Preliminary study on the presence of diatoms on lichens

Auteurs

J. Bertrand, C. Coste, R.L. Cohu, J.-P. Renon, and L. Ector

Référence

Botany Letters, vol. 163, no. 2, pp. 93-115, 2016

Description

Lichens are symbiotic relationships between fungal filaments (mycobiont) and either unicellular algae or (sometimes) cyanobacteria (phycobiont). Diatoms have shown the ability to colonize fruticose lichens via aerial dispersal, but these lichenicolous diatoms have been rarely studied. In the present work, 18 lichen samples from five genera (nine Evernia, three Usnea, three Ramalina, two Cladonia and one Pseudevernia) were collected from trees in diverse locations in France (Alps and Centre-Val de Loire regions). The lichen-inhabiting diatoms were quantified and identified. Lichens of the mentioned genera are almost always colonized by diatoms. Absolute diatom abundances were low on lichens, but species richness was quite high compared with other neighbouring micro-habitats (Populus and Acer bark, Sphagnum bogs, plankton). The number of taxa per sample ranged from 39 to 105 (mean 85), and high numbers of diatom genera (50) were found among the 313 diatom species identified. Comparatively, we found only 29-35 species on adjacent barks covered with lichens and 37 species on average per sample in rivers. Of the diatoms identified, 41% were present in only one sample and only 16.9% of species were common to more than 10 samples. Some diatoms, mainly aerophilic, were particularly abundant, and the community was generally dominated by one or two species, including: Pinnularia borealis (54%), Achnanthidium minutissimum (39%), Diadesmis gallica (31.5%), Hantzschia amphioxys (27%) and Luticola goeppertiana (24%). Diatom composition differed between the supporting lichen genera, and independent diatom colonization of lichens was shown. Species composition discriminated between sampling locations and was negatively correlated with distance to species sources such as ponds, lakes and rivers. These results suggest that aerial diatom dispersal from surrounding freshwaters by spray, wind and rainfall mostly contributes to colonization. Over shorter distances, soredia dispersed by the lichens may contribute to the local homogenization of the observed diatom flora. Ambient humidity appeared to favour diatom growth in situ, as evidenced by the presence of cells containing vital elements (chloroplasts, lipid droplets). Observation of fresh material also suggested that diatoms are likely to be captured by the mucus exuded by the symbiotic algae and/or by the mycobiont. In lichen aerial parts, 97%of diatoms were dead and only 3% were found alive and free. By covering them, soralia may limit desiccation and promote survival. On the other hand, the high number of dead diatoms and of fragmented valves could be interpreted as a result of mechanical pressure applied by the mycelium on the diatom cells, and subsequent digestion of their cell content. This work highlighted that lichens are commonly colonized by diatoms through aeolian dispersal. Altogether, the results obtained tend to indicate that the occurrence of diatom populations on lichens is random, but in some cases opportunism might occur when aerophilic diatoms grow and multiply before being digested by the lichen. 

Lien

doi:10.1080/23818107.2016.1156573