Many thousands of amoebae 
come together to form a migrating slug that orients towards light and in heat gradients. The 
adaptive reason for these orientations is presumably to find the optimal place to form a small 
fruiting body (a spore mas s lifted up into the air on a slender stalk; Fig. 1) to disperse the 
spores. If they are near the surface of the soil they will be better able to be in the path of 
passing invertebrates to which the spores will adhere and carry them to new patches of bacteria. 
It is well known that that these migrating slugs will go towards light of extremely low 
intensities, but also they are extraordinarily sensitive to small temperature gradient&--so small 
that they can detect a difference of less that 0.0005 oC between one side of the slug and the 
other (Bonner el al., 1950). Our earIy results were not only confirmed by Kenneth Poff, but 
with one of his students he showed that if the temperature range of the gradient was aboye the 
temperature at which they were grown, they would go towards the warmer side (positive 
thermotaxis); but if they were in a temperature range cooler than the growth temperature, they 
would orient towards the colder side (negative thermotaxis) (Whitaker and Poff, 1980). They 
argued that during the warm daytime the air would be warmer than the soil, which would mean 
that the slugs would migrate up towards the surface; and in the cool of the night the air would 
be colder than the soil, which would still mean that the slugs would move upwards. By this 
intriguing mechanism of a sign reversal of the thermotaxis, they will go towards the surface 
night and day. 
12 Boll. SOCo His/. Nat. Balears. 42 (1999) 
~ ,~ 11. ' . ", . ,',' ' 
 "J) ~ .IJ ~~.}. V y~) , ..•. y' 
Fig. 1. Cicle vital d'un fong mucilaginós unicel-lular. L'etapa social comen<;a quan les amebes individuals 
(al fons a ¡'esquerra) es comencen a agregar en un conjunt central de punts i es distribueixen en Himacs 
migradors. Després d'un període de migració,el mucílag es redre<;a i les cél'lules anteriors donen 1I0c 
a un brot que s'al<;a cap amllnt,elevant així a les ceHules posteriors que es converteixen enespores. 
(Dibuix de Patricia Collins, en Bonner, Scientific American, juny 1969). 
Fig, J. The lije cycle of a cellular sUme mold. The social phase begins when individual amoebae (far 
left) begin to aggregate into central collection points and gather into migrating slugs. After a period 
of migration. the slug rights itself and the anterior cells form a stalk that rises up into the air, lifting 
the posterior cells which turn into spores. (Draw[ng by Patricia Collins from Bonner, Scientific 
American, June J 969), 
The third story is the recent work from the Iaboratory of Richard Kessin (Kessin el al" 
!996). He and his associates investigated the interactions between nematodes-another common 
soil organis~nd cellular slime molds. They mixed the two in culture dishes and found that 
nematodes were voracious predators of amoebae, gobbling them up and digesting them. 
However the spores went right through the nematode gut unharmed and in this way were 
carried some distance away--they were dispersed. Furthermore, once the amoebae had 
aggregated into a mass they become protected by a slime sheath and the nematodes were unable 
to eat them. So here we see a true predator that also acts as a disperser, but the slime mold 
amoebae are protected both as spores and once they become multicellular. But there is more. 
Nematodes have their own dispersa! system: they have small, non-feeding individuals that serve 
as a dispersal stage in their life cyele. To improve their chances of dispersal these larvae will 
climb up the stalk of a slime mold fruiting body and mili around the spherical spore mass 
(rather like the snakes of Medusa's hair), taking advantage of the fact that the fruiting bodies 
stick up into the air and provide more effective sites for dispersa!. There is give and take 
between these two opponents. 
When one considers how many different kinds of organisms there are in a small lump 
of soil, besides bacteria, slime molds and nematodes, the possibility for rich and fascinating 
ecological interactions seem enormous. But instead of piecing together the puzzle with the 
naked eye, or with a pair of binoculars, it requires a microscope and some elever detective 
work. 
13 
ReferencieslReferenc.es 
Bonner, J.T., W.W. Clarke jr, C.L. Neely jr and M. K. Slifkin. 1950. The orientation to light 
and the extremely sensitive orientation to tempemture gradients in fue slime mold 
Dictyostelium discoideum. J. cell. Comparo Physiol. 36: 149-158. 
Coleman, D.C. and D.A. Crossley, Ir. 1955. Fundamentals of soil Ecology: Academic Press. 
Hom, E. G .. 1971. Food competition among the cellular slime. molds (Acrasiae). Eco/agy, 52: 
475-484. 
Kessin,. R.R., G.G .. Gundersen. V. Zaydfudim, M. Grimson, and R.L. Blanton. 1996. How 
ceUular slime molds evade nematodes .. Proc. HatL Acod. Sci. USA, 93: 4857-4861. 
MacArthur, R.R. 1958. Population ecQlogy of s.ame. warl>lers of Northeastem coniferous forest 
Ecology. 39: 599-619. 
Whitaker, B.D. and K.L. Poff.1980. Thermal adaptation of thermosensing and negative 
thermotaxis in Dictyostelium. Exp. Cell Res., 128: 8J-9J. 
(1 would like. ta tbaM ro}' coUeague,· Professor Hemy Hom. fOl his helpfuI suggestionstln an 
earlier draft) 
J. T .. Bonner 
Professor Emeritus 
Department of Ecology and EvolutlQnary Biology 
Princeton University 
Princeton, New Jersey 08544 USA 


A future for soil ecology

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