Substrates On Which Lichens Grow Appear To Act As Reservoir Of Lichen Photobionts

On the 10^th September 1867, at a meeting of the botanical section of the Swiss Society for Natural Sciences, Simon Schwendener announced his opinion that “lichens may not be autonomous plants, but fungi in association with algae” and subsequently in 1869 he proclaimed that “lichens do not, from my investigations, constitute a separate, higher division of cryptogams, but merely a subdivision of the great order of Fungi: they are ascomycetes parasitic on algae“. Since then, lichens have been traditionally defined as symbioses in which the ecologically-obligated mycobiont (mainly an ascomycete fungus) provides a microhabitat for the extracellular photobiont (green algae [chlorobiont] and/or cyanobacteria [cyanobiont]), which fixes carbon through photosynthesis and, in the case of being a cyanobacterium, has the additional ability to fix nitrogen.

In general, this symbiosis is considered as mutualistic, since the mycobiont obtains carbohydrates (and nitrogen in the case of cyanobionts), and the photobiont may benefit from being less exposed to adverse environmental conditions. In the past decade, this traditional definition has been revolutionized, since several new members, mainly additional bacteria, and fungi, have been proposed to be part of this symbiosis.

The process of lichen formation is largely unknown. Although reproductive strategies of lichens have been widely studied, little is known about how they acquire and transmit their symbionts. Lichens reproduce both sexually and asexually, and the transmission of traditional components is different in each case. If the lichen reproduces asexually by producing vegetative propagules including both components (i.e. mycobiont and photobiont), vertical transmission of these symbionts occurs. If it does so sexually, by producing mycobiont spores, then the germinated ones must restore the symbiosis with a compatible photobiont (horizontal transmission or relichenization). In summary, if the pairs are vertically transmitted, the symbiotic association is maintained for several generations, but if they are horizontally transmitted, the association is decoupled and must be restored after fungal reproduction.

Even in the case of asexual reproduction, it is possible that decoupling between the components of the symbiosis occurs, allowing the photobiont to be replaced by another one available in the environment. The potential photobionts may be available in the free-living state, symbiotically associated with other lichens or even with other organisms. However, it is still not clear if the substrates where lichens are growing are a source of potential photobionts. For that reason, we studied the diversity of cyanobacteria in 186 terricolous Peltigera-Nostoc cyanolichens (Peltigera, lichen; Nostoc, cyanobacteria) and their underlying substrates, obtained from different localities on a latitudinal gradient along the south of Chile and maritime Antarctica.

Our results showed that Nostoc dominated the cyanobacterial communities of the studied substrates and many of them matched those present in the lichens, which indicate that the substrates where lichens are growing might act as a reservoir of lichen photobionts. An integral overview of the different potential photobiont sources might help us to understand the complex process of lichen formation and how it influences the distribution patterns of these organisms.

These findings are described in the article entitled Substrates of Peltigera Lichens as Potential Source of Cyanobionts, published in the journal Microbial Ecology. This work was led by Julieta Orlando, Catalina Zúñiga, & Diego Leiva from Universidad de Chile.