The heathland lichen Cladonia portentosa was collected from sites in mainland Britain differing either in rates of wet N deposition or in annual mean N concentration in rainfall based on a modelled data set. Methanolic extracts of thalli were analyzed by liquid chromatography time-of-flight mass spectrometry to yield metabolic profiles. Differences between sites in metabolite concentration were quantified using multivariate statistical tools and used to identify potential biomarker molecules. The abundances of three structurally related betaine lipids showed an increase with increasing modelled N deposition to a threshold of 22.3 kg ha-1 year-1 after which they remained constant. In contrast, the abundance of a phosphatidylcholine (PC) lipid showed concomitant decrease. Correlations of the identified biomarkers with N deposition and precipitation were stronger than those with N concentrations. The results presented in this study clearly show that N enrichment associated with tissue P limitation changes lipid composition, leading to shifts from PCs to betaine lipids, and that these lipids identified have the potential to be used as biomarkers for nitrogen enrichment.
Modelled data on wet nitrogen deposition and concentration were kindly made available for this work by Dr Rognvald Smith, Centre for Ecology and Hydrology (Edinburgh), Penicuik, Midlothian. Special thanks are to Prof. Stuart Gibb, Director of the Environmental Research Institute, for securing the funding for the project through the UHI Millennium Institute’s ARC Programme (Addressing Research Capacity in the Highlands and Islands). The ARC Programme was generously supported by the European Regional Development Fund, Highlands and Islands Enterprise and the Scottish Funding Council. The laboratory work was also supported by the UK Natural Environment Research Council (NERC) Biomolecular Analysis Facility at the University of Birmingham with FT-ICR MS analysis conducted by Ulf Sommer and Mark Viant. The substantive body of work presented here was carried out under the guidance of Dr Simon C Thain.