Beleza que mata - Beauty that kills

Líquens do Parque Marapendi...
Lichens of Marapendi Park ...

...Trabalhando...
... Working ...

...Devorando.
...Devouring.

Os líquens são seres vivos muito simples que constituem uma simbiose de um organismo formado por um fungo (o micobionte) e uma alga ou cianobactéria (o fotobionte). Alguns taxonomistas classificam os líquens na sua própria divisão (Mycophycophyta), mas isto ignora o fato de que os componentes pertencem a linhagens separadas. Os líquens desenvolvem-se como lâminas ou placas de várias cores na superfície de árvores ou de pedras, expostas à umidade e ao sol. São geralmente estudados pelos botânicos, apesar de não serem verdadeiras plantas.

A verdadeira natureza desta simbiose é ainda tema de debate pelos cientistas, em que alguns afirmam que o fungo é um parasita do fotobionte; no entanto, em muitos casos, a alga sozinha não sobrevive no habitat ocupado, assim como o fungo isolado também não sobrevive e, portanto, não é realístico usar o termo parasita.

O micobionte dos líquens pertence, na sua maioria, à classe Ascomycetes (acima de 95%), sendo os restantes Basidiomycetes. Cada espécie de líquen tem uma espécie diferente de fungo e é com base nessa espécie que os líquenes são classificados. A classificação baseia-se, em geral nas características do talo e órgãos reprodutivos. Estão descritas de 13.500 a 17.000 espécies de líquens, de acordo com diferentes sistemas de classificação. Cerca de 20% das espécies de fungos conhecidas pertencem a líquens.

Os fotobiontes são muito menos numerosos que os micobiontes, ou seja, a mesma espécie de alga pode fazer parte de vários líquens diferentes. Muitas espécies de algas, se não todas, podem existir isoladamente em alguns habitats, mas quando fazem parte de um líquen, apresentam uma distribuição muito maior.

Lichens are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.[citation needed] Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.[citation needed] Recent ESA research shows that lichen can even endure extended exposure to space.[3] Some lichens have the aspect of leaves (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (illustration, right).[4]

Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.

There is evidence to suggest that the lichen symbiosis is parasitic rather than mutualistic (Ahmadjian 1993). The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (ibid.)

Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness.[5] This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it is not itself cellular.[5] In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia/cephalodium. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium.[citation needed] Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows.[citation needed] Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex.[citation needed] Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.

photos by marcos santos

fonte/sourse: Wikipedia.org