Margalef, Ramon

Biblioteca Digital de les Illes Balears

SOCIETAT D'HISTORIA 
NATURAL DE LES BALEARS 
Editorial 
On fine 
Algal mats, estromatogels i 
estromatolits: una relíquia a conservar 
Corresponc de bona gana i agraTt a la invitació de la Societat a col.laborar en 
aquesta secció, també perque em dóna una ocasió per a recordar I'interes científic 
d'un tipus d'ecosistemes que encara es troba, si bé en condicions precaries, a 
Mallorca, i el convenient que seria tractar de que no es perdés, tant més que la 
qualitat de les comunitats residuals representades a I'illa és excépcional. 
Tots sabe m que la Terra difereix deis altres planetes, i de manera molt 
demostrativa deis seus ve'ins (Venus i Mart), pel caracter oxidat i oxidant de la 
nostra atmosfera. S'accepta que la primitiva atmosfera de la Terra fou també 
reductora. La vida nasqué o aviat es conforma com un sistema d'oxido-reducció, 
apte per a captar fotons de la lIum del Sol i oxidar I'entorn, tot donant més poder 
reductor a la propia vida. El ritme dialnit pogué ser responsable de la necessitat 
d'assolir unes dimensions mínimes. D'aquesta manera es forma una fina peLlícula 
-descomposta en petites unitats discontínues-, anticipació de la moderna biosfera, 
que anava oxidant cap a dalt, i mantenint el propi poder reductor com una 
característica fonamental de la química de la vida. 
Antiquíssimes formacions mineralitzades que corresponen a la forma més 
senzilla d'aquest esquema s'han reconegut entre els indicis més vells de la vida i 
han rebut el nom d'estromatolits. És ciar que aquestes formacions quan eren vives, 
i les actuals equivalents a elles, no eren pas totalment rígides, encara que podien 
contenir partícules de sorra i diposits minerals locals: les que avui dia segueixen 
creixent en molt lIocs es diuen "algal mats" o "tapetes algales", o potser hauríem 
d'inventar per elles el neologisme estromatogels. 
En diferents localitats de la Península Ibérica, prop deis lIocs on rius importants 
arriben a la mar, s'en troben d'aquestes formacions i s'han publicat treballs que 
poden servir com a instrucció general i per situar més referencies. Pero la seva 
estructura esdevé més perfecta si el niveli del líquid es manté relativament constant. 
Aquesta condició es dóna en uns estanys de poca fondaria que hi ha prop d'unes 
velles salines a la Colonia de Sant Jordi, al SE de Mallorca. Entorn de les Salines 
més importants entre Es Trenc i Campos, antigues formacions analogues es veuen, 
ara, molt més mal meses. 
12 BoII. Soc. Hist. Nat. Balears, 40 (1997) 
L'estructura típica d'una d'aquestes formacions, qual part més activa té d'uns 
mm a un cm de gruix, consisteix en un estrat (verd) del cianofit o cianobacteri 
Microcoleus, d'uns pocs mm de gruix, fet de filaments incolors. Barrejats amb 
aquesta mena de teixit fonamental sovintejen altres cianofits deis generes Spirulina 
i Aphanothece i, per damunt, encara més cianofits, deis generes Lyngbya, 
Chroococcus i d'altres que, quan són envoltades de beines mucilaginoses, les tenen 
en general de color groguenc o torrat, indici de que en aquell nivell hi ha prou 
oxigen per oxidar les substancies que donen color. 
Davall de I'estrat de Microcoleus es troba una varietat molt gran de bacteris, 
entre els que predominen rodotiobacteris, més o menys rosats, que viuen en un 
ambient reductor. És molt notable la transició que es manifesta just cap al límit 
inferior de I'estrat de Microcoleus. La mesura contínua del potencial d'oxidació-
reducció, que es pot fer movent cap avall un electrode adient a través de la massa 
compacta del que en direm "estromatogel", pot marcar una diferencia de més d'un 
volt en poc més d'un mil.límetre. Passem d'un món a un altre. En aquest nivell, 
com en un endoll, la vida pot aprofitar la diferencia de potencial de manera gairebé 
directa. Potser els organismes que més se n'aprofiten són les Beggiatoa, organismes 
filiformes de diametres molt diversos, incolors, que es mouen amunt i avall, i oxiden 
els compostos redurts de sofre en forma de granets solids d'aquest element dintre 
deis filaments del nostre organisme, qual diametre va d'1 fins a més de 20 f-tm 
(micrometres). El teixit organic continu i compacte, representat principalment per 
Microcoleus isola efectivament el món inferior redurt, i el món superior oxidat, que 
avanga cap avall, com I'esperit de Gaia o Gea, la deessa de la Terra en la versió 
divulgativa promoguda per Lovelock. 
Es reconeix una identitat total entre tot aquest conjunt d'organismes i els deis 
mateixos grups que s'han reconegut en les corresponents formacions geologiques 
més antigues que són exemples d'ecosistemes de persistencia excepcional a través 
del temps. 
