Journal Article

Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis

Frieda L. Henskens, T. G. Allan Green and Alistair Wilkins

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 110, issue 3, pages 555-563
Published in print August 2012 | ISSN: 0305-7364
Published online May 2012 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcs108
Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis

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Background and Aims

Cyanolichens are usually stated to be bipartite (mycobiont plus cyanobacterial photobiont). Analyses revealed green algal carbohydrates in supposedly cyanobacterial lichens (in the genera Pseudocyphellaria, Sticta and Peltigera). Investigations were carried out to determine if both cyanobacteria and green algae were present in these lichens and, if so, what were their roles.

Methods

The types of photobiont present were determined by light and fluorescence microscopy. Small carbohydrates were analysed to detect the presence of green algal metabolites. Thalli were treated with selected strengths of Zn2+ solutions that stop cyanobacterial but not green algal photosynthesis. CO2 exchange was measured before and after treatment to determine the contribution of each photobiont to total thallus photosynthesis. Heterocyst frequencies were determined to clarify whether the cyanobacteria were modified for increased nitrogen fixation (high heterocyst frequencies) or were normal, vegetative cells.

Key Results

Several cyanobacterial lichens had green algae present in the photosynthetic layer of the thallus. The presence of the green algal transfer carbohydrate (ribitol) and the incomplete inhibition of thallus photosynthesis upon treatment with Zn2+ solutions showed that both photobionts contributed to the photosynthesis of the lichen thallus. Low heterocyst frequencies showed that, despite the presence of adjacent green algae, the cyanobacteria were not altered to increase nitrogen fixation.

Conclusions

These cyanobacterial lichens are a tripartite lichen symbiont combination in which the mycobiont has two primarily photosynthetic photobionts, ‘co-primary photobionts’, a cyanobacterium (dominant) and a green alga. This demonstrates high flexibility in photobiont choice by the mycobiont in the Peltigerales. Overall thallus appearance does not change whether one or two photobionts are present in the cyanobacterial thallus. This suggests that, if there is a photobiont effect on thallus structure, it is not specific to one or the other photobiont.

Keywords: Zinc; heterocyst; carbohydrate; ribitol; photobiont; lichen; cyanolichen; cyanobacteria; green algae; phenotypic plasticity; Pseudocyphellaria; Peltigera; Sticta

Journal Article.  5970 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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