Coexistence of different intrathalline symbiotic algae and bacterial biofilms in the foliose Canarian lichen Parmotrema pseudotinctorum

Abstract

Parmotrema pseudotinctorum (des Abb.) Hale is a foliose lichen able to colonize large areas on rock surfaces in semiarid and warm localities in the Canary Islands. In this contribution, we investigate the phycobionts of this successful lichen under these extreme environmental conditions using ultrastructural and genetic methodologies. Two populations from La Gomera and La Palma islands were analyzed. After TEM analyses, three algal types were clearly distinguished in intrathalline symbiosis, provisionally named Ph1, Ph2, and Ph3. Two of them (Ph1 and Ph2) were Trebouxia showing a well visible pyrenoid corticolatype the chloroplast thylakoids being very different in both. The type Ph3 could be a taxon included in the genus Asterochloris. Our molecular approach consisted in sequencing two different DNA loci: a portion of the chloroplast psbA gene and nuclear ITS. Sequences of the psbA gene resulted in electrophoretograms showing double peaks when DNA extracted from the whole lichen thallus was used as template. Such double peaks were interpreted as single nucleotide polymorphisms (SNPs). This interpretation was confirmed by cloning. However, no intrathalline polymorphisms were detected among the nrITS sequences. Phylogenetic analyses on the basis of the psbA gene revealed three distinct clades. It is likely that these clades corresponded to the the three different morphotypes revealed by TEM. One of these clades, was closely related to T. corticola, other was related to Asterochloris glomerata and the third did not grouped with any specific taxa. These results are the first piece of evidence that algal coexistence may even be established between species of different genera of the Trebouxiophyceae (Asterochloris and Trebouxia at least). Moreover, the coexistence of several microalgal taxa evidenced in this study appears as a consistent character among the populations of this foliose lichen. Further isolation and cultivation of the three different algal types and physiological studies should shed light on the ecological plasticity of the entire holobiont. Along with such variety of intrathalline coexisting algae, another unexpected result was the observation of an almost continuous layer of bacterial-communities coating the lower cortex in all the studied samples of P. pseudotinctorum. The function of these biofilms in the lichen symbiosis remains to be elucidated. The existence of such particular symbiosis involving different algal species and bacteria could be explained by an increased fitness in particular habitats or under specific environmental conditions.