El teixit fonamentalment representat per Microcoleus é:: molt efectiu en isolar 
el món redurt del davall, on es troben productes generac :xs de males olors, 
principalment sulfhídric. Pero la coberta viva, si es conserva contínua, és estanca 
i deixa al damunt aigua clara i inodora, amb una vida aquatica normal, amb variats 
protozous, rotífers, petits crustacis i fin s insectes, com les larves de les mosques 
de les salines (Ephydra) quals adults apareixen en primavera i es mantenen de 
costat en colles nombroses damunt el mirall de I'aigua. En aquestes formacions de 
Mallorca es troba també un interessant dinoflagel.lat, versemblantment toxic, que es 
passa la meitat de la vida mudant successivament les cobertes de les respectives 
cel.lules. En fi, un món del més gran interés científic i educatiu, que bén bé 
justificaria un museu o alguna exhibició per il.lustració local. Insisteixo que es tracta 
d'un tipus de comunitat biotica excepcional i que pot fornir infinitat de suggeréncies 
per a I'estudi de la vida i de la seva historia. 
Que passa quan posem un tros d'aquesta catita org¿mica dintre algun petit 
aquari? Doncs que el circuTt electro-químic es tan ca per la vora i tota I'estructura 
s'en va per avall. Predomina la influencia reductora, I'aigua fa mala olor i sovint tot 
s'omple de bacteris del genere Chromatium. El sistema intacte in situ manté al 
damunt aigua netíssima i inodora mentre segueixi viu i actiu. Microcoleus es pot 
considerar com una moqueta ideal per mantenir isolat el pudent submón inferior. 
Editorial 13 
Podríem usar recobriments de Microcoleus per a confinar o embolcallar masses de 
residus, digerint-Ios lentament o, almenys, evitant que facin pudor ? 
Es comprova com, si algun any el nivell de I'aigua baixa excessivament, en 
I'argila que fa de substrat profond s'obre un retícul d'esquerdes profondes. Al tornar 
I'aigua, aquestes esquerdes són 1I0cs de circulació de nutrients cap amunt, i ells 
permeten el desenvolupament d'algues filamentoses verdes (Cladophora, 
Rhizoclonium) o rojes (Johannesbaptistia), localitzades precisament en la vertical de 
les esquerdes del fango No és estrany que els estromatolits formats en costes 
sotmeses a marees tendeixin a fragmentar-se en masses o "caps" distints. 
Seria una pena que formacions biotiques tan interessants tinguessin de 
retrocedir davant I'empenta que va destruint progressivament la major part de les 
nostres comunitats naturals autoctones. 
Algal mats, stromatogels and 
stromatoliths: a relie to be preserved 
It is with great pleasure that I accept the request of the SHNB to participate 
in the Editorial section. Indeed, I welcome this opportunity to focus on the scientific 
interest implied by certain kinds of ecosystems which can still be found in Mallorca, 
although in most cases they remain under precarious conditions. And also, I would 
like to remark on the convenience of maintaining them under good condition, mainly 
as the quality of the communities remaining on the island happens to be an 
outstanding matter. 
It is common known that the Earth differs from the other planets, and 
particularly from its neighbours (Venus and Mars), by its oxidized and oxidizing 
atmosphere. It is generally accepted that the primitive atmosphere of the Earth was 
reducing. Life was born, or it was quickly conformed as a system of oxidation-
reduction, suitable to capture the sunlight photons and oxidize the environment, so 
that it conferred a higher reducting power to life itself. The day/night cycle should 
be responsible for the need to achieve minimal dimensions. In consequence, a thin 
layer -decomposed in small discontinuous unities- appeared, as the ancient 
predecessor of the modern biosphere. Such a film was oxidizer on the upper strata 
and, simultaneously, maintained the reduting power as a main characteristic of life's 
chemistry. 
Among the most ancient witnesses of life, very old mineralized structures have 
been recognized which follow the aboye pattern and which have been named 
"stromatoliths". It seems obvious that when these structures were alive, as well as 
their present equivalent ones, they were not at all hard formations, although they 
might have containen sand particles or local mineral patches: those which are at 
present still growing in many locations are called "algal mats" and maybe we could 
propose for them the neologism "stromatogels". 
14 BolI. Soco Hist. Nat. Balears, 40 (1997) 
These structures have already been found at different locations in the Iberian 
peninsula, always close to the mouth of important rivers, and so me works on them 
have also been published, which may serve as a reference. But their structure 
happens to be even more perfect when the level of liquid remains relatively constant. 
Such conditions are present in some shallow ponds close to the old Colonia de Sant 
Jordi salting ponds, SE of Mallorca. In the surroundings of the most important salt 
marshes, between Es Trenc and Campos, similar primitive formations can be seen, 
although they are at present rather damaged. 
The typical structure of one of these formations, the most active part of which 
ranges from a few mm to one cm in thickness, consists of a (green) layer, only a 
few mm thick, of the cyanophyte or cyanobacterium Microcoleus. Associated to this 
sort of main tissue are frequently also other cyanophytes belonging to the genera 
Spirulina and Aphanothece. On top of them, there are even more cyanophytes of 
the genera Lyngbya, Chroococcus and others which, when covered by mucilages, 
they are usually of yellowish and brownish colours, showing that in such level the 
oxygen rates are high enough to oxidize those colouring substances. 
Below the Microcoleus layer there is a great variety of bacteria and among then 
pinkish rodotiobacteria, living in reducting environments, are predominant. Really 
remarkable is the transition existing just on the lower limit of the Microcoleus layer. 
A continuous measure of the oxidation-reduction potential, which can be easily made 
by moving a suitable electrode downwards through the compact mass of the 
stromatogel, could show a diference of over one volt across hardly one mm. We 
move from one world into another. At this level, as would be in a socket, life is able 
to profit directly from the potential difference. Maybe the organisms which take the 
higher profile are the Beggiatoa, filiform and colourless beings of variable diameters, 
which move up and down between the filaments of the stromatogel and oxidize the 
reduced sulphur compounds giving little so lid grains of sulphur inside the threads 
which, diameter ranges from one to over twenty micrometers. This organic tissue, 
continuous and compact, mainly represented by Microcoleus, can efficiently isolate 
the lower reduced world from the upper oxidized one, which boundary tends to move 
downwards, emulating Gea's spirit, the goddess of the Earth in the popular version 
given by Lovelock. 
A perfect analogy can be found between this bunch of organisms and those of 
the same group which have been recognized in equivalent older geological 
structures, thus being a clear example of the remarkable persistence of this kind 
ecosystems throughout time. 
The main tissue represented by Microcoleus is, though, very efficient when it 
comes to keeping the lower reduced world isolated, a world that smells of hydrogen 
sulphide: Consequently, once the upper covering remains intact, it allows a layer of 
clear and odourless water to exist aboye. Within this water a normal aquatic life 
takes place, involving a great variety of protozoa, rotifera, small crustacea and even 
some insects, such as the larvae of the saltponds flies (Ephydra). The adults appear 
in spring and gather sidewards in massive patches on the surface of the water. In 
Mallorca, within these structures can also be found interesting dinoflagellates, which 
are likely toxic, and must spend half of their lifetime casting off the covers of their 
respective cells. In short, a most interesting world from the scientific point of view 
which could be worth of a museum or, at least, of a local exhibition. Surely it is an 
Editorial 15 
exceptional type of biotic community which may stimulate and contribute to the 
knowledge of life and its history. 
What does actually happen when a bit of this mat is put into a little aquarium? 
Well, the electrochemical circuit appears to be interrupted all along the edges and 
the whole structure decays. The reducing influence becomes predominant, the water 
starts to stink and very often all the mass is taken over by the bacterium 
Chromatium. When the system keeps itself intact in situ, it maintains its top layer 
of pure and odourless water while it remains active and alive. Therefore, Microcoleus 
could be considered a perfect carpet to maintain the fetid subworld totally isolated. 
Could coverings of Microcoleus be used to confine or pack masses of residues, so 
that they could be slowly digested or, at least, they would be kept odourless? 
It can al so be observed that the year in which the water level is excessively 
low, a net of deep fissures appears in the clay substrate. When the water level rises 
again such fissures become a path for nutrients, that are transported upwards. 
These nutrients allow the development of green (Cladophora, Rhizoclonium) or red 
(Johannesbaptistia) threadlike algae on the vertical above the fissures in the clay. 
It is not astonishing, then, that those stromatoliths that grow in coasts influenced by 
tides show a tendency to divide into different pieces or "heads". 
It would be sad that such interesting biotic structures would suffer under the 
pressures which, gradually are destroying, most of our autochthonous communities. 
Tradui"t per la Junta de Publicacions 
Algunes referencies 
Bertrand-Sarfati, J. i Monty, C. (edits.) 1994. Phanerozoic Stromatolites 11. 460 pp. 
Kluwer Acad. Publishers. Dordrecht. 
de Wit, R. i van Gemerden, H. 1987. Oxidation of sulfide to thiosulfate by 
Microcoleus chtonoplastes. FEMS Microbiology Ecology, 47: 7-13. 
Margulis, L., Ashen, J.B., Solé, M. i Guerrero, R. 1993. Composite, large spirochetes 
from microbial mats: Spirochete structure review. Proc. Natl. A cad. Sci. USA, 
90: 6966-6970. 
Mir, J., Martínez-Alonso, M., Esteve, 1. i Guerrero, R. 1991. Vertical stratification and 
microbial assemblage of a microbial mat in the Ebro Delta (Spain). FEMS 
Microbiology Ecology, 86: 59-68. 
Risatti, J.B., Capman, W.C. i Stahl, DA 1994. Community structure of a microbial 
mat: The phylogenetic dimensiono Proc. Natl. Acad. Sci. USA, 91: 10173-10177. 
Rothschild, L.J. i Mancinelli, R.L. 1990. Model of carbon fixation in microbial mats 
from 3,500 Myrago to the present. Nature, 345: 710-712. 
Prof. Dr. Ramon Margalef 
Departament d'Ecologia 
Facultat de Biologia 
Universitat de Barcelona 
Avgda. Diagonal 645 
08028 Barcelona 


Algal mats, estromatogels i estromatolits:

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