Freshwater and coastal lichens

We are analysing the patterns of functional and genetic diversity of lichenised fungi, algae and cyanobacteria in freshwater and coastal habitats across the world.

We are focusing on the ecology, biogeography and taxonomy of poorly known but species-rich groups (e.g. the family Verrucariaceae). Fieldwork is being carried out in Europe, tropical areas in Costa Rica, Venezuela and Malaysia, and the Valdivian temperate rainforest in Chile.

Lichen symbiosis is particularly successful in habitats with frequent changes in water availability. In contrast, long periods of constant immersion in freshwater or seawater is tolerated only by a small number of specialists.

Trait-based analysis

We are searching for traits that help to explain why these lichens cope better with aquatic and semi-aquatic conditions compared to terrestrial relatives in order to understand the benefits and limitations of lichen symbiosis.

Understanding the role of specific traits in stress tolerance helps us to identify candidates for the exploration of novel chemical compounds and gives us insights into the genetic basis for coping with gas diffusion-limited conditions. We also hope to develop a system of biological indicators of patterns of water level changes, stream bed stability and water chemistry.

As a consequence of the research we are revising the taxonomy of critical species groups, particularly within the family Verrucariaceae, and producing guidebooks and keys for freshwater lichens.

Methods

• Taxonomic revisions of critical species groups using molecular phylogenies, morpho-anatomical characters and chemical compounds as taxonomic markers.
• Physiological experiments on inundation tolerance, surface hydrophobicity and heat resistance.
• Assessment of the overlap between and the efficiency of randomised and targeted sampling techniques in tropical and temperate streams and rivers.
• Population density assessments and time series studies on the response of coastal lichens in the UK to environmental change.

Cryptic diversity

Cryptic diversity (closely related taxa which cannot be distinguished by morphological characters) at the species level, and even among non-related lookalikes, appears to be characteristic of some freshwater species groups with a high degree of adaptation to subaquatic life, for example the Verrucaria elaeomelaena complex.

Ongoing taxonomic work focuses on endemic freshwater lichens in Chile, and tropical species in the genera Hydropunctaria andStaurothele sensu stricto.

Study bias

We have addressed the current bias towards study in temperate areas of the northern hemisphere in most ecological and taxonomic studies of freshwater lichens through targeted fieldwork and collaborations with colleagues in tropical countries (Costa Rica, Venezuela, Brazil, Malaysia) and in areas of temperate rainforest in Chile and Alaska.

The material collected in these areas is now being identified and DNA-barcoded for comparison with the better-known temperate species from the northern hemisphere.

Data from randomised and targeted sampling are used to analyse community structures and distribution patterns of freshwater lichens in different climates.

Environmental change

Quantitative assessments of the population density of Heterodermia leucomela in coastal communities of southwest England have been carried out. The lichens show signs of compensation for population losses due to storm damage, through colonisation of new localities and an increased population density in surviving colonies.

Excessive trampling in the areas surrounding popular lookout points is likely to have contributed to the extinction of some local populations.

Algal partners

Lichens in the aquatic and subaquatic family Verrucariaceae associate with specific freshwater algae as a photosynthetic partner. This is a key innovation in their evolution.

The flexibility needed to adopt unusual algal partners seems to be linked to a simple crustose morphology without a clearly developed cortex. This flexibility may have facilitated the diversification of freshwater and marine species in certain lineages of this family (Thüs et al. 2011).