Endolitiska lavar. (Bryonet 070413)


Dear Scott,

endoliths are organisms which are able to form boreholes, furrows 
and channels within the substratum by mean of an active process of 
dissolution of the matrix (?euendoliths? sensu Golubic et al. 1981). 
For these peculiarities, they do differ from other organisms living 
within the rocks: the ?chasmolithics?, which simply adhere to cracks 
of the substrata, and the ?cryptoendoliths?, which colonise the 
structural holes of porous substrata (most ?endolithic? lichens from 
Antarctica belong here).

Some endoliths are able to penetrate quartz crystals, but the 
heaviest colonisation typically occurs in carbonatic rocks, such as 
(in order of decreasing frequency) sedimentary and metamorphic 
limestone, dolomitic rocks and sandstone. Endolithic lichens are, 
together with cyanobacteria, the most common endoliths of European 
carbonatic outcrops (W. Pohl, in Schneider & le Campion-Alsumard 
1999). The symbionts, but particularly the mycobiont, penetrate into 
the substratum to a depth of 2-3 mm (Pomar et al. 1975, Pinna et al. 
1998). The colonisation of the rocks is often recognizable only by a 
light fading or pigmentation of the substratum (Doppelbaur 1959, 
Gerhmann-Janssen 1995, Gerhmann & Krumbein 1996, Pinna et al. 1998), 
and a careful observation is needed to detect the presence of their 
fruiting bodies.

Surprisingly, the mechanisms used by endolithic organisms to 
dissolve the substratum are still unknown, although different 
hypothesis have been proposed (Pia 1937).These may be divided in two 
main categories, 1) biological corrosion, consisting in the chemical 
dissolution of the substratum, and 2) biological abrasion, 
consisting in the mechanical removal of carbonate (Golubic & 
Schneider 1979). In calcicolous endolithic lichens biological 
corrosion prevails (Pinna et al. 1998). As it is well known that the 
carbonatic matrix is easily attacked by acids (including weak 
acids), it was hypothesised that the penetration of endoliths might 
be related to the same factors that are at the basis of the chemical 
processes involved in the biodeterioration of any rock. These 
factors, that are nor mutually exclusive, are: 1) the secretion of 
organic acids, such as oxalic acid; 2) the secretion of chelating 
compounds (e.g. some lichen substances); 3) the production of 
carbonic acid derived from respiratory CO2 dissolution (Syers and 
Iskandar 1973). Although these processes have been described in many 
epilithic organisms, in our opinion there are not yet sufficient 
evidences supporting their role in calcicolous endolithic lichens. 
For this reason, our research groups have concentrated their efforts 
to identify the mechanism of calcite dissolution in endolithic 
lichens from the Trieste Karst (N Italy). Histological, biochemical 
and biomolecular techniques applied to thalli still immersed in the 
substratum, or free from it showed that all the species produce 
specific carbonic anhydrases (CA). CAs are particularly frequent in 
the lithocortex and in the oil-hyphae of the pseudomedulla (see Pinn 
et al. 1998 for tan explanation his terminology). CAs are important 
for several fundamental metabolic processes, from CO2 transport to 
acid-base balance, because they catalyze the reversible hydration of 
CO2. In our opinion, CAs might play a significant role in the 
substratum dissolution, because their secretion in the sites of 
active growth would significantly intensify the chemical activity of 
respiratory CO2, whereas their presence in the hyphae would favour 
lipogenesis, that might explain the high content in triacylglycerols 
typical of endolithic lichens.

We hope that this answer satisfied your curiosity. Do not hesitate 
to contact one of us for further information: as you can imagine, 
part of this text was taken from a manuscript (in prep.)!

Ciao ciao,

Mauro Tretiach (Trieste), Paolo Modenesi (Genova) and Rosanna 
Piervittori (Torino), ITALY


At 18.05 12/04/07, Scott LaGreca wrote:

Dear All,

I am helping our Exhibitions team with a new "Antarctica" exhibition 
they're working on and they want to include some Antarctic lichen 
specimens, especially "endolithic lichens".

Can anyone on this list please provide me with a definition of 
"endolithic lichen"?

I assume it means a lichen that has its entire vegetative thallus 
within a saxicolous substrate--and only its fruiting bodies can be 
seen above the surface of the rock.

Of course this would mean that many common species of lichen, e.g. 
Porpidia albocaerulescens, are endolithic lichens. True?

Another question which the team has asked me: Are endolithic lichens 
only found in Antarctica? I told them "'no, but the endolithic habit 
is a common form for Antarctic lichens in really harsh parts of the 
continent."

Can anyone comment please? Any help would be appreciated.

Thanks very much,

Scott

____________________________

Scott LaGreca, PhD
Curator of Lichens
Herbarium, Botany Department
the Natural History Museum
London SW7 5BD
United Kingdom
telephone: [+44] 20 7942 5250
fax: [+44] 20 7942 5529
____________________________

Prof. Mauro Tretiach
Dipartimento di Biologia
Università di Trieste
Via L. Giorgieri, 10
I-34127 Trieste
Italy
tel. +39 040 558 3886
fax +39 040 575079
mob. 329 0688